Sunday, August 24, 2014

ABC System

Welcome to my guide for the Mercedes Active Body Control (ABC) Suspension System.

In July of 2014, I purchased a used 2003 Mercedes SL500 with 80,000 miles on it. And within
days the infamous "ABC Drive Carefully" message started appearing on the dash. That was
my introduction to the ABC system. While my car was in the shop, I got the distinct impression 
the dealer was grasping at straws trying to figure out what was wrong, so I figured I would educate
myself about this system to avoid the dealership throwing parts at the problem at my expense.

Afterwards I decided to share what I have learned since I have encountered a lot of confusion and 
bad advice out there about how this system works and what to do to fix it when something goes 
wrong.  The online owner forums have been very helpful to me, and I figured I would give 
something back to the community. 

Troubleshooting Summary:
Here is a cheat sheet to help determine the cause of a problem and what action you should take. Or if you are having a repair shop do the work, this table can help you verify the shop has made the right diagnosis and is attacking the problem correctly.   

Action to Take
Any time "Drive Carefully" or "Visit Workshop" appears and it isn't related to a known issue you are dealing with.

The ABC control module has sensed something wrong and is warning you.  Blue or white is a minor warning, and red is a severe warning and usually indicates the system has shutdown.  

If the message only appears briefly, the problem was likely a momentary drop in system pressure. The system recovered and is working normally again.

If the message stays on the dash, something has gone wrong with the system. 

If the message stays on the dash, and this is the first time you've seen this error message in a while, pull the car over immediately and shut it off.

Check the fluid levels and make sure the reservoir is not empty or extremely low. If so, do not start the car again until you have added more fluid and restored normal pump operation.

If you attempt to drive the car without adequate fluid, the pump will be destroyed in very quick order (a $2,500 repair), while shedding metal debris into the system that will damage other components as well.

It is NOT worth trying to drive the rest of the way home!!!! Even if you do make it home, you will have drastically shortened the remaining life of the pump.

If the problem was not fluid level or pump related, you can continue to drive the car while you sort out what the problem is.

"Drive Carefully" appears for a few seconds when hitting a bump or dip in the road.

This may or may not be accompanied by fluid overflowing from the reservoir.

One or more of the accumulators has failed and is no longer supplying pressure to help fill the struts when needed.
Start paying close attention to the fluid level in the reservoir. If it starts to drop and the messages start coming more frequently, then you have blown an accumulator. 

Pull and inspect the front and rear axle accumulators - each are part number 2203270115 (the large ones). It's likely both are blown. Replace as necessary.  

The life expectancy of these accumulators is around 10 years or so. So if one accumulator has worn out, the other accumulators in the system are likely to be in similar condition and will fail soon. Given these accumulators only cost about $150 each, you should seriously consider just replacing them all as a preventative maintenance investment. 

One or more corners of the car sags or lowers when parked.

One or more corners of the car raise shortly after the car is shut off. 
Per MB, it is normal for the car to sag after a couple of weeks given the design of the valve blocks. If a corner sags after only a few days, then you should be concerned.

Possibilities are:

1) The strut may have developed a leak (less likely)

2) The valve block is not making a good seal, allowing fluid to escape out of the strut and return to the reservoir, (most likely). Possible causes are:
   a) Gunk has built up in the valve block (in the shutoff valve in particular) (less likely)
   b) The o-rings in the valve have deteriorated or are worn out. (most likely)
If the strut is visibly leaking, then it must be replaced. Also be careful not to let the ABC fluid levels get too low. Start carrying extra ABC fluid in the trunk until fixed.  

If the valve block is not holding a good seal, then the first action is to change the ABC fluid filter, perform a procedure called a "rodeo" that exercises the system, and then replace the filter again. This will sometimes clear up the problem, at least temporarily.  This will cost around $300. If the fluid is dark, you should strongly consider having it flushed, which will cost another $500.

If the filtering fails, there are DIY guides on the internet showing how to pull these blocks and rebuild them. Labor cost is around $1500 and parts costs is negligible. Although not a MB approved procedure, it is usually successful. 

You last option is to have the valve block replaced.  This will cost around $3,000, or around $1,500 if DIY. 

You can also find rebuilt valve blocks on ebay for a few hundred dollars.  

"Drive Carefully" or "Visit Workshop" appears on the dash every time you start the car and stays on.  It never goes away
There is a malfunction in the ABC system somewhere. The ABC system noticed a problem when starting up and has disabled itself, locking the struts in their current position.  This may result in what many people describe as a "tuna boat" ride (very bouncy).
Possibilities are:

  1. A loose connection to a sensor
  2. A sensor has failed
  3. The pump has failed
  4. The system has a fluid leak and the fluid level in the reservoir is extremely low, preventing the system from getting up to operating pressure.

Check the fluid levels in the reservoir. If empty do not attempt to start or drive the car. If you run the pump dry it will be ruined...a $2,500 repair! Go the parts store for a quart of "Pentosin CHF 11S" fluid and fill to the upper mark before starting the engine. 

If in fact the pump was run dry, the pump may require priming to get operational again. This is done by pressurizing the reservoir to help the pump get the fluid moving again.

Check the underside of the car for any signs of fluid leak.

If the system is unable to hold it's fluid long enough for a drive to the workshop, then have the car towed.

You will need a scanner capable of accessing the ABC control module through the OBDC port to get the ABC related codes, see sensor values, and run diagnostic routines. 

"Drive Carefully" or "Visit Workshop"  comes on the dash and stays.  Stopping and starting the car sometimes resets things back to normal for a while.

Possibilities are:
  1. The pump is weak, resulting in inconsistent performance. Sometimes the ABC system reaches operating pressure, sometimes not.
  2. A strut travel sensor or ride height sensor may be going bad. Some motion from driving gets it working again.
You will need a scanner capable of accessing the ABC control module through the OBDC port to get the ABC related codes, see sensor values, and run diagnostic routines. 

If it is the pump, low pressure error codes will have been logged.

Confirm the pump by monitoring pressure during a rodeo. If it can make it through the rodeo without the pressure dropping below 100 bars, the pump is fine.

A sensor problem will usually have error codes logged that point to the offending sensor.
You can hear a hum or whine under 2000 rpm. Is present both in gear and out of gear.  
The pulsation dampener has worn out.
There are lots of pumps and other non-ABC components that produce noises like this on these vehicles, so try to located the source of the noise using a stethoscope or long screwdriver. 

The ABC component most likely to cause this noise is the pulsation dampener. The location varies by model and year. On the R230 it is located in the front-left wheel well. On the W215 it is attached to the undercarriage near the transmission.
On later models with the updated ABC system (2007+), the pulsation dampener is on the ABC pump.

Replace the dampener. Cost is around $175 and is a relatively simple DIY project.  Or around $600-800 if you have a shop do it.

You hear a hissing or whistling coming from the valve blocks

On R230, they are located in the front left wheel well on the side nearest bumper, and the rear wheel well on the side nearest driver's door.

On the W215,  they are located in the front left wheel well on the side nearest bumper, and on the rear underside of the car. Follow the line from the strut to find it. 

Air is trapped in the line between the strut and the valve block.
Correcting the problem involves lifting the vehicle or jacking up the vehicle to remove weight on the struts. Then opening the bleed valve located in the wheel well to let any air out. You can find the exact procedure with a google search. 

The remainder of this document goes into more detail on how each component of the system works, how to recognize when they fail, and what courses of action to fix them. There is also advice on how to reduce the ownership costs related to the ABC system, how to maintain it, and other valuable information.

I will occasionally repeat certain concepts and advice in this document, since some concepts apply to more than one area, and readers may be jumping around or just partially reading the document.

General knowledge all owners of ABC equipped cars should know

The ABC system is a computer controlled hydraulic system. A control module monitors sensors and sends commands to the valve blocks to add/remove fluid from the struts. The control module's goal is to keep the chassis level at all times. Should any problems occur, it will inform the driver with a message on the dash.  A blue or white error message is considered a warning. The control module detected a problem but the system is still operational. A red message is more severe, and if it stays on the dash it means the system is not operational. Some errors can be be transient in nature, in which case the error message will disappear and the system will function normally again. "Drive Carefully" and "Visit Workshop" are the most common messages. 

"Drive Carefully" is a warning message saying the system is not operating at 100%. Low pressure is the most common cause. "Visit Workshop" is more severe. It indicates a component has failed and is in need of repair. 

Should the ride height of any of the 4 corners of the car fall to an unacceptable level, the ABC system will display a "Too Low" warning. Typical situations where this message may occur are 1) the car has sagged while parked and the height is below the minimum level. Pressing the ride height button should raise the vehicle and clear this message  2) When cornering or braking hard, the message may appear briefly which indicates the system is having trouble keeping up with demand (low pump pressure or failed accumulators the typical cause), and 3) The message comes on and stays, indicating some sort of leak or valve block failure or other component failure is preventing the corner of the car from being raised back up to an acceptable level. You should pull over or risk the tires coming into contact with the wheel linings or fender. It is better to deal with the inconvenience of having the car towed rather than incur expensive repairs to the car.

Error messages will have associated codes that will be logged, and they can be retrieved later by diagnostic tools. 

If an ABC error message comes on the dash for the first time (it is not related to a known issue) and stays, you should pull over immediately and shut off the engine. Seconds count. Check the fluid reservoir and make sure it has fluid. If not, do not attempt to start the car until you have added fluid back. The reason being if the problem is a fluid leak, there will inadequate fluid to lubricate the pump. Should the fluid run dry the pump will be destroyed (a $2,500 repair), and it will shed debris with sharp edges into the ABC system. This will generate problems with downstream components for years to come. Don't even think about trying to make it home without fluid.

Driving the car while the ABC warning message is on the dash can be very dangerous, especially at highway speed. Hence the "Drive Carefully" message. The system is in limp mode allowing you to get the car to the workshop. It is not to be ignored.

The reservoir dipstick has two marks. The upper one is when the engine is off. The lower one is for when the engine is running. It takes a few minutes for the system to depressurize and the fluid to return back to the reservoir when the engine is shut off. Be sure to wait before checking the fluid level. Fresh fluid is clear with a green tint. If it has become brown or black, you should replace. Contamination is extremely hard on hydraulic o-rings and seals and pumps and will lead to frequent and costly repairs. 

I strongly recommend driving the car at least once per week during the winter months. Inactivity is really hard on hydraulic systems, even more so than daily use. Other components on your car will appreciate it as well.  

ABC System Design

The ABC system can be found predominantly on the S, SL, and CL models.

R230 SL Class (2003-2012 : SL320, SL500, SL550, SL55 AMG, SL600, SL65 AMG)
R231 SL Class (2012+ : SL350, SL400, SL550, SL63 AMG, SL65 AMG)
C215 CL Class (2000-2006 : CL500, CL600, CL55 AMG, CL63 AMG, CL65 AMG)
C216 CL Class (2006+ : CL500, CL600, CL55 AMG, CL63 AMG, CL65 AMG)
W220 S Class (1998-2006: S280, S320, S350, S430, S500, S600, S55 AMG, S65 AMG)
W221 S Class ( 2005-2013 : S280, S300, S350, S400, S450, S500, S550, S600, S63 AMG, S65 AMG )

The system consists of the following components: The exact location will vary depending on what series and model you have. But the design is the same.

     W215 Component Locations

      R230 Component Locations

ABC Control Module
Controls the system. 
Supplies hydraulic pressure for the system
Pulsation Dampener
Smooths out the pressure from the pump (P/N 2203270215)
Pressure Limiting Valve
Bleeds off any pressure in excess or 200 bars
Pressure Sensor
Reports the system pressure to the control module
Front Axle Valve Block
Controls letting fluid in and out of the front struts, based on
commands from the control module
Rear Axle Valve Block
Controls letting fluid in and out of the rear struts, based on
commands from the control module
Front Axle Accumulator
Stores fluid and pressure for the front struts, assisting the
pump by supplying the on-demand pressure needed to fill the
front struts. (P/N 2203270115)
Rear Axle Accumulator
Stores fluid and pressure for the rear struts, assisting the
pump by supplying the on-demand pressure needed to fill the
rear struts. (P/N 2203270115)
Connects the chassis to the wheels and the coil spring inside absorbs vibration. Pumping hydraulic fluid into the strut raises the vehicle.
Return Accumulator
Evens out the pressure on the return side of the hydraulic
system (once the fluid leaves the struts) (P/N 2203270415)
Return Pressure Check Valve /
Temperature Sensor
Maintains a minimum return side pressure of 10 bars, and reports the oil temperature to the control module
Oil cooler
A small radiator that cools the hydraulic fluid
Stores extra fluid for raising the ride height and pressurizing the accumulators and replacing any leaked fluid.
Strut position sensors
Reports to the control module the position of each strut
Ride height sensors
Reports to the control module how high each corner is
Motion sensors
Reports to the control module what the car is doing motion-wise (accelerating, slowing down, cornering)

ABC Hydraulic Design

Pictured below is the schematic for the hydraulic portion of the ABC system. I've found this diagram to be the most informative of any of the diagrams out there on the ABC system. Taking time to understand this diagram is key to understanding how the ABC system works, and will help you determine if the repair tech's assessment of the problem is correct.

Going on a brief tour of the diagram…the ABC fluid starts its travel at the fluid reservoir(2). From the reservoir it is drawn into the pump(1). The pump pushes the fluid to an assembly(52) containing a pulsation dampener(52a) that reduces vibration, a pressure limiting valve (52b) that regulates the pressure at 200 bars, and a pressure sensor (B4/5) that reports the pressure to the control module.  From there the fluid travels to the front and rear valve blocks(Y36/1 and Y36/2) which are used by the control module to manage the amount of fluid in the struts(40,41). Accumulators(4,14) are connected to each valve block to store fluid and pressure for filling the strut. The control module commands the valves to open or close which allows fluid to enter or leave the struts. When the fluid leaves the struts, it travels through a temperature sensor(B40/1),  and then through the oil cooler(9) and back to the reservoir(2).  An accumulator (53) helps even out the spikes in the return side pressure caused by the struts letting out fluid.

There are two generations of the ABC system. The first generation is from 2000 - 2006. The second generation was introduced in 2007. The main differences between the two generations are the valve blocks being improved and the pulsation dampener relocated to be attached directly to the pump.  

While the locations of the components is generally the same across all models, there tends to be variation  in how the components are positioned in terms of brackets, orientation, and so forth. Many of the pictures and diagrams are for the SL500, but the concepts apply to all ABC equipped models. 

I know the SL model got the new ABC design starting in 2007. I'm not sure if the CL and S models got the update at the same time or not. The easiest way to see which ABC generation is on your vehicle is to look at the ABC pump on the upper right side of the engine. If a black sphere is attached to the pump, your have the second generation design.  

ABC Electronics Design

There is also the electronics side of the design.

Each wheel well has a ride height sensor reporting to the control module how high the corner is. There is a sensor in each strut that reports how far extended the strut is. There are acceleration sensors that report how fast the car is speeding up or slowing down or changing direction. The control module monitors all these sensors and decides whether to add or remove fluid from each strut, with the goal of keeping the chassis level. It reevaluates 10 times per second.

All these sensors and valve solenoids are wired to the ABC control module. Failure of any of these sensors will disable the ABC system, causing an "ABC Drive Carefully" or "ABC Visit Workshop" message on the dash.  Electronic issues with the ABC system are rather rare. The majority of issues are hydraulic related.

Mercedes dealerships and other workshops that work frequently on Mercedes vehicles will have the STAR Diagnostic System, referred to as SDS or STAR. It is software that runs on a laptop along with various interface cables. It was developed by Mercedes for their vehicles. It connects to the various control units on the car (like the ABC system), and can retrieve error codes, examine the current values of sensors, execute diagnostic routines, calibrate sensors, view the error history logs, etc. Fixing an ABC issue will often require SDS to get the error codes, so a trip to a workshop with SDS may be necessary.  Handheld scanners on the market from Foxwell, ANCEL, etc are getting better every year and may be able to access the ABC controller and run diagnostic routines.

The major ABC components in depth


The power steering and ABC pump are integrated into one unit, referred to as a Tandem Pump. Although the two pumps share the same housing, they are separate components otherwise. It is possible for the ABC portion of the pump to fail but the power steering portion of the pump is fine (and vice versa). But if one portion fails, both have to be replaced since they are one unit.

There seems to be some consensus that the average life of a pump is around 60-80K. Like any component, some will fail sooner and some may last much longer. Pumps cost around $1,500  and $1,000 in labor to replace. There are rebuilt pumps on the market for around $500, so you can get your cost down to around $1,500 if you go that route and with an independent shop. There is also a seal kit available if you want to tackle rebuilding the pump yourself. 

Integrated into the pump is a suction restrictor(Y86/1) or throttle valve. It is wired to the control module and open and closes based on the voltage supplied to it. The opening and closing of this valve controls the rate of flow from the reservoir into the pump.

The pump may fail one of two ways.

  1. It goes completely and cannot generate any pressure. 
  2. The pump wears and cannot maintain steady pressure as it did before. It progressively gets worse, making error messages on the dash more frequent and more persistent.

The pump failing completely should be pretty obvious to diagnose. The ABC "visit workshop" or "drive carefully" message will appear shortly after the car is started, and the message will stay on. The car will not raise on command either. ABC control module error codes will indicate inadequate or no pressure. 

If the pump is weak, you should also get ABC "Drive Carefully" and "Visit Workshop" messages, but they will be intermittent in nature. In many cases you can restart the car and the system will pressurize successfully, and the error message will clear. The car works normally again for a while. There will also be pressure related error codes logged as well.

Some owners report the ABC system operates fine when the car is cold, but the error messages start appearing after the car has warmed up. The reverse also seems to be reported. Temperature does seem to be a factor.

It is also suggested the suction valve may be the culprit, and not the pump itself. But unfortunately the valve is not sold separately, it comes with the pump. Check to make sure +5 volts or more is present at the suction valve if the pump is not producing pressure in order to rule out wiring or control module issues.

Some owners report a grinding or growling sound when the pump goes bad. A pump that is noisy at idle but get's quiet when under load (by pressing the ride height button) is also a sign of a worn pump. 

It should also be noted that low pressure codes do not necessarily mean the pump. Accumulators can fail leading to intermittent low pressures (when hitting bumps). The suction restrictor valve could be malfunctioning. The pressure limiting valve could be malfunctioning. The pressure sensor itself could be malfunctioning.

If air gets into the system for some reason, such as accumulator failure or repair work or running the fluid reservoir dry, it is possible that there is an air bubble in the pump, preventing it from generating pressure. I recommend ruling out this scenario before spending $2500 to replace a pump. The procedure (priming the pump) involves pressuring the reservoir to force fluid down into the pump to help get it going again. If that fails, another option for pre 2007 models is to remove the pulsation dampener to eliminate any flow resistance, then start the car and wait for fluid to stream out the opening. Then stop the engine and replace the dampener. 

The best way to know for sure if the pump is bad is to monitor the pressure while doing a rodeo. The rodeo will stress the system, and even a good pump will see about a 1/3 pressure drop at times. So if the car can get through the rodeo successfully, then the pump is probably fine. If there are still pressure related codes being generated after passing a rodeo, I would recommend investigating some of the other possibilities mentioned earlier.

Pulsation Dampener / Pressure Limiting Valve / Pressure Sensor Assembly

W215 (under passenger floor)
R230 (left front wheel well - towards rear of car)

These three components are grouped together into the same assembly. They are the first set of components immediately after the pump.

There is a pulsation dampener(52a) attached to the assembly, part number 2203270215. It is a black sphere. It is similar in design to the other three accumulators in the system (nitrogen gas behind a rubber membrane), but much smaller. Since the fluid flow from the pump is "choppy" given the nature of its design, something is needed to smooth out these waves or vibrations in the fluid. This is the job of the dampener. Air behind the rubber membrane acts as a cushion and evens out the pressure, much like a gas shock absorber removes road vibration.

There is also a pressure limiting valve (52b) integrated into the assembly. It is a passive device, not actively controlled by the control module. It will open when the pressure exceeds its designed limit (~200 bars), allowing any excess pressure to be bled off. Its job is to help regulate the system pressure.

Lastly, there is a pressure sensor (B4/5) attached to the assembly, and it is wired to the control module. A resistor inside the sensor alters the voltage passing through the sensor based on the amount of pressure applied to it. At zero pressure the voltage is around 0.6 volts. At full pressure it is at 5 volts. The control module monitors this voltage, and infers the system pressure from it.

There is an inverse relationship between the voltage from the pressure sensor and the voltage supplied to the pump suction valve. When the voltage from the pressure sensor is low (the system needs more pressure), the voltage to the suction valve will be high (open up the valve and give me more), and vice versa.

On 2007+ models with the updated ABC design, the pulsation dampener is attached to the pump instead. Replacing is a little more difficult that on older models. The MB workshop instructions say to remove the pump first, but other owners have reported that loosening the bolts that attach the pump to the engine will create enough clearance to get the dampener off.  No need to disconnect any hydraulic lines or remove the ABC reservoir.

So what can go wrong with these components?

1) The pulsation dampener fails. The rubber membrane inside of it eventually breaks down, and the dampening ability is lost. You will hear a humming or whining sound caused by the fluid vibration. The ABC system will function normally though, although the vibrations will stress the system if not addressed. I don't believe the control module will notice this, so no error codes or warning messages will appear. Just an annoying hum or whine, most noticeable with the top up and driving at slow speed around parking lots.

2) The pressure limiting valve could be opening at too high a pressure. There aren't any error codes for excess pressure, so the control module will likely not notice this. The control module will still regulate pressure by controlling the intake valve (suction restrictor) to the pump, so the system will probably work fine, except for spikes in pressure at times which would add stress to the ABC components. Valves of this nature are generally pretty reliable, so I don't this this is a very likely scenario. 

3) The pressure limiting valve opens at too low a pressure. I don't think it is a likely scenario but it is theoretically possible. In this scenario the control module will sense the need for more pressure and tell the suction valve on the pump to open up, and meanwhile the pressure limiting valve will constantly route all that extra fluid back to the reservoir in an endless loop. If the pressure that this is occurring at is below the acceptable pressure to operate the ABC system, the control module will shutdown the ABC system and display a warning on the dash. If this endless loop is occurring at a pressure above the minimum but below the ideal, then the system will operate normally but the pump will be working extra hard. I'm guessing the control module would not notice this situation. It won't be able to tell the difference between a weak pump and a pressure limiting valve letting off pressure too soon. But a constant 5V at the suction valve would indicate the pump is working full throttle all the time.

4) The pressure sensor is not working correctly. It could fail. There could be a loose connection between the sensor and the control module. It could be sluggish in responding to pressure changes. Or it could just be wrong about the pressure it is sensing. The control module is able to detect a loose connection or a completely failed sensor and log an error code to that effect.  If the sensor is bad, the ABC control module will report no pressure and/or an error code for the sensor, but the car will still rise when pressing the ride height button. If the sensor is reporting higher than normal, you will likely see frequent "too low" error messages since the system can not keep up with demand. The ride will cause get bouncy and handling will deteriorate since the system cannot add fluid to the struts fast enough to meet demands. If the sensor is reporting lower pressure than actual, then either 1) in extreme cases the control module will think the system is below normal operating pressure and will shutdown the ABC system along with dash warnings and error codes. It could mimic a pump failure. 2) if a minor case, the normal drops in pressure will be more exaggerated from the control module's perspective, leading to intermittent "drive carefully" messages and occasional ABC system shutdowns.

5) Occasionally the o-ring that provides the seal for the pressure sensor will fail, causing a fluid leak. There is a repair kit available (part number A2203201158) for approximately $80. Don't let the workshop convince you that you need to replace the entire assembly, a $1,250 part. If the kit is not available, the shop should be able to improvise something.

Here are some DIY resources:


The accumulators (#4,#14,#53 on the hydraulic diagram above), are often referred to as a "air cell" or "nitrogen ball". They are black spheres that contain nitrogen gas (air) trapped behind a rubber membrane.  Hydraulic fluid is allowed to travel in and out of the sphere based on the pressure differential between the rest of the system and the air on the other side of the membrane. The compressed air in the accumulator pushes back against the fluid and can either absorb pressure or supply pressure. 

There are three accumulators placed strategically in the system. The two larger ones (4 and 14), part number 220 327 01 15, are connected to each of the two valve blocks and they provide the pressure necessary to add fluid to the struts when the valves open. The pump's job is just to keep these two accumulators topped off. These two are the most critical ones that will cause problems when they fail. The third accumulator(#53), part number 220 327 04 15, is often referred to as the "center" or "return" side accumulator. It is smaller than the other two and it's job is to smooth out the spikes in pressure that result when fluid is being let out of the struts. These accumulators are often overlooked since many techs do not understand their true function in the system. They are much more than just fluid repositories.

The pulsation dampener(52a), part number 220 327 02 15, is arguably an accumulator as well. The design is the same. Its size and position in the system has it serving a different purpose though. It is to even out the vibrations in the fluid from the pump. When it fails you get a hum or whine noticeable below 2000 rpm. 

When you ask shop techs how many accumulators the system has, you often get different answers ranging from two to four, depending on whether they are considering the return accumulator and pulsation dampener in that count. So when talking to the shop about accumulators, it is best to clarify what they are referring to. 

These accumulators wear out. Like any sort of wear part, how fast depends on a lot of factors. 60K-80K miles or about 10 years seems the norm from what I read. Your mileage may vary based on the age of the car, driving conditions, and so forth.

So, what can we take away from all this?

1) The dampener and return accumulator are important in that they smooth out the system pressure. Keeping those healthy will reduce stress on the system.

2) If the two larger accumulators(4 and 14, P/N 220 327 01 15)  that provide pressure to fill the struts were to weaken or fail, then there will be momentary drops in system pressure. It will be most noticeable when the car hits a bump or drop in the road, which requires fluid to be added quickly to the struts to compensate. The control module is monitoring system pressure and when it sees the pressure drop, it puts the "Drive Carefully" warning on the dash. When the pump catches up moments later the message goes away.  You may also get frequent "Too Low" warnings, since the system can't fill the struts fast enough to meet demand. 

If an accumulator has failed, the accumulator will fill with hydraulic fluid and the air will begin to work its way out of the system. While this process is occurring, the free air in the system expands whenever the system depressurizes, pushing fluid to and out of the reservoir. The fluid level in the reservoir will drop during this phase.  Eventually all the air gets out, and the overflow behavior will cease. But by now you should be getting very frequent "Drive Carefully" messages on the dash due to the lack of available accumulator pressure to help fill the struts. The air in the accumulator has now been replaced by fluid, and it cannot accept any more fluid when the car is started and the system pressurizes. 

The ABC system can run fine with only one main accumulator working. If you look closely at the hydraulic diagram, you'll see there is a clear path for fluid to flow from the rear accumulator to the front valve block, and vise-versa. The two accumulators can assist each other. So if "Drive Carefully" messages only appear during extremely large bumps or drops in the pavement, then it is likely you have just one blown accumulator. If the problem has reached the point that you are getting drive carefully messages even on modest bumps, then it is likely both main accumulators have failed. 

Shop techs who see "low pressure" codes in the logs often conclude that the pump needs replacement, when in reality one of the main accumulators have failed.

So how do you tell if your accumulators are in good shape?

Observing the dipstick levels. There are two notches on the dipstick. The lower one is when the engine is running, the upper one for when the system is off and fully depressurized. If your fluid levels are set correctly for when the car is shut off(the higher mark), and then you start the car and don't see the fluid level drop to at least the lower mark, then there is likely a blown accumulator(s).  On my 2003 SL500 with 4 fresh accumulators, the level drops from the top notch to the bottom notch and an additional 1/3 below that, Each of the two main accumulators account for about 1/3 of the level drop. The pulsation dampener and return accumulator are smaller and together make up the last 1/3.  

RPM drop when cycling through the ride heights. The accumulators, when healthy, have enough capacity to lift the car to the next level when the ride height button is pressed. If you press the ride height button once, and you hear a RPM drop during the raising process, then you may have a blown accumulator. The pump is having to step in and help lift the car. 

Visual inspection. This will require removal of the accumulator.If an accumulator has blown, it will be full of fluid, and you can reach a considerable distance into it with a wire or screwdriver. But if you are going to go through the work to pull the accumulators, I recommend just replacing them regardless of what you find, as a preventative maintenance investment. Accumulators cost about $150-200 each. 

Dash warnings and low pressure error codes in logs. If just one of your accumulators is blown, you will start seeing occasional dash warnings when hitting only very hard bumps or drops in the pavement. When the second one blows the messages will start appear very frequently and on even modest bumps in the pavement. 

The locations of the valve blocks and accumulators vary between the S, CL, and SL models.

The front accumulator and pulsation dampener in the front wheel well is a fairly easy job to replace. It takes about an hour for a SL500, and maybe a little longer for other models if you need to loosen some brackets to the valve block to get better access to the accumulator. The pulsation dampener on the opposite side of the wheel well is easily accessible. A set of slim or thin wrenches will be required to break it loose. From there in screws off/on like a light bulb.  

The rear accumulators are fairly easy as well. For the R230 they are both located in a rack that slides out of the left rear wheel well, just behind the driver. There is a bracket below the rack that holds some hoses in place. Remove that and then a nut at the top of the rack that holds the rack in. After those are removed you can slide the rack out and get easy access to both the rear accumulator(the larger one) and the return accumulator (the smaller one). Once replaced, slide the rack back in, put the top not back and the bracket on the hoses, and your done. About 60-90 minutes. A set of slim wrenches are necessary to break loose/tighten the return accumulator.

To recap, there are two common symptoms that point to your accumulators failing. The brief "ABC Drive Carefully" messages on the dash, and the reservoir overflowing a few minutes after the car is shut off. 

Valve Blocks

The valve blocks control the amount of fluid in each of the 4 struts. There are two valve blocks, one for the front struts and one for the rear struts.

Pre 2007 Valve Block
2007+ Valve Block

    This model has been discontinued.

    This is the updated design and is compatible with 
    the pre 2007 ABC system as well. This is 
    the valve block you get when you order a new 

For each strut, there are two valves (Pre 2007 version shown here)

The main control valves (y1,y3 in schematic, lower valve in picture) is a 3-position valve.  In the outer position it allows fluid to enter the strut, in its center position it closes off the strut, and in the inner position it allows fluid to leave the strut.  When the ABC system is active, this valve is doing all the work.

The other valve (y2,y4 in schematic, upper valve in picture) is the shut-off valve that sits between the main control valve and the strut. It's purpose is to lock the struts at their current fluid levels when the ABC system is not in operation.  When the car is not running or the transmission is not in gear, this valve is closed. When the car is put into drive or reverse, the valve will open and allow the fluid levels to be managed by the main control valve. If the control module senses a malfunction and disables itself, it will also close this shut-off valve for safety reasons.

So each valve block has 4 valves in it in total. These valves open and close based on voltage being supplied to them by the ABC control unit.

These valve blocks rarely fail outright. What happens is the o-rings in the valve deteriorate (most likely), or gunk has built up in the valve block (less likely). Either way, the valve no longer makes a good seal. As a result, fluid slowly escapes out of the strut, past the shut-off valve (y2) and the main control valve(y1) and returns back to the reservoir. This causes the strut to lower and the corner of the car to sag while parked.

It is important to remember that the car sagging after a couple of weeks is completely normal, according to Mercedes.  The tolerances in the design of the valve block will allow some leakage to occur over time. The height should return to normal when using the ride height button or putting the car in gear.  

If a o-ring in the valve block is bad, it is also possible for a corner(s) of the car to raise a little shortly after you shutoff the car. The reason being is that it takes a few minutes for the pressure in the system to dissipate, and during that time fluid could get past the seal and flow INTO the strut.  

It is possible (although rare) the car sagging is a fluid leak, such as the line between the valve and the strut, or a leak in the strut itself. If the leak is severe enough to cause noticeable sagging while parked, there should be obvious signs of the leak, so visually check the strut to rule out it leaking fluid. If the strut is leaking, there should also be a drop in fluid level in the reservoir. 

When the car is running, the control module will compensate for any leaks in the valves. It is constantly monitoring levels and adjusting as necessary. The sagging would only occur when the car is parked and shut off.

If you are having issues with a corner of the car not being at the correct level when running, or exhibiting other odd activity, then you probably have a sensor issue or calibration issue.

Sometimes a "C1531"suspension strut moves although locking valve is closed" error code will get logged..most likely while is stop and go traffic or idling at stop lights. It generally indicates the valves are sticking or jamming from being held still too long. 2007+ model years have a software update to periodically move the valve a little to reduce this. So you probably shouldn't get too concerned about these errors in your logs unless you notice symptoms.

Unless the sagging problem while parked is severe, it is not an urgent problem that you have to rush to the repair shop for. You can monitor the situation over time and decide when it has reached the point you want to fix it. In the meantime be careful not to let the corner sink all the way down while parked, as the wheel well may come into contact with the tires. Be sure to park with the wheels oriented straight ahead to avoid wheel well damage, and to start the car periodically to pump up the strut.

You should also be sure to keep an eye on the reservoir fluid levels. If too much fluid leaves the struts, it may overflow the reservoir. Then when you start the car and the struts are pumped back up, the system may be extremely low on fluid, which may cause pressure problems and/or damage the pump. It would be wise to carry a spare quart of ABC fluid in the trunk for this situation.

If and when you have to fix the valve blocks, you options are to:

1) Filter the fluid (requires two filters) and perform a rodeo (a test that exercises the system) . Cost would be around $200-300. If the cause is debris rather than an o-ring, then it may dislodge some debris from the valves, but the results will be marginal and probably temporary. Sort of like trying to clean a fry pan by just running water over it. If the ABC fluid is older that 40K miles, many on the forum would suggest replacing the fluid as well (about $250+labor). 

2) Overhaul the valve assembly (Pre 2007 version). This is not an approved MB procedure. Many owners have reported success in pulling the valves and cleaning them. Replacing the o-rings is also a good idea. There are DIY write-ups and a youtube video as well. Your local independent shop may be willing to do this for you, with no guarantees of course. Parts cost would be minimal and labor cost around $1,000. 

Although usually successful, there is no guarantee of success with this. Sometimes the cylinder walls may be scored or deformed, so new o-rings may not be enough to allow the valve to create a good seal. 

Some good o-ring sources are APG (, and "The O-Ring Store" ( You can also find rebuild kits for $30-$50 on the internet with all the o-rings and gaskets needed. 

O-Rings Required for Rebuild: Buna 90 (Nitrile)  O-Rings
  APG part #    Cross-Section    Inside Diameter    Outside Diameter    Height    Quantity per block 
H90018 .070 7/8  (0.739) 1 1/16 4
H90016 .070 5/8  (0.614) 3/4 1/16 6
H90015 .070 9/16 (0.551) 11/16 1/16 2
H90014 .070 1/2 (0.489) 5/8 1/16 4
H70013 .070 7/16 (0.426) 9/16 1/16 4
There are also some spacers that may need replacement - (Buna-N 70 square cut o-rings)
They are APG part numbers SH70013, SH70014 , SH70015 , and SH70016. One set per block.

Tip: Be careful to note which o-rings came from which position on the valve when removing them.
Tip: The square spacers are prone to breaking when installing. Ordering a few extra may be wise.

3) Replace the valve block assembly. This will run you around $2,000. If it is a warranty repair or saving money isn't a goal, then this is the best option to fix the problem. You can also find rebuilt or refurbished valve blocks on the internet for $200-$300 each.

Anyway, valve blocks leaking is a very common ABC problem and is also the easiest problem to diagnose. If the car sags when parked, and there are no signs of fluid leakage, then you have a leaky valve block. There aren't any other explanations. 

I've read numerous reports where repair shop told owners they need new pumps or struts to fix this issue. If you are told this go to another repair shop since the tech clearly doesn't understand the design of the ABC system and is grasping at straws(at your expense).

Here are some DIY resources:


The ABC strut contains a spring and a shock absorber, just as standard struts do. As in typical struts, the spring does the work of supporting the vehicle's weight and absorbing impacts from bumps in the road, and the shock absorber keeps the spring from oscillating or "bouncing". 

The ABC strut goes a step further, and adds a hydraulic fluid chamber so the height can be adjusted by adding or removing fluid. This enables the ABC control module to adjust the height of each corner of the car, and it does so  at the rate of 10 times per second.  

While many suspension systems offer the ability to adjust the stiffness of the shock absorbers, it is still a compromise between ride comfort and handling. The ABC system approaches the problem differently. It 
constantly reads the inertial sensors and the ride height sensors for each corner, and adjusts each strut's height independently to keep the chassis level and provide optimal handling. It's why the car doesn't lean when cornering, and why the nose doesn't dive when you hit the brakes.  And it does all this without compromising comfort. 

But these benefits do come at a cost. It is a more complicated system, and like most hydraulic based systems, it is prone to hose leaks and for seals and o-rings to break down and pumps to wear out.  But in the final analysis, I think the benefits are worth it. 

But back to the strut in more detail.

Inside each strut there is also a electronic sensor, referred to as the strut travel sensor. It reports to the control module how far extended the strut is. The control module compares this to the ride height sensor in each wheel well, and to the inertial sensors to decide how much fluid should be in the strut at that moment. This travel sensor can fail. Generally when this happens the control module will put the ABC error message on the dash, and will log an error code pointing to the offending sensor. If the ABC system is having difficulty maintaining the correct height for a corner of the car, this travel sensor reporting incorrect information is one of the possible explanations. If errors pop up on the dash intermittently while entering or leaving your driveway, this sensor is often the culprit. Fixing it requires replacing the entire strut. It just isn't cost effective to disassemble a strut to replace the sensor. 

These struts will eventually wear out. The seals and o-rings will eventually develop leaks. To get as much life out of them as possible, make sure you are using fresh hydraulic fluid, and inspect the boots (the rubber accordion like cover) to make sure there are no rips or tears. Dirt or dust inside the strut will wear out the seals quickly. Once they do leak, there isn't much you can really do about it except to replace the strut.  Mercedes charges around $1,250 for a replacement strut, and around $800 or so in labor to install. A popular route is to go with Arnott struts, which is a remanufactured strut with a lifetime warranty for around $500, and have an independent shop install it.

Sagging issues are not caused by a bad strut (unless fluid is leaking externally). There is only one entrance/exit for fluid into/out of the strut, and the control of that flow is at the valve blocks. The typical sagging issue occurs when the seal in the valve is bad, causing fluid to escape out of the strut and go back to the reservoir. There are no components within the strut itself where the failure would lead to sagging issues. 

Replacing the strut is fairly straightforward. There is just one hydraulic connection and one sensor connection. Beyond that it is not much different than any other strut to physically remove and install. After installation, it will require extra steps to fill the strut with fluid and bleed the air out of the line. And the SDS system (or a scan tool capable of executing ABC control module commands) will be needed to recalibrate the ride height.  An alignment may also be necessary.

When the ABC system shuts down due to a system failure, owners report what they describe as a very bouncy ride, nicknamed the "tuna boat" ride. The reason for this is that the gas shock absorber integrated into the ABC strut is smaller than usual and only handles oscillations at frequencies above 5 Hz (5 oscillations or 5 bounces per second). The ABC control module is responsible for handling oscillations below 5 Hz, and it does this by rapidly adjusting the fluid level in the hydraulic fluid chamber.  When the ABC system is shut down, the dampening ability below 5 Hz (the larger oscillations or bounces) are not being handled anymore.  You are in effect now driving a car with no shock absorbers! 


The purpose of the reservoir is to provide a place to store fluid when not in use.  The green arrow points to the dipstick location. The dipstick has two marks. The lower one for when the engine is running. A higher one for when the system is shutoff. When all is well and there are no leaks, you should never have to top off the fluid levels. Dropping fluid levels indicates a leak in the system somewhere or a blown accumulator.

The fact that the dipstick mark for when the car is running is lower than the one for when the car is off indicates it is normal for some fluid to flow from the ABC system back to the reservoir when the car is shut off and the system depressurizes. Remember that fluids do not compress. Only the air in the accumulators will compress. What happens is that the accumulators hold fluid when the system is under pressure, and the amount of fluid returned is proportional to the amount of fluid being held there during operation. It can take 5-10 minutes for the system to depressurize, so be patient before taking measurements or beginning repairs.

Sometimes, the reservoir will overflow after a drive. If this happens, there are two possible explanations:

  1. It is a common to interpret the marks on the stick an cold and warm like coolant levels markings, which is incorrect for ABC fluid levels. If the reservoir was filled to the top level on the dipstick while the engine is running, the reservoir may overflow when the engine is shut off. (remember fluid is drawn into the accumulators when the engine is running, and the fluid returns back to the reservoir when shut off). 
  1. An accumulator membrane has blown. Then when the system depressurizes the gas expands, pushing fluid into the reservoir and overflowing it. Eventually the air works out of the system, and fluid replaces the air in the accumulator. As a result you will see a drop in the fluid level in the reservoir. This is the most common reason. 

So the take away is that the reservoir overflowing fluid out the dip stick cap is a pretty clear sign an accumulator has failed. If you also find you need to keep your fluid levels somewhat below normal to keep the reservoir from overflowing, it indicates a failed accumulator in the process of losing all its air.

It is also important to keep an eye on fluid levels if you have a blown accumulator or a leak in the system. If the level of fluid gets too low in the reservoir, the pump will start ingesting small quantities of air along with fluid, leading to a loss of pressure and ABC warnings on the dash. 

It is important to keep this fluid clean and fresh. As time goes by, the o-rings, seals, hoses, accumulator membranes, struts, and other rubber components shed microscopic debris into the fluid. This debris shortens the life of the components in the system, especially the valve blocks. This gunk tends to deposit on the valve surfaces preventing a good seal, and interfere with optimal lubrication within the pump. Microscopic metal shavings will also accumulate in the fluid, acting like sandpaper for your pump bearings and valve block o-rings. 

It is extremely important to keep contaminants out of the ABC fluid when checking the level. Wipe the area around the dipstick clean, and use a lint free cloth to check the fluids. Any contaminants that get into the reservoir will make a trip through the ABC system before encountering the filter.

Some owners claim that by keeping the ABC fluid pristine they have been able to make it to well over 120,000 miles without ANY repairs needed to the ABC system. The evidence is anecdotal, but it does make some sense. So flush the fluid when it shows signs of wear (turns dark in color).  


There is a considerable amount of hoses and piping traversing the vehicle. Each connection point presents an opportunity for a leak to occur. A typical hose leak repair will cost you $200 - $500.  

Some people like to speculate that a hose blowing will cause the car to drop suddenly onto its wheels and cause a crash. This is highly unlikely. The control module has to energize the shutoff valve for each strut to allow fluid to enter or leave the strut. If the control module senses anything wrong (like a pressure drop), or the control module itself would fail, then the voltage to the shutoff valves will be interrupted and the valves will close, locking the fluid in the struts. Conceivably the line between the valve and the strut could fail, or the strut itself could fail, but I suspect these parts are designed not to "blow" altogether, giving you enough time to pull over.

Some sections of hose/piping that run through tight spaces near the engine can be extremely expensive to repair..sometimes over $2,000. An alternative to replacing these lines is to repair the existing line by visiting a hydraulic shop and have them patch the bad section of line by fitting a couple of compression joints. Or even run a completely new line along a different route between the two ABC components. Some independent repair shops may have sufficient hydraulic skills to perform the repair as well.  You may be even to find some kits on-line to replace commonly failed lines...particularly that expensive line that runs through tight spaces under the engine. 

The control module is the brains of the system. It is constantly monitoring the inputs from the sensors, and adjusting how much fluid should be in each strut to meet the needs of the current situation.
The "sport" switch changes how aggressive the control module should be about countering the leaning effects.

The driver can also increase the ride height by either 1/2 inch or 1 inch. This is useful when more clearance is needed, like driveways and speed bumps.

If the control module senses inadequate pressure to operate, or if any of the sensors provide in-plausible data, the system will shut itself down for safety reasons. The offending sensor will be logged for later retrieval by the SDS system (or a scan tool capable of accessing the ABC control module).

Loose connections can happen - sometimes corrosion builds on the connectors, so pulling the connectors and cleaning the contact points may help. Bad sensors or loose connections will often result in an error code with "fault in component" as part of the description. Values of 255 from a sensor generally means "no reading" or "bad input".

Ride Height Sensors

In each wheel well you find a ride height sensor. It reports to the ABC control module how high the corner of the car is. If a corner of the car gets too low, an "ABC Car Too Low" message will appear on the dash. 

Usually this message indicates a low pressure issue preventing the ABC system from keeping up with demand, but the sensor can go bad and give false information or no information to the control module.  If you suspect one of these sensors is the problem, do a visual inspection to make sure the linkages are not damaged or loose. Also check the wiring connector to make sure is on tightly and the connection surfaces are not corroded.  If you suspect the sensor is bad, you might try swapping it with one from one of the other wheel wells, and see if the problem moves. 

Strut Travel Sensors

Imbedded inside each strut is an electronic sensor that measures how far extended the strut is. It reports this to the ABC control module. The control module uses this information along with other sensor inputs to decide how much fluid it wants in the strut. 

These sensors can fail. They can report incorrect information, or can fail altogether. When these sensors start generating errors, the ABC warning message will appear on the dash. If dash messages start appearing intermittently on the dash and the messages correspond to strut movements, it is likely the strut travel sensor is failing or the ride height sensor. 

When the travel sensor starts to go, you will often see symptoms like the errors happening when the car is first started, and after driving a short distance the car is fine for the rest of the day. Or the errors occur or clear up when parked on inclines, or when entering/leaving your driveway. 

These sensors cannot be replaced. You will need to replace the entire strut. 

Using SDS (or a scan tool capable of accessing the ABC control module) is the best way to identify the bad sensor. If you want to try to identify the bad sensor without SDS, I suggest the following:  First try to find the corner of the car with the bad sensor. Try triggering the error by pushing/pulling up/down on each corner of the car to exercise the sensors to cause the error. Keep in the mind the car will need need to be running and in gear while you do this. You also might try using a jack and try raising the corners of the car one at a time to see if you can reproduce the error message. Once you identify the corner with the bad sensor, the next step is to determine if it is the ride height sensor or the travel sensor. Try swapping the ride height sensor with the one on the other side. If the problem moves to the other side, the issue is probably the ride height sensor. If the problem stays at the same side, then it is likely the travel sensor. 

ABC Control Module Error Codes

The control module will log errors, which can be retrieved later by the SDS diagnostic computer (or a scan tool capable of accessing the ABC control module). Below are the most common ABC codes and what they indicate. 

Error Code
Critical Vehicle Level

C1525-001 Front Left 
C1525-002 Front Right
C1525-004 Rear Left
C1525-008 Rear Right
The corner of the car got too low. This can be caused by:

1) the corner sagged while parked, and when the car was started the control module noticed the corner was low and logged the error code

2) The ABC system was unable to keep up with demand due to inadequate pump or accumulator pressure. If so, there will usually be pressure related error codes logged as well. 

3) A control valve in the valve block is sticking, causing erratic control of the fluid level in the strut. 

4) Although rare, it could be that the level sensor is reporting incorrect values. If everything checks out as far as pump pressure and accumulator health, then consider the sensor or wiring could be bad.  Try swapping with the sensor on the other side to see if the problem moves to the other side. 
Fault in hydraulic circuit

C1526-001 Front Left 
C1526-002 Front Right
C1526-004 Rear Left
C1526-008 Rear Right
The valves within the valve block that controls the strut fluid levels may not be functioning correctly. 
suspension strut moves although locking valve is closed

C1531-001 Front Left 
C1531-002 Front Right
C1531-004 Rear Left
C1531-008 Rear Right
This usually indicates a sticking valve block. Some of these errors are to be expected if in stop and go traffic. But if these error codes are frequent or other observable problems are occurring, then should address the issue.

The block is likely dirty or the o-rings have deteriorated.
Fault in component Y86/1 (ABC suction restrictor valve)
The valve that controls the amount of fluid to let into the ABC pump has an issue. Either the valve has failed or there is a wiring problem. 
malfunction in PSI supply
Indicates the pressure in the system dropped too low. Could be either the pump or the accumulators.
C1525 - 032
Piston Stroke sensor calibration failed
there is a piston stroke sensor (strut travel sensor) inside each strut. Check the wiring to the sensor. If the sensor has failed you will need to replace the strut. 
C1526 -064
Load Calibration failed 
(system needs to be calibrated using SDS (or a scan tool capable of accessing the ABC control module))
Try to recalibrate the ride height. 
The calibration settings entered into SDS may be invalid.
Fault in component - Plunger travel Sensor

C1126 - left front
C1127 - right front
C1128 - left rear
C1129 - right rear
The travel sensor inside the strut has failed or there is an issue with the wiring or connections. 
Fault in component B4/5 (ABC pressure sensor)
The pressure sensor has failed or there is a wiring issue or loose connection. 
Fault in component - oil temperature sensor 
The oil temperature sensor has failed or is sending data intermittently or there is a loose connection. 

Fault in component - level sensor

C1132 - left front
C1133 - right front
C1134 - left rear
C1136 - right rear
There is a ride height sensor in each wheel well that reports the ride height to the control module. Either the sensor has failed or there is an issue with the wiring or a loose connection.

Try swapping the sensor with the one on the other side of the vehicle and see if the problem moves or not. If problem moves then replace the sensor. Else look into wiring issues. 
Fault in component - strut control valve

C1343 - left front
C1344 - right front 
C1345 - left rear
C1346 - right rear

The control valve (the 3 position valve that controls the fluid level in the struts) has a problem.

It may be jammed, or has failed, or the electrical connection is bad.

Cycle the ride height to see if the corner raises and lowers. If so the problem may be intermittent .

If it is not a simple electrical issue then your options are to try rebuilding the valve block on the assumption it is sticking at times, or replace the valve block. You can't replace individual valves.   
Fault in component - strut blocking valve

C1347 - left front
C1348 - right front 
C1349 - left rear
C1350 - right rear

The shutoff valve (the 2 position valve that closes when the ABC system is not in operation) has a problem.

Remedies are the same as for the control valve above on the line above.
Fault in ABC Suction Restrictor Valve
The valve that controls the rate of fluid flow from the reservoir into the ABC pump has an issue. The valve is integrated into the pump and if it fails, you need to replace the pump. 
Level Calibration was not successfully carried out
Invalid settings were entered
System pressure too low
The ABC pump may be weak, or one of the two main accumulators may be blown.  

On extremely harsh bumps this error may occur so not a major issue unless these codes are frequent. 
Oil temperature too high or sensor is faulty
Check to make sure air flow to the ABC fluid radiator is not obstructed.


Mercedes says this system is maintenance free. But nearly everyone on the message boards agree that the ABC fluid should be replaced on a regular basis. How frequently is a subject of debate.

Clean fluid is clear with a green tint. If the fluid has turned brown it is because it is full of loose microscopic particles floating with the fluid. If it is black it will likely lead to problems. These particles will settle on surfaces you do not want them too, especially in the valve blocks. My personal opinion is that the fluid and filter should be replaced when the fluid begins to darken.  Sometimes it may take two flushes to get the system clean. 
It will cost around $200-300 dollars to purchase the 10 pints of Pentosin CHF 11S fluid, and about $50 for a new filter. There are DIY write-ups on the web on how to change out the fluid.

Here is an excellent description (message #6 of this thread) on why the fluid should be kept clean:
 It is somewhat biased in that it is from a company that sells a filter with a magnet in it, but I agree with their conclusions for the most part. I'm not sold on the need for a magnet, since the filter should catch the same particles the magnet does. But it can't hurt either. They concern themselves more with metal shavings, but the rubber debris that all the rubber components shed over time also contributes to the eventual failure of components.  The fluid also absorbs moisture over time which will lead to rust accumulation within the pump. So if you decide to add the magnet filter, don't let it be a substitute for regular flushes. 

Dirty fluid will cause the valve blocks to develop leaky seals and the car will start sagging overnight. The pump bearings will also experience excessive wear and will lead to the pump failing sooner than normal. So ignore the MB dealer when they tell you flushes aren't necessary. Flush this ABC fluid regularly!

Like brake or coolant or transmission fluid flushes, sometimes problems appear shortly afterwards. There may be a cause and effect relationship, or just plain coincidence. It is hard to know one way or the other. The flushing procedure for ABC doesn't introduce any pressure or unusual flows or anything outside the norm. It is simply diverting the return line to a bucket while adding fresh fluid to the reservoir until the fluid clears up. It is hard to imagine the process would dislodge gunk that would otherwise have stayed put. But there have been reports of problems after flushes, and I suspect this has more to do with the rodeo that shops often perform after flushing the fluid (see caution on rodeos below). The risk/reward decision is yours to make.

For checking the fluid levels, I recommend the following: 1) Make sure the ride height is at the normal setting (no lights on the switch) before taking the measurements. 2) With the engine off for at least 10 minutes, note the fluid levels relative to the upper dipstick mark. Adjust if needed. 3) Replace the cap, start the engine and let run for a few minutes. 4) Check the dipstick again. It should be near the lower dipstick mark. You can infer the overall health of the 4 accumulators in the system by the amount of fluid level drop. The more it drops, the better. 

You should also drive the car regularly, at least once per week if not more.  The reason being is that hydraulic systems do better when used regularly. All the hydraulic o-rings and seals do not do well with prolonged inactivity. Seals dry out faster and stick to the cylinder wall, and microscopic pieces are torn off the next time it is moved. The o-rings in the valves become deformed from being held in the same position for extended periods of time. Gunk gets the opportunity to settle and harden on surfaces and then damage o-rings when they move over that surface. I understand that this is often the 2nd/3rd/4th car for many and only driven during summer months. Just understand there is considerable cost to leaving the car sit over the winter. The most likely impacts being worn/dirty valve blocks causing corners to sag, and seals around the strut piston have excessive wear and develop leaks.

Purging air from the system
The system will purge itself of air in the system over time. If you were doing some work and want to get the air out immediately, you can do so by using the ride height button. Cycling through the levels about 15 times is sufficient to get most of the air out of the system. The air escapes through a pinhole in the dipstick cap. The rest will work its way out over time. A rodeo procedure can help speed the process along, but it is NOT necessary.  While the system is generally self-purging, it is possible for air to get trapped in the struts since the hydraulic path to each struts is a dead-end. There are bleed screws located in each wheel well that can be open to bleed air from these segments of the hydraulic circuit. 

Caution on Rodeos
Rodeos are a useful diagnostic procedure, sort of like a cardiac stress test is. It puts the system under heavy load and the SDS (or a scan tool capable of accessing the ABC control module) tool monitors pump pressure and watches for errors to occur. You should use this procedure sparingly. It puts the struts through travel ranges and components under loads that are not typically seen in day to day driving. Sometimes a component may break down during the test. Or a piece of gunk or metal shaving may break loose and then get lodged in a valve block, causing sagging issues. Now it can be argued that the rodeo exposed a component that was about to fail anyway, but who really knows for sure. 

Also, some shops or techs will often perform rodeos as part of fluid flushes or replacing components in the ABC system. And to their defense...for many repairs the workshop manuals call for a rodeo to be performed afterwards to purge air and check for normal operation. But it really isn't necessary...especially for flushes and simple repairs. Any air that got into the system during a repair can be purged from the system by simply cycling through the ride height levels and adding back any lost ABC fluid. The shops are going to do what they think best. But my advice is to perform rodeos sparingly and discuss with the shop whether or not a rodeo should be done as part of the repair.  

Typical Repair Costs for each ABC component (Dealer Pricing)

Valve Block
Front Accumulator(R230,W215)
Rear Accumulator (R230)
Rear Accumulator (W215)
Return Accumulator (R230)
Return Accumulator (W215)
Pulsation Dampener (R230, W215)
Pressure Limiting Valve + Pressure Sensor Block
Note: remanufactured pumps are available for around $500, and rebuilt struts are available for around $500 each as well. Independent shops can reduce your labor cost 20% - 40%. 


With the exception of the pump, the other components of the ABC system (the valve blocks, accumulators, and struts) are in easy to access locations, and replacing them requires only basic mechanical skills. There are do it yourself write-ups and videos on the web, and support forums where other owners can provide advice.

So if you are the adventurous type that doesn't mind getting his hands dirty, then you have lots of options to cut down your repair costs. And you really don't have much to lose other than a tow bill and eating a little humble pie. If you don't succeed,  just put things back the way they were and get the car to a good independent shop and have them do the repair...hopefully with your supplied part.    

Conversion to Standard Coil-Over Struts.

Another option is to replace the ABC system altogether with an aftermarket set of coil struts. Strut Masters and Rebuild Master Tech sell conversion kits.  The kits may or may not include replacement pumps or sway bars. 

Cost will be around $3,000-5,000  to have installed.  Assuming you wait until an expensive ABC repair bill comes up, then the net cost becomes considerably less. You can also sell your used ABC components on ebay and recoup some of this cost.

Of course installing this kit eliminates the advantages of the ABC system. The ride will be a little harsher and the handling not as tight. You also may not have any anti-sway bars, so if you are an aggressive driver on corners, the car will have a greater tendency for oversteer(the rear end swinging out) and make it more difficult to recover from such a situation, or perform avoidance maneuvers without losing control . Odds are you would never notice the difference unless you drive as if you are at the track, but safety will suffer. You will also need to deal with the ABC tandem pump. It may be advisable to replace it with a standard power steering pump off a non-ABC car that uses the same engine. 

So having the ABC system replaced is an option if the added ownership costs for ABC equipped cars is a deal-breaker for you, or you were unfortunate to have just bought a car with an extremely neglected system. I strongly recommend doing your homework first on what the ride and handling quality will be like by seeking out other owners on the forums who have performed this replacement.

Pre-Purchase Inspection (ABC items to check)

As the price for older SL, CL, and S classes equipped with ABC have come down to earth, it is getting more frequent for these cars to be owned by people who cannot afford the maintenance/repair costs. Some owners will see the warning signs of an expensive ABC repair, and decide it is time to sell the car or trade it in rather than pay for the repair.  So be diligent. 

Also, in some cases owners unfamiliar with ABC get the error message on the dash, google it, and then encounter some of the more inflammatory posts or horror stories. They then rush to unload the car and are willing to take a significant loss, thinking they are avoiding a massive present or future repair bill. Their overreaction can sometimes be a great buying opportunity, especially for a DIY'er.  

Start your inspection by checking the fluid in the reservoir. Clean fluid is clear and green. Worn fluid will be brown. Excessively worn fluid will be black and should raise concerns about the valve blocks being dirty, which would cause the struts to sag overnight. MB says the system is maintenance free, so it really isn't the owners fault if the fluid is dark. But it is a concern nevertheless.

Check the fluid level, both with the engine running and off for about 5 minutes. If the distance on the dipstick between the two fluid measurements is significantly less the distance between the two marks on the dipstick, it indicates one or more blown accumulators. Check for signs that the reservoir has been overflowing, which also indicates a blown accumulator.

Listen to the pump for grinding sounds. Have someone cycle the ride height using the ride height switch and listen to the pump for unusual sounds. If the pump makes noise that disappears when under load, it indicates excessive wear within the pump. If you hear a RPM drop while cycling through the ride heights, it indicates the pump is having to kick in to help lift the car, a sign that one of the two main accumulators is blown. 

When driving the car, go to a parking lot and go over a few speed bumps. Go over them a little faster than normal but not fast enough to damage the car. If a message comes on the dash saying "Drive Carefully", there is a blown accumulator.  An even better test is to find a drop or hump in the road (like the transition on/off bridges usually have). Find a section that bounces you into the air a little. That puts a large demand for fluid for all 4 struts simultaneously, and if a "Drive Carefully" message appears, then the pressure dropped momentarily indicate an accumulator is blown. 

Put the car on a rack and remove the under panels. Check for any signs of fluid leaks from the hydraulic lines. Check for any fluid leaks around the ABC pump area. Also check the strut boots for any rips or tears, fluid leaks, or signs that a leak has been cleaned up prior to your inspection. The boots should be dirty but dry. Check the wheel well linings and look for signs the tire has been coming into contact with the lining - a possible sign the corner has been sagging while parked. 
If you do not have a scan tool capable of access the ABC control module and running diagnostic tests, you may  want to take the car to a dealer or independent shop that has the diagnostic software for Mercedes, generally referred to SDS or STAR or DAS.  It can put the system through what is called a "rodeo" and monitor pump pressure and watch for ABC error codes to be thrown. It can also pull any Mercedes specific error codes  (not available to a basic OBDC scanners) that may have been logged. There is a small risk that a rodeo will cause a component to fail, so to be respectful to the current owner, I would ask permission from the owner first. 

If possible, come back the following morning, or go look at the vehicle in the morning (without any advance notice). Pay close attention to the height of each corner of the car and make sure the car is sitting level. If the valve blocks are dirty, fluid escapes out of the strut overnight and goes back to the reservoir, causing the corner to sag. When the car is started, the struts pump back up. This is the most common issue with ABC. 

Finding a repair shop.  

As you can see, this is a highly complex system. The ABC system is only installed a few MB models, so you will find that it is very hit and miss on finding a shop (including MB dealerships) that have experience with it. It is very common for the workshops to misdiagnose the problem. For example, I had a problem with intermittent "Drive Carefully" messages on my dash. The MB dealership first though it was leaking hoses ($1,000), then they tried the pressure sensor($1,500), and finally replaced two accumulators, which was the problem ($1,000). This experience is typical from talking to other owners on the message boards. It also seems that shop techs are too quick to assume that pressure related error codes mean the pump is the problem, when the accumulators or other components might be the cause.

The shop having a SDS system (or a scan tool capable of accessing the ABC control module) is a requirement for troubleshooting  and working on the ABC system. If they cannot pull ABC specific error codes or run the ABC diagnostic routines, they will not be able to fix your problem.  These codes cannot be read using standard OBDC scanners. 


After reading all this, I wouldn't blame some owners or potential owners if they were scared away by the ABC system. Yes, it is prone to breaking down, but hopefully you have learned from this guide that most repairs can be taken care of yourself if you have some basic mechanical skills. And this guide should help keep your ABC repair bills reasonable by reducing trial and error at your expense.

More importantly, let's not forget the advantage of the ABC system. Cars equipped with it have excellent handling despite the car's weight, and without sacrificing comfort. Many cars have buttons to adjust the suspension stiffness, but few take it to the level that the ABC system does. 

So I wish you luck in keeping those ABC error messages from popping up. And if you dealing with a current issue, I hope you found this guide helpful. 


P.S. - Bear in mind I am not a mechanic, so my understanding of the ABC system may not be perfect. It is more academic than hands-on experience. The support forums like and are good places to get additional advice, which I highly recommend. 


  1. Thanks for posting up this comprehensive description of the ABC system issues. Just took my 05 SL500 in - handed a print out of this to my mechanic. He said he wished he had this 6 months ago when working on a CL500.

    Much appreciative of the work you did on this.


  2. 2002 CL500 - 'Airmatic Visit Workshop' in white display.
    No ABC warning lights on, car is equipped with ABC, not airmatic.
    Car can display both ABC and Airmatic in both white and red.
    Lift button does not work. Limp mode?
    Power steering feels a little tight.
    No leaks or peculiar sounds, reservoirs correct level.
    Anyone ever encounter these symptoms?
    Valve block locked up at power steering relay/pulsation damper?
    There is a pressure sensor at this block and it will throw a
    red ABC warning light if compromised.
    What else to look for besides star diagnostics at the workshop?

    1. The ABC system is clearly disabled. Only SDS will be able to show the error code explaining why. If you take the car to a shop to get an estimate for repairs, the first thing they will do is hook up SDS to determine what is failing. You can then go from there. Pump is probably the most likely culprit, but it could be something as simple as a bad sensor or loose connection.

    2. Hello Darren, I recently installed a rebuilt abc pump on my cl500 (2003) I notice three of my struts raises and lower fine except for the rear left. And also the abc warning light comes on when I ran over a bump or drive on the highway but it goes away when I shUT off the car and restart it. What could be the problem thanks!

  3. Great write-up. Thank you for sharing all that info.

  4. Hi Darren, I have an 02 CL500 with 90K miles. Two months ago, as part of the preventative maintenance, I pulled & inspected the ABC accumulators. Only the left front (#4) had failed (no ABC warning message) but since I've already bought all the parts from the MB dealer, I went ahead & replaced all 4. Since there was very little fluid lost from the removal & the ABC is a closed-loop system w/ the return line back to the reservoir, I didn't flush nor performed a rodeo. Car ran fine until recently, while I was driving in the rain, I had to turn on the headlights & the blower then all of a certain, the ESP, ABC, BAS & ABS popped up on the display then the car went into a limp mode (1st gear). I pulled over, shut off & inspected the ABC & other components but couldn't find anything abnormal so I re-started the car, did a reset by turning the wheels left & right then all messages went away. The problem turned out to be a faulty battery as it couldn't hold charge & the additional load from the headlights & accessories had caused the output voltage to drop & trigger all those MIL lights. The next day, I replaced the faulty battery w/ a brand new top-of-the-line Bosch deep cycle AGM (Abosorb Glass Mat) battery. To retain all the settings in the ECU, I had a battery tender attached to the battery terminals, however during the removal process, the battery tender got disconnected so all the settings were lost. I cleared all the messages, reset everything using the reset procedures but now noticed the driver side is sitting at about 1/2" higher than the passenger side (passenger side is at its normal height). I've tried to re-adjusting the height by working the up/down button a number of times but no change. I've also noticed the car is soft & bouncy when I did a push test on the front & rear bumpers. Car is drive-able with no messages but still soft & sitting slightly uneven. No OBDII error codes & all buttons work except the ESP as I can't toggle it on & off. Yesterday, I was thinking the softness & bouncy may be due to air still in the ABC system so I went ahead & flushed the ABC & replaced the filter but still no change. I'm about ready to pull & inspect my new accumulators, one by one before I take her in for a Star Diagnostic check & possibly a rodeo. Please advice. Thanks, John

    1. Sounds like you might need to recalibrate the ABC system (via SDS). I'm thinking maybe the settings got lost with the power interruption. Normally you only need to do this when replacing a strut, but the lack of calibration would explain what you are seeing. There should be error codes in the logs indicating the calibration is needed if I am right. Or it can't hurt to do it again if the car is already hooked up to SDS.

    2. I believe the power interruption had disable to ESP & ABC functions even after I had gone through the reset procedures, however what strange to me is if there is/are any error codes, how comes the message is clear w/ zero messages? An OBDII scanner had revealed no MIL error codes but it's only for generic codes so I'll need to take the car in for a Star diagnostic scan. I'll keep you posted.

  5. Hi Darren, I have a 02 CL 500 that drops at various wheels when I turn the car off for more than a hour. A mechanic changed the ABC pump and hydraulic fluid about 6 months ago. I have only driven the vehicle about 4000 miles since that repair. Now I am experiencing the tuna boat effect intermittently after start up. Based on what I am reading it appears that I should take the car to a SDS certified mechanic that can test the accumulators first then the valve blocks. I am a veteran retiree on a fixed income. Would you recommend that I attempt to troubleshoot and fix this problem or go with the strutmasters conversion kit. If your recommendation is to diagnose and repair the problem, could you kindly recommend someone in the SoCal area that can do this type of diagnostic and subsequent repair. Thanks again for the education and knowledge share.

    1. Yes, your first course of action should get the car hooked up to SDS to look for error codes and perform diagnostic tests for the pump, accumulators, and so forth. Even if you need to pay a MB dealership a "diagnostic fee" of $100 or so.

      It sounds like both valve blocks are in need of overhaul or replacement , which could run you around $4K. But leaky valve blocks shouldn't cause error codes or dash messages, which suggest you have multiple problems. It's possible the valves are sticking which would generate error codes, which would be included in the valve block replacement cost.

      Or you have a pressure problem. Either the pump is failing again or the accumulators are worn out. It's hard to say without knowing what the error codes are and what SDS is saying.

      As far as converting to coil over struts. I'm not sure what to tell you. I would recommend googling the solution to see whether owners are satisfied with the results or not. You do sound like a good candidate in that you might be facing a steep repair bill here and this ABC system has a way of reaching into owners pockets and pulling out thousands at a time.

      I live in Colorado so I am not familiar with any good Indy repair shops in the So Cal area. I suggest you study up a little on the ABC system and then take the car to a few shops that specialize in MB to get estimates, and in the process strike up a conversation with the mechanic and test their understanding of the ABC system. The MB related message boards might have some useful leads on good shops.

  6. Hi Darren. Astonishing blog. Very thankful you put it together and that i found it. I have a 2003 CL600 with 27k miles on it and the white ABC visit workshop light is on. It came on in September 2014 and has been on every since.. I had diagnosed by local Keeler Mercedes Dealer and my impression after the diagnosis was it was a half-assed diagnosis and recommendation by them. They recommended replacing the tandem pump at a cost of $2700...and that this is a “good starting point, but they wouldn't guarantee it will resolve it.” I cannot for the life of me find the service report, but i recall it saying the pump pressure slightly out of range, but not much. I don't know what codes were logged. Anyway, I parked the car a week later and it sat until 3 weeks ago (March 1 2015). I ahd to jump start it - was completely dead. The whole car was basically sunk down. I started it, pushed the height button and it immediately rose perfectly and stayed at proper height. I drove it about 9 miles with NO ABC warning. I thought it had magically fixed itself, but alas after a heavy acceleration it turned on and now remains on just as it had in september. Anyway, I start the car, i can go 10 feet and it comes on.....or on a good day 1/10 of a mile before it comes on. Driving it is like the old tuna boat. I was going around a corner today and it rocked side to side so violently i actually thought i was going to wipe out (and decided that moment to get this done). I have never seen any leaks under the car, but i have not been able to see the plastic panels underneath yet. The high pressure hose below the pulley in the front of the engine is hard to see, but it looks a little wet and aged, but i can't be sure. The car can sit for about 2 days and then the front sags - rear seems to remain at correct height. Fluid seems quite clean and at correct level. I changed the filter a few weeks ago and added small amount of pentosin but it didn't make any difference. I was thinking of buying two accumulators and replacing those myself AND i was going to take the front valve block out and do the DIY clean and change of the o rings for that and hope that fixes it. Im also about to purchase a SDS on ebay. My questions are these and hopefully you can answer for me:
    1. Based on my description, do you think its likely the tandem pump? My steering is totally fine. Even with the light on, the height button works – raises the car properly and it stays there when on. Car off, like I said, after about 2 days the front sags.
    2. I was primarily going to do the front end, do I need to have the entire car jacked up to do the front accumulator and the valve block?
    3. Do you have any information on where i can locate/purchase O-rings for the DIY valve block clean and repair?
    4. Is the process to remove the accumulators and the valve block difficult or is just a few bolts and screws?
    4. When I change the accumulators, are you supposed to pour some pentosin into them? Or just fill the reservoir after?
    5. I was also going to a do a flush…do you know if this abc flush took is worth the $120? Is there a simple DIY for a flush that doesn’t require this tool?

    Any input or answer is sincerely appreciated. Thanks!

    1. I think you are on the right track by starting with the front valve block and accumulator. If both of your main accumulators are bad it can mimic pump issues since there is no buffer or reserve of pressure. You will have erratic system pressure. After 10 years it is likely both accumulators are worn or shot, so definitely replace the front accumulator while you are pulling the front valve block. Once you have the hydraulic lines disconnected from the valve block, replacing the accumulator is simple. Adding fluid directly to the new accumulator before reattaching will help with keeping air out of the system.

      I've been keeping my eye out for someone to post a good o-ring source, but haven't seen one yet. If you find one, please let me know and I'll update the blog so can help others out.

      This is the first I've heard of the ABC flush tool. All the write-ups I've seen on flushing the ABC fluid involve disconnecting the return line from the reservoir and extending the hose enough to reach a bucket on the ground. Owners use a turkey baster or something similar to empty the contents of the reservoir before adding new fluid. The process isn’t perfect but it is good enough. You’ll never get 100% of the old fluid out anyway. Some will remain in the struts.

      I think you are fine with jacking up just the front, but I would recommend playing it safe and using jack stands or other supports on all 4 corners in case of a sudden drop.

      If the front valve rebuild and accumulator replacement does the trick, I would suggest you consider doing the rear as well in the near future. Odds are they are in the same condition as the front, and the preventative maintenance will pay dividends.

    2. All sounds good and I will update if I get any O-ring good info. I just found my benz dealer service receipt. States the following: "Found C1526 PSI supply code. Check and found 175 bar a bit low. Check PSI sensor and found connector not seated completely. Inspected and found normal. Check electrical to sensor - normal. Lamp drops out and PSI reading comes and goes. Recommend replace sensor as tet and connector prior to pump replace." Do you know what the PSI sensor is (This is pressure sensor in blog diagram?), where it is and maybe a part #?

    3. Yes, there are referring to what I called the "pressure sensor". Definitely double-check the wiring to make sure no shorts or breaks. Replacing the sensor will run you around $1500 since you have to replace the complete assembly. But the good news is that the pump doesn't appear to be the issue. 175 bars is still very good.

  7. UPDATE: I found O-rings that can be used for the DIY ABC Valve block service. They are available at Harbour Freight. Metric O-Ring Assortment - Nitrile rubber construction, Resists oils, hydraulic fluid and water. Comes in a case of 397 pieces for less than $10.00. Here is a link to website. The part number is $67580 for the 397 Piece Metric O-Ring Assortment set. The sizes needed for the valves are in there.

    1. Excellent. Let me know how they work out. From what I read the valve block is pretty sensitive to having the right o-ring dimensions. Save the originals just in case.

  8. Hello there. I have CL600 2002 lift model. One of the hydraulic pipes has been blown so mechanic replaced the oil and pipe. After that car is bend very low the right side front and back. After doing pitch, roll and rodeo car is straight however putting to drive results in bending the car again. Front right is very low where back left is very high. ABC errors are level callibration not successfully don same as losd adjustment. Any ideas?

    1. Not sure what the LIFT model or system is. I assume you mean ABC?

      It sounds like the tech is having difficulty getting the ABC system to calibrate. Without seeing error codes and screens and so forth, is hard to know what is going on. Each strut has a travel sensor inside reporting to the CPU how far extended the strut is. There is also a ride height sensor at each corner. If you have adequate pressure (you would have dash warnings if not), then the problem has to lie with those sensors and or the CPU, which stores values on what it thinks is level for the values of the sensors. Level is established through calibration in SDS.

  9. WOW you my friend deserve a price !!.

    Quick question my rear suspension is fairly soft and once i go over a hump usually the back touches. This a sign of the accumulator being bad ? also i thought each strut had a accumulator ?
    I own a 2003 CL500

    1. It is possible the rear accumulator is weak and not filling the rear struts fast enough to compensate for the "hump". But usually if the accumulator is bad enough to interfere with normal operation of the vehicle, you should be getting intermittent ABC warnings on the dash.

      There are four accumulators, but only two are used for filling the struts. One handles the front wheels and the other the rear wheels. The third accumulator helps even out spikes from the struts letting out fluid. The 4th is referred to as the pulsation dampener that evens out the choppiness of the pump pressure.

      Are all corners at the correct height when standing still? Maybe it has been calibrated at such a low height that there isn't enough range for the struts to operate effectively.

      Or are you talking about speed bumps? I generally use the ride height button going over them to add an extra inch of clearance, just in case.

  10. Hi Darren,
    Just wanted to give you an update on my CL. Last week, I took it back to my indy shop for a re-scan. Vrooman's Automotive in Artesia, Ca. Mike is the owner of the shop & a very nice gentleman. Anyway, everything is fine, no error codes. I've decided to fix the height as I can't stand the look of it. Mike doesn't have the level ramp so he made an appointment for me at a local MB dealer, House of Imports in Buena Park, Ca. While I was there, I talked to the service adviser that the height sensors are in need of recalibration & told him I also want to drop the car another inch. They kept the car for a day, got all 4 corners leveled & dropped the car to the lowest height while still meeting MB specs. With 18" CLS stagger wheels, my car looks sick. The gap went from 3", 3.5" down to 1". I love the aggressive stand & the ride is great now. Thanks to Mike Vrooman & the House of Imports.

  11. Hi Darren,
    WOW, I am so blown away by this fantastic right up you did! It has really helped me with my decision on weather or not to finally purchase a CL. Sadly it looks like the answer is NO NO NO! It looks like the bottom line is I will be guaranteed many MAJOR and COSTLY repairs And thats just not right. I can see it if you have to make major repairs on a particular system once along the age of the car. I have owned several "Merc's" and I still am the proud owner of an 01 E430 and even it has given my share of woes. But not like what I'd be looking at with a CL. I am so disappointed. I can see one time, but then over and over again?

    I am curious, I noticed it seemed most of the replies here and on the forum have been owners of 2000-2001-2002 models. Does that play a part? I am looking at 2006. Are there any improvements to the system by that point? Or was Mercedes really that stubborn? I probably partially answer that question by mentioning I just went to look at a 2006 CL600 with 62k miles yesterday and as I approached the car it was clear the front right side had totally sunk. ;(

    I was originally thinking I'd only consider cars that had recently already had repairs to the system "documented" of course. But that was before seeing that even if you get a full system replacement failure is still pretty much guaranteed at ANY time! WOW! Well even if I did take the leap and buy one I now know for sure I will also keep my 430! Looks like I'd never know when I may not be able to drive my CL. Very disheartening.

    Lastly, I am curious, you mention the system was used on 2000-2006 models. So what about the 2007 CL's? Different system? Better System? NO system?

    Again I thank you SO much for this information and enlightenment!

    1. There was an update to the ABC system, in 2007 I believe. The valves in the valve blocks were updated, and the pulsation dampener was moved up to the pump. There was also a software update applied to the controller. Yes, the older year cars seem to have more problems, but it is hard to say whether it is the design or just the age of the vehicles.

  12. hi great write up.
    I have a 2002 CL500 coupe W215 5.0 V8

    The left rear ABC pipe burst near the bleed nipple, as you can imagine the whole car dropped to the deck, I have replaced the pipe with a new one. This is where my problem is.
    I have topped the fluid up (3 litres so far) and been using the button on the dash board to raise and lower the suspension at least 20 times with the engine ticking over to try and bleed the system whilst keeping an eye on the fluid level.
    If I start the car the left front and rear raises and drops as it should when I press the button, The right rear raises and lowers roughly 1 inch and the right front doesn't move at all. If I then start to drive the car (doesn't matter whether 1, 2 or no red lights are on the switch) the car levels out after roughly 1/4 of a mile, All corners raise to normal height, once this has happened no matter how many times you press the button the suspension stays at this height. If I turn the car off it stays at this height (normal height) when I start the car it stays at this height until I do 1 of 2 things, Either press the button or put it into gear and actually move (even a couple of mm) then the right hand side drops again, front right goes right down over the wheel and the rear right drops about half the way. Again if I carry on driving 1/4 of a mile it raises and levels out. The ABC warning message illuminates white when it drops and goes off and on when it likes after this. Any ideas and many thanks for your reply in advance. Paul.

    1. Wow, that is a interesting problem. Did all four corners drop when the line burst? My understanding of the system is that the shutoff valve for each strut would have prevented the others from lowering. But I might be wrong.

      Assuming everything worked prior to the segment of hydraulic line being replaced, one would think the problem is related to air in the system. I think MB has a more thorough procedure for bleeding the system involving using the bleed nipples. If you google for it you should be able to find the procedure on one of the forums. I would start there.

      You also might want to have it hooked up to SDS to see if any error codes are being logged. Recalibrating the ride height for each of the four corners may also help.

      Please report back what the eventual solution is so I can correct the blog and/or add what you have learned.

    2. Many thanks for your reply. When the pipe burst the whole car dropped but more so on the drivers side even though it was passenger side that burst. We was able to bleed left front and rear and right front but the right rear nipple is too corroded. We have today dropped it off at a Mercedes specialist who will hook it up for us. They are charging £50GBP to do this but if we have them carry out what work it needs we don't have to pay the £50GBP. I will post back here when it is sorted.

    3. Hi just a quick update for you. I have the car back now running spot on. The garage did the rodeo on it, re calibrated the sensors on all the shockers and found the right front rod had snapped on the sensor. he cable tied this for me and running like a dream.

  13. Your posting is amazing!! Having issues with the ABC and reading the information you posted will save me hundreds if not thousands of dollars. Local dealer also quoted me $7000 to fix the problem. Thanked them for the info and went to an indy. He said the pump was not leading at all (which was what the dealer told me). In reading your info, feel it's the accumulators and plan on ordering them and having my new fav car guy install them for me. I've always checked the internet before taking my car to the shop to see if it's something I can do myself. I replaced the door handle and fixed an issue with my secondary latch by doing a little research. As a female owner of this car, I now feel as though I understand the ABC system a little better and will be able to talk to my new fav car guy about the issue. Thank you!!!

    1. Thanks for the feedback. It feels good knowing this blog is making a difference.

      The most expensive repair job for the ABC system should only be around $3000. If more than that the shop probably isn't sure what the problem is and is taking a shotgun approach to the repair (at your expense). You were wise to get a second opinion.

  14. Thank you for this insightful and informative guide. However I have a situiation that you may have some sort of insight to as I will probably have to do the majority of my ABC repairs myself... Here is the situation as follows.... 2002 CL500 .....The ABC system was working fine for a while but my front passenger strut would randomly raise to an extremely high position in idle or it would do the same thing if i went into a dip or over a bump too fast... did not get any kind of ABC warning when thus happened... I would have to cut the car off then back on and the strut would come back to normal level..... a few months later my ABC (white then red) light came on while the car was raising high and restarting it didnt solve the problem and the car proceeded to sink to the ground... i checked the fluid and it was black and smelled burnt and fluid had come back up through the resivoir(accumulator failure possibly).... i started the car the next day and it rose up enough for me to get it to the shop that i go to repair my vehicles (specializing in only mercedes but i dont think they are completely well versed on ABC systems as in they know alot about them but maybe not enough) and they ran the star diagnostic and found that tge pump was dead(no pressure)..... so i had them order a NEW pump... and during the time i was waiting for the pump and getting it back to the shop my rear driver side strut completely went down (no known leaks anywhere either)... so they put the new pump on and there is no response when the level adjustment is pressed... and the abc light came back on... and the front right strut is locked in an extremely high position... so they tried to find out what was going on with STAR but couldnt find out what the problem was. They even swapped an ABC control module from another CL500 and still the same thing.... so now i am on my own with it and am going to let a dealer diagnose it and go from there... from what I read here I am leaning towards a valve block/accumulator/pressure (any or all) type malfunction but i was hoping that you might have some insight on this situation.... also I dont think the fluid was not flushed when they replaced the pump...just added 3 liters of new fluid..and I know how important flushing the fluid is because im almost certain that my system is contaminated being that the pre repair fluid was blackish brown... once i take it to the dealer and get the codes then i will have a better idea and be able to get some assistance..... I DIY repaired/replaced the airmatic suspension (struts,valve block, pump)on my S430but ABC is a whole different ball game and I need all the help i can get...I have been studying the forums but this blog us by far the most detailed piece of info... thanks in advanced and any indight woukd be greatly appreciated

    1. If a corner of a car sinks while the car is parked and off, then the valve block is leaking or gunked up. So you are looking at rebuilding or replacing one or both valve blocks. But that shouldn’t cause problems with the corners staying at their correct height while driving unless they so gunked up that the valves are also sticking open or closed at times.

      Another strong possibility is a bad ride height sensor on the front passenger side. In each wheel well area, this is a ride height sensor. There is also a strut travel sensor inside the strut. The ABC computer compares these two sensor readings to decide how much fluid to put in the strut. I would guess the front passenger ride height sensor is bad, giving erratic readings to the ABC computer. See if it can be easily swapped with another corner of the car and see if the problem moves. If it is the strut sensor, you are looking at a new strut, but you can get them from Arnott for reasonable pricing.

      What you really should to do is hook the car to SDS again and run the full battery of ABC tests. I’d focus on trying to recalibrate the right height for the 4 corners first. In the process you may find the bad sensor. If successful at the calibration, then try cycling the ride height while watching the sensors values and make sure none of them are producing erratic data. If they pass, then run the accumulator and pump related tests to see if they pass. If so then try a rodeo while watching the ride height and strut travel sensor values again and look for a sensor that is going erratic or giving wrong info.

      If your accumulators are shot, you might have some difficulty passing the rodeo, but it shouldn’t get in the way of diagnosing the sensor issues.

      I’m a little surprised you don’t have a lots of error codes in your logs relating to “too low” or unexpected strut movement.

      I would try to sort out the sensors first, then if/when necessary tackle the valve blocks and accumulators.

      Good luck.

  15. Ok and if the vavle blocks are the culprit would I need to flush the old fliud to clean the system before or after replacing the valve blocks because the fluid is probably still dirty. The car does not respond at all when I press the height level adjustment button or would I even need to use the button during flushing? And how about bleeding the system before or after replacing or repairing the valve blocks..I was reading about it but needed some clarity... I am taking it to the dealer tommorow abd should have the codes so we can have a much better idea of whats going on and what to do.

  16. Ok here is the latest.... i took it to the dealer and indeed it was the rear valve block so I flushed the system with fresh fluid and bled the struts and proceeded to pull and clean both valve blocks and replaced worn o-rings (front too just in case)... I put everything back together and cycled the system and the rear of the vehicle did not respond at all when i pushed the height adjustment button but the front responded and operated as it was supposed to (held pressure with no sinking and responded to height adjustment button accordingly) .... so am I to assume that there is a solenoid (electrical perhaps)(more than one of the 4) problem in the rear valve block that is preventing it from responding??..... I ordered a rear valve block and will try it out when it arrives...

    1. I'd say you are on the right track! I would be interested in feedback on hour much work was involved to pull each valve block and reinstall, as well as any other insights you gained from the repairs. The more people that respond with what the final solutions were, the more value the blog will provide.

  17. This will be mu 3rd attempt to get this to post so if all of a sudden 3 comments pop in (all the same) that's why, When it hit "publish" it throws errors.
    Great information here, and thx to Darren as you have put some time in to this. I can't find anyone with my issue. I have 2005 SL500 R230. The car sits perfect. No sagging over time. When I push the button to raise it, it does nothing. When I drive to say 40 mph and faster, it raises to the max. When I stop at a light, after a few seconds it drops. Sometimes it won't drop and I can put it in reverse and start to roll back. And it drops. If I park it and leave it, it stays up. And I mean UP.. to the max. When I return , start it, and start to roll forward it drops. I was about to send the ECU off for testing and I found this forum. Any feedback would be greatly appreciated. Thx Lee

    1. Sounds like you have a sensor issue. In each corner of the car, there is a ride height sensor and inside each strut is a travel sensor. The ECU reads these values and decides how much fluid to put in each strut. Odds are one of these sensors is sluggish or is sporadically giving bad output to the ECU.

      Try visually checking the ride height sensors in each wheel well for damage or loose connections. Then have the car hooked to SDS and try viewing the values of the sensors while raising and lowering the vehicle to see if a sensor is not behaving normally. Then recalibrate the ride height.

    2. It wont raise or lower by using the button by the gear shift. I didn't think it would be a problem at a certain wheel , being as they move all together,. When I drive it, all 4 sides jack up. Up 5" or so. Then it will drop all of a sudden when I stop. Or sometimes I have to reverse it, or move it around some and it suddenly drops. I would disconnect the whole system if I could . I just want it to stay at one level.

    3. The ECU is a small computer, and like all computers...garbage in = garbage out. The fact that the ride height button is disabled indicates the ECU knows something is wrong and has disabled some functions, There is probably a bad sensor or short in the wiring or something that is confusing the ECU. The logs will likely have error codes that will point to the offending input. It is far more likely the ECU is getting bad input than for the entire ECU to be bad. Either way SDS should tell you what is wrong.

  18. Hi Darren great information i learning quick... i recently bought a 02 sl500 R230 with 72k miles the car is dropping on the passenger front which through your blog i have an idea.... couple days ago a leak from a unit located on the passenger side behind the wheel inside the inner arch... i think it is called pressure relif valve? can this be repaired or is it a replacement and what is the main function of this... i will try upload a photo of it
    thank you John

    1. If the assembly has a small black sphere attached to it, then yes it is the assembly that contains the pressure check valve, the pressure sensor, and the pulsation dampener. It is common for the seal around the pressure sensor to develop a leak. There is a repair kit, part number A2203201158. Or see if an independent repair shop is willing to try a gasket or crush washer first to get it to seal.

  19. You have given excellent report with graphics illustrated over the faults that occur in Mercedez Cars, and the type of sensors that may fail due to which the system malfunctions. For more on these sensor seals you can check out the website to know more customary information

  20. 2004 sl500 lost enough abc fluid while driving causing the abc to stop working. Culprit was anot improperly seated filter seal at the cap. Refilled resevoir and reset filter seal. Abc will not come back online. Is there a way to reset the abc or does the pump need to be primed?

    1. Yes, if the fluid got so low as to cause the ABC system to shutdown, then priming the pump will be needed to get the system to generate pressure again.

      I do not recommend driving the car until you get ABC pressure restored, or you may ruin the pump (which uses fluid for lubrication). Hopefully you didn't drive too far without fluid.

      The ABC should resume automatically once the pump is primed. No "reset" is necessary.

  21. Hi Darren, I have a 2003 SL500 and over the past few months, every time I hit a small bump or dip in the road it gives me the 'drive carefully' signal on dash. Could it be that it just needs some more ABC fluid? Or is it something more serious? Also, I am about to drive the car 800 miles straight in a couple of weeks. Do you think I can make it to destination in its current state with that signal going off or is it signaling a greater problem? The other day, I started up the car and it gave me a 'car too low!' signal. I raised the car by pressing button and it is holding at that level just fine. I believe the car lowered itself because it had been sitting for a few weeks untouched. Thanks!

    1. Sounds like you have a blown accumulator. They supply the pressure needed to fill the struts quickly. When you hit a bump, it creates a sudden demand for fluid, which the pump alone can not supply.

      Assuming the warning message is not staying on the dash, the car will probably stay drivable although handling will suffer. Safety is also a concern in that the ABC system cannot react fast enough during emergencies maneuvers like sudden turns and so forth. It will be like driving a car with bad shock absorbers or no shock absorbers. It will be very bouncy. Plus by the time you get back from your 800 mile trip the chime will probably be going off multiple times per minute.

      If you have another vehicle at your disposal, I would take that. If you desperate, the car can probably make it there and back if you are careful on off-ramps and other situations where handling will be important.

      Of course check the fluid level and make sure it is at the high mark when the engine is off, or the low mark when the engine is on. Don't be surprised if the reservoir starts overflowing when you shut the car off, another classic sign of blown accumulators.

    2. Thanks! After the car was sitting for a few hours last night, I checked the ABS fluid level and it was quite low -- less than an inch on bottom of dipstick. So I'm heading to dealership to pick up more fluid -- will add more today -- I'm hoping that's all it needs and is the source of the problem. If it is an accumulator looks like..based on your research, it wouldn't be too expensive of a job

  22. Thanks for posting all this information. The better informed a buyer or potential buyer is, the better decision can be made. This ABC system is an intimidator to buyers. I figured worse comes to worse, I'll just pay and have the system swapped to coil overs and sway bars. I am purchasing the STAR system when I buy a used SL500. I hope to recoop the cost in a few maintenance item repairs and diagnoses

  23. Put simply, AMAZING! Thank you, Richard from La Verne, CA...

  24. Hi Darren. I have a 2003 SL55. Just recently the red drive carefully light comes on. If the car has been sat for a day or so and you start it and wait a few seconds the system works ok and the car rises and drops as required. If you start it and press the button quickly the red drive carefully light comes on and stays on. You then have to wait a long time before starting it again and waiting a few seconds and it will work properly again. Today I started it and it worked fine. I raised the suspension and lowered it a few times. The red light came back on and currently it is stuck in the fully raised position. Do you have any ideas what might be going on? Thanks. Marc

    1. Hi Marc, I hate to say it, but it sounds like your pump is failing. Try listening to it closely and see if it is making any unusual noises. You are going to need to take it into a shop with SDS have have them rodeo it and monitor the pump pressure.

      There is a remote chance it is a sensor problem, and if so an error code will be logged as to which one.

      Good luck.

  25. Hello I think I may need some advice on my 2007 Mercedes s550. I've recently replaced my power steering rack within new aftermarket unit by a local independent shop here in the city. The steering feels great and I do not have any more leak. But I have taken on some new symptoms.
    1. While driving or braking I'm getting a intermittent groan/vibration noise coming from the front end. It only last about 10 seconds and will usually stop when I press the brake.
    2. While driving or braking I am getting a "whoosh/ squeaking" noise. Usuallyhappens when im braking. It sounds like a rush of air with a rubbery squeak.
    I do not have any alerts or lights on the dash and air ride level seems to be just fine. I am concerned this might be something more serious as this is my daily driver. Do all of these cars have the ABC system? I really don't recall ever seeing a hydraulic fluid reservoir anywhere. Please help as I am apprehensive to take it to the stealership.

    1. A few questions, when you say they replace the power steering rack, did that include the pump? I'm not much of an expert on the steering related systems, but maybe there is some air in the system and it needs to be bled, or it is low on fluid.

      Obviously we need to figure out if you in fact have the ABC system on your car. I suggest you take a look at the pics for the fluid reservoir, the pump, and the struts, and then see if you recognize them on your car. The struts should be a dead giveaway.

      If so, and if the power steering pump was replaced, then it is possible there is air in the ABC lines. There are bleed valves in each wheel well to let air of the line between the valve blocks and the struts. The shop may not have bled the system after installing a new power steering pump, which is actually both a combination power steering pump and ABC system pump in the same housing.

  26. Great post! Thank you,
    My Autoshop is stating that I need a new ABC control Unit on 2001 S600, part number 0325455432, a new one cost like 1400.00, the same module is available for much cheaper at various places, do you suggest purchasing a used one and replacing it? Also what is the exact location of ABC control unit module?

    1. I don't know exactly where it is, but the diagrams suggest it is in the left foot well on the right side (nearest the middle of the car). You'll probably need to google for a more exact locations and what panels to remove to get to it.

      I would probably go with a used one, but which ever way you go, make sure it is refundable. It may not be the control module but rather a sensor problem or wiring problem. I've rarely, if ever, came across a discussion thread where the problem ended up being the control module.

      You might also want to try cleaning the contacts and pins on the control module, and check for broken solder points. I know one owner who saved himself the expense of a new one by resoldering a bad connection point he found.

      Of course check your fuses if the shop is saying their diagnostic software can't communicate with the control module. Unseating and reseating the unit might restore operation as well.

      Sorry if I stated the obvious at some point. Just noting all possibilities I can think of.

    2. Thank you Darren for all your help and quick reply

  27. Darren, is it safe to remove the nitro contained damper to see if it still in working condition via the typical method mentioned by various mech's out there? They typically suggest to just unscrew and put a small screw driver in the ball hole, it should not go in all the way if the condition is good. I have had the left front one replaced recently but I feel may be front passenger side may also gone bad as typically the life is like 5 years

    1. The mechanics you talk to are correct.

      Be sure to weigh the labor costs to pull/reinstall them and the cost of a new one (about $175). If I were you I would just replace, as a maintenance investment, any accumulator you plan to pull for inspection.

  28. My 2003 had a leaking rear strut with fluid on my garage floor. After checking fluid level I drove it to my dealer. While in route it started tuna boating and on arrival smoke was coming from the hood. I opened the hood and the holding tank blew up with me getting lots of warm fluid from waist to head (my eye glasses saved my eyes). The car was traded in the next hour with the dealer giving me a more than generous deal. This wasn't the first time for problems, most fixed under extended warranty! I always wondered what was that problem all about?

    1. That is a new one. The fluid in the reservoir is not under pressure. I suppose the pinhole vent in the dipstick could have been clogged, but I would think the dipstick would just pop off under pressure rather than the entire reservoir blowing up. Also the fluid is just mineral oil and is not flammable or has explosive characteristics.

      An accumulator bursting will cause air to get into the system, and air does compress under pressure(fluid does not). It could be that an air bubble found its way to the reservoir at the time you were standing there (I'll bet it was a few minutes after you shut the car off).

  29. Hello Darren, thank you for a very informative post.
    I also have the ABC message in red, with an additional, drive carefully message. I won't afford any expensive repairs... Can I sort this out myself? I only drove for 10 miles after this message came on and haven't driven the car since.


    1. If you have some do-it-yourself skills, yes. You'll likely need the car scanned by SDS to know where to focus on. And you should be able to find a step by step guide for almost any ABC repair on the net.

  30. Hi Darren,

    Great info! I have a 2006 SL 500 that runs great except for the suspension. To make a long story short, the current symptoms are a very ruff ride - road bumps go bang, bang - and lots of engine and road vibration and the red ABC light comes on only occasionally. It seems worse at high speeds and in the rain the ABC lights are more likely to come on and stay on. There are no codes coming up on the SDS computer at Mercedes and because the mechanics who test drive it don't own an SL, don't know how it should drive so they think it's alright - it's not. I have had my mechanic replace the engine mounts, transmission mount, all three accumulators and the front pump and I well over 6 grand with no results. I don't want to waste any more money on the car at this point but I know if diagnosed correctly it will be a simple fix and it's a shame now because otherwise the car is great. Would greatly appreciate your insight on this.

    Thank you,

    1. I would first try to figure out why the occasional abc error is coming on. There should be error codes in the logs. Knowing that might explain the larger issue.

      I've seen some other posts on forums about the rough ride of the R230. No answers though. Conversation usually goes into whether the expectations are realistic. It is a roadster, not a sedan. Comparing to a MB sedan tuned for comfort may not be realistic. Low profile tires make for a rougher ride. I tend to run my tire pressure on the low side of the range for extra cruising comfort.

      The only way to know for sure is to go test drive another R230 and compare. I can't think of any explanation why the ABC system would give a rough ride without an error message staying on the dash.

      Sorry I can't be of more help.

    2. Another thought...I recently had my rims checked and 3 out of 4 were bent to various degrees from potholes. I had them trued up and noticed a much better ride. But that doesn't really explain the harshness of bumps you describe. Just brainstorming here...

    3. One last thought...ABC errors more prevalent with rain...maybe a sensor is getting wet and shorting out? There are ride height sensors in each wheel well. You might try checking them for damage and that the electrical connectors are in good shape. But I would expect error codes or "Too Low" messages to appear on the dash if they were the problem. But who knows...

    4. Thanks Darren. I really appreciate your time to respond. Yeah, I did replace one sensor but that was not the answer. If I just bought the car I would have nothing to compare the ride to but since I've owned it for several years now, I'm comparing it to the way it used to ride, as recently as a few months ago. FYI, about year after I got the car maybe in 2010, I had to replace the rear valve block. It never had a lowered corner; both sides went down symmetrically on the rear wheels. This was a successful repair. It had been doing the same thing on the front wheels for years but didn’t want to spend the money because as soon as I moved or hit the height button on the console it would come right up. I finally replaced the pump recently because my mechanic said it was not the valve block. It can still go down now but not very often and when it does it too is always symmetrical. I don't know if this gives you any clues. I would say the shocks are shot but there have been no codes to say that's what's happening.

    5. Did they ever rodeo your car. If not it might uncover something. At a minimum it should verify all your struts, valve blocks, accumulators, pump, and so forth are all working correctly and responding to commands from the control module..

      Another possibility is the ABC struts have worn out, just like a standard strut would. The ABC strut contains a spring and a shock absorber, just like regular struts do. But I think a worn strut would give a bouncy ride, not harsh, I'm not sure what tests a shop would do to evaluate if a strut needs replacement, but I would think those tests would also be valid for an ABC strut.

    6. Another thought is that if they did a lot of work recently on ABC, there could be air trapped in the system interfering with normal operation. A little bit of a long shot, but it doesn't cost much if anything to open the bleed screws in each wheel well to let out any air that might be trapped.

  31. Thanks again Darren. The rodeo has been done a couple of times but no results. I'm just so unhappy with the harsh ride. It's also noisy as if there is no oil in the engine and transmission and all the sound proofing has been removed which of course is not the case. There's another mechanic I plan to take it to and will suggest the possibility of worn out struts and the bleed screws.

  32. What an amazing write up. I which I came across this long ago. I have being overseeing some work on my friends SL 55 AMG. We have had all the seals changed in the valve blocks and now it keeps drooping on the back which never had a problem until now. My question is in relation to the locking valves in the valve block. I going to have the seals replaced again so I’m wondering if you can give me the sizes of them and the order they are fitted as I want to make sure there fitted the correct way around, I believe there was a mix-up last time the mechanic fitted them. the fault code comes up as locking valves locking but car still dropping. Thanks for your help.

    1. I have the specs posted for the o-rings in the valve block section of this write up. Be sure to get the square "lathe cut" (tetraseals) o-rings which are square rather than round. I don't have any advice or orientation or proper order.

      I personally have never rebuilt them. I am just relaying the information that I have gathered from other sources. There is someone on the forum that will rebuild them for you for a fee. I forget the username but you can either search the site or do a general post looking for someone to rebuild the block for you. Or if you send him a message he may have those specs for you.

      Rebuilding them is not always successful. There could be other wear or deformations that might be preventing a good seal. It is also important to clean the block extremely well and do the work in a clean work area. It doesn't take much to cause a leak. A small piece of hardened sediment , a strand of hair, a grain of sand, is all it takes to prevent a good seal and cause the corner to sag overnight.

      It is also possible the solenoid that moves the valve could be worn or weak or not applying enough pressure to hold the valve closed. There is also a spring that you might try stretching out a little to add back some lost strength.

  33. Anyone know of good mechanic in so cal that can change out my ABC struts with the Strutmasters coil over system? In my opinion the car is too old to ABC troubleshoot (02 CL 500). Thx.

    1. I suggest calling Strutmasters directly and see of they can recommend some shops that have purchased the kit in the past.

  34. Hi Darren, great write up on the ABC suspension. My local Mercedes garage don't know much about ABC as they don't get many in! I have an S600 2012 with 90k miles. It has a full service history and I know the pulsation dampers, rear struts and accumulator have been replaced not long ago. I bought this after reading that the suspension is the best out there. However the ride is hard, especially when hitting potholes and it seems to bounce and shift a bit when going round corners if I hit a hole. It has had the fluid replaced recently and has had a rodeo. There are no error codes either. I don't notice any difference between sport or comfort, it certainly doesn't 'waft' along which I was lead to believe it would. Any idea? Is it possible to adjust the dampening in any way? Would be grateful for any suggestions. Regards steve

    1. I've heard this complaint many times. I wish I knew what is going on. I don't know if it is an expectation thing or something else. I know my R230 doesn't float along either. I feel the cracks and bumps in the road too, but not nearly as badly as some other roadsters I've driven like the z4 or audi TT.

      The spring inside the strut should absorb the impact from bumps. The shock absorber should keep the spring from rebounding or oscillating afterwards. The ABC part where the fluid chamber that raises/lowers the strut height really shouldn't have much effect on the spring/shock operation. So whatever ever would cause a regular car equipped with struts to have a harsh ride should also apply to a ABC equipped car. I would investigate along that line. Maybe the front struts are worn out and need replacement, the same as non abc struts wear out.

      If you do find an answer, please let me know so I can share what you learned with others having the same issue.

      Best of luck,

    2. Having done some research on suspension in general, you might want to investigate the following:

      1) the shock absorber portion (the lower half) of the ABC strut might have seized and is no longer dampening the smaller bumps and vibrations.

      2) suspension bushings may be worn and in need of replacement.

      3) out of round tires or bent rims can contribute to an overall rougher ride. I know the alloy rims will bend over time from pothole abuse. I recently had 3 rims trued back up ($75 per rim).

      I suggest taking the car to a shop that is good with suspension and have them check out the various components. Although the ABC may throw them a few curves, they should be able to find any problems.

    3. Hi darren, thanks for your reply and happy new year. I've had lots of problems with this car and considering it is the flagship model, it shouldn't be this bad. However Mercedes have recognised this also and are attempting to fix the many faults! I will give them the suggestion you have made and let you know the outcome. One question though, I thought the fluid in the ABC suspension played a bigger role in the overall suspension then just affecting the height?

    4. In addition to adjusting strut height, the fluid lubricates the pump and lubricates the o-rings in the valve block as they move back and forth within the valve blocks. Contamination leads to increased wear.

    5. One of the downsides to a MB flagship model is that it gets loaded with all the high tech stuff. ABC suspension, SBC brakes, soft close trunk, keyless go, and so forth. Some of these systems, like ABC and SBC, were not designed to hold up well in the later years of ownership, and are insanely expensive to repair. An entry level MB will probably have lesser repair bills in the later half of its life.

  35. Hi Darren,
    I have a Mercedes S55amg a year ago to break the axis of the pump ABC. I replaced the pump.
    Then I replaced high-pressure hose because there were drops.
    This year I cleaned all Axle Valve and o rings changed. I changed the front Axle Accumulator (P / N 2203270115). I replaced with new hydraulic fluid.
    I visited the service and made rodeo.
    They did test the car with start test and there were no errors.
    All these procedures learned from you, for which I am very grateful.

    Now, at the moment of ignition of the engine noise of the pump for 2-3 seconds is somewhat strange, stronger. Then the sound subsided.
    At 1800-2000 rpm car gets strange light vibration.
    There is noise that resembles a defective Pulsation Dampener, but not as strong.
    8 years ago in the movement of the car, the membrane burst and then the noise was similar but stronger.
    Sometimes a place or in motion control includes ABC in white light, but everything works.
    Now I think of membranes Pulsation Dampener is amortized and should be replaced with a new one.
    Again, thank you for the extensive information for ABC system and would expect and your opinion.

    1. I'm not sure what to say about the pump. As long as the sound lasts only a few seconds and all is well after that, I wouldn't lose too much sleep over it. If there is a problem (like a bearing going or something), there probably isn't much you can do anyway except replace the pump again. Maybe the belt tension needs adjusting.

      My car at the moment currently needs the pulsation dampener replaced as well. The sound is most noticeable with the top up and driving at slow speed (around parking lots). On a left side drive car, the sound seems to come from the foot well. So I agree with you it may need replacing. As far as ABC repair jobs go, it is the easiest repair.

      As far as the occasional white error messages, you might want to have the car scanned to SDS for errors. There may not be any errors logged for white error messages though. Not sure on that. You may have a strut travel sensor or ride height sensor or something else that is in the very early stages of going bad. I wouldn't worry about it too much until symptoms start to show.

      I'm mostly speculating here. It sounds like you gave a good grasp on how it all works, and you can trust your own judgement on what to do if anything at this time.

    2. Also, the pump noise could be coming from the power steering side of the pump. Something to keep in mind.

    3. Hi, Darren
      I changed pulsation dampeer.
      Noise and pulses declined.
      Yes, I have a strut travel sensor or ride height sensor to right rear wheel.I will visit workshop to discover the defect.
      In workshop for repair of hydraulic pumps made prevention of the rotor pump. Only the pump filter was dirty and renovated one of the valves. Now the pump run more quietly.
      ABC is really a very precise system.

  36. Pretty spot on break down of the ABC system. I was very impressed. I have been working on these cars for over 10 years, and found your blog pretty spot on, except for a leaky strut will indeed cause a ride height issue (sagging, etc). I have seen two bad rear struts sag, because the ride height is controlled by the fluid level in the strut, not the spring. Otherwise, spot on.

    I actually have a custom display made which I show to my customers that has the main components of the ABC system on it dissected so as to better explain the parts operation, and failure. I use it to upsell preventive maintenance work. I also saved your blog, and if you don't mind, i would love to pass it on to my customers so they can have additional resources to fall back on.

    1. Thank you. I'll update the blog to include checking for strut leaks when corners sag.

      Of course you are welcome to distribute it. I wrote it to educate owners. There is a lot of fear and hysteria about this ABC system, and knowledge is the best remedy.

    2. Thank you. I'll update the blog to include checking for strut leaks when corners sag.

      Of course you are welcome to distribute it. I wrote it to educate owners. There is a lot of fear and hysteria about this ABC system, and knowledge is the best remedy.

  37. Hi Darren,

    THANK YOU VERY MUCH FOR YOUR ARTICLE !!! You have done a great favor for owners of SL-Class, CL-Class, and S-Class vehicles with ABC system.

    I have been looking at 2004-2005 MBZ SL55 AMG with between 30,000-60,000 miles from and since November 2015 to the present. However, I am not yet decided to own this type of vehicle due to the potential of between $2,000 - $3,000 per year for repair and maintenance costs due to age-related failures of complicated parts and components for this year and type of car. I will be using this vehicle as a weekend car and for leisure, so the yearly mileage may be between 4,000-6,000 miles per year. My daily driver cars are three (3) 20-year old vehicles (1995 LEXUS SC400, 8-Cylinder, 4,000-cc engine) ranging in between 110,000-140,000 miles, and they have been bullet-proof except the Timing Belts that fail between 90,000-120,000 miles, but everything else have been very reliable, and the quality, fit-and-finish, and reliability are world-class.

    As far as the ABC system, I have been educating myself on alternatives such as coilover systems to replace the ABC system that has way too many parts to fail. I have found 2 coilover systems on the Internet that may work, but still need more refinements and improvements due to the heavy car weight of approximately 4,300-lbs for SL500 or SL55 applications:

    1. REBUILD MASTER TECH Coilover System Kit for $2,495.00 + $1,500.00 Core Deposit, which includes EIBACH Pro-Kit Springs, BILSTEIN B6 Shock Absorbers, Front and at Sway Bars with Links, and bolt-on Power Steering Pump.
    2. STRUTMASTERS Coilover System Kit for $2,200.00 (but no Core Deposit), which includes EIBACH Pro-Kit Springs, and KYB Shock Absorbers (no other additional parts are included). Labor / installation are not included.

    Of course the coilover system will not be as high-tech and sophisticated as the ABC system, but will eliminate lots of repair/maintenance issues. But for those that are okay with the performance and low-maintenance requirements of coilover systems, it would be an excellent alternative in the future.

    What are your ideas or opinions on alternative suspension replacement systems for the ABC system?

    From what I have read in your article, there are far too many parts and components in the ABC system that will be susceptible to aging-related problems (i.e.: computer circuit boards / wiring / electronics, hydraulic pumps, solenoid valves, sphere accumulators, proprietary MBZ hoses and connections, hydraulic struts, ride height sensors, rubber components, hydraulic oils, etc.). Do you think that after 10-years, most of the parts and components of the ABC system need replacement?

    I am an intermediate DIY-er, so I am willing to learn and perform most of the ABC maintenance that you have covered in your article. I may have to invest in electronic Fault Code diagnostic tools, and some Special Service Tools such as the “All German Auto (AGA)” custom-made ABC hydraulic system hydraulic oil flush tool.

    Based on your ownership of 2003 SL500 with 80,000-miles on it, what parts, components, and systems have you personally performed on your vehicle, and are they doable for an intermediate-level DIY-er on these cars?

    Thank you very much for any comments.
    Elmer Catalan – 01/08/2016

    1. I recommend you post your question to the R230 forum or site and get opinions from multiple owners. The topic is discussed quite often, and while you will find a few "haters" of the ABC system that will tell you to run away, most owners are aware of the coil-over alternatives and have chosen to "suck it up" and live with the extra ownership cost that ABC contributes.

    2., not

  38. Hello Darren. I think your post on abc troubleshooting is awesome. I have a 01 s55 and have replaced the tandem pump about 2 years ago. Last summer, I started experiencing red abc light frequently, but found could turn the car off and restart and it would usually clear out. Last November, came out to the car from Office Depot and the rear was all the way down on the tire. I started the car and kept working the ride height button up and down and finally got the car to raise and was able to get home. The car has sat since them with me only starting it and running for a few minutes every week or two. I plan on buying the conversion kit to replace all that, but it would be nice to be able to drive the car again for about a month until I could afford to have converted. Now that it is sort of cold in TN, when I start it and has for a few weeks, the red abc light does not come on and the car appears to sit normal, but the ride height no longer raises or lowers. I am afraid to drive it anywhere in fear of it dropping and having to tow it in. With what I have told you, what would you feel might be my problem? I feel, after reading your info, that it may be accumulator or ride height sensor. I don't find any leaks anywhere and the reservoir appears to have the right level of fluid. I did have a couple instances where I drove it last summer and when I stopped, I found fluid had been forced back out the reservoir. Any help, I would appreciate. You can also email me at if you would prefer.

    1. Since you multiple issues going on, I would recommend having the car rodeo'd and scanned for error codes.

      I'm reasonable sure your accumulators are shot. Your two main ones are almost surely shot, and the pulsation dampener and return accumulator are probably gone as well, so if you have to replace both main accumulators, you should replace all 4 for about the same amount of labor.

      The rear sinking may be a second issue. Was it a one time thing or still going on? If still going on, sounds like you may need to rebuild or replace the rear valve block as well.

      The rodeo and error codes may tell a different story, but based on what I have heard I would replace all the accumulators and rebuild/replace the rear valve block. There is a considerable labor savings if the rear accumulators and valve block are done together.

  39. Great blog, extremely helpful to me to troubleshoot my 03 SL500. I have symptoms of bad accumulators - intermittent 'ABC drive carefully' errors and overflowing reservoir after switching off the engine. My SDS laptop occasionally reports 'System pressure low' and I need to re-prime the tandem pump with air pressure in the reservoir as it seems to have developed an air lock again. I pulled both large accumulators but neither seems to have a damaged membrane - when I look inside, it looks like a tire valve stem and you cannot insert anything beyond about a hard cup-like piece 30mm into the part. I put a brand new accumulator at the front, replaced the front left strut (bad ball joint), flushed ABC fluid with CHF11S and replaced the filter. Not sure what to check next.

    1. I have to admit I am stumped. I've seen multiple pictures on the internet showing a failed and good accumulator side by side with sticks or screwdrivers extending out the top demonstrating a failed accumulator. Fluid overflow of the reservoir is also considered a classic tell of accumulator failure. Maybe the return accumulator or pulsation dampener has failed causing air to get into the system, but since they are so much smaller I would be surprised if the cause.

      I suppose when the system is off, air bubbles in the system would seek the highest point where the pump is and can collect there, causing you to have to prime it again. It could be that some air in the system is causing havoc. I good long drive might get it all out along with frequently cycling the ride height, and maybe performing the strut air purge procedure might help as well.

      If you do figure this out, please let me know. I can only do such remotely, and I will share what you have learned with future readers.

      Good luck!

    2. Thanks, I'll report back after I switch out the remaining accumulators and maybe the tandem pump too. I am trying to locate the hydraulic quick release tools so I can remove the rear valve block more easily next time.

    3. I replaced the tandem pump and wrote a DIY for that here:

  40. So, where did you get the orings and how did they work? Do you need both kinds?

    1. jnash on is someone with hands-on experience rebuilding them. He can probably advise you or even do the rebuild for a fee. See

  41. I must be the only lucky one i must say lowering the w215 cl improves the reliability of the abc system it puts less stress on the system all together mine is lowered via links purchased from aptires ive owned my cl55 for 3 years with absolutely no problems whatsoever with the abc system, i do of course maintain my car strictly and baby the car more than anything i love it the ride is fantastic i would not let this article scare anyone out of purchasing an amg version i dont know about the regular cl's as ive never owned one before . The key i would say is avoiding potholes at all costs and checking the fluid levels in the reservoir regularly even though if your abc system is functioning correctly there should be no change for some reason i believe the abc system only breaks down if you worry about it too much lol everyone enjoy your cl's they are the best cars ever . Thing ive had to change are motor mounts transmission mounts a/c evaporator temperature sensor a pulley rotors and brakes cost $1200 at the very least labor included with a good indy mechanic which to me is cheap but please only purchase these cars if you can afford to maintain the car well or else you will go through hell and back with it the car has 100k on the odo still going strong anyone who wants to see my vehicle my Instagram is @poloreligion

  42. And Darren btw your article is a gold mine to have for mercedes owners that own an abc equipped vehicle i screenshotted everything to have just incase for the future thank you for shedding light on this complex system that makes an awesome ride possible . You are greatly appreciated by all of us commenting on here .

    1. Darren you are a heaven sent figure! Thank you for your time and energy in delivering this message to the unknown and unlearned people the ABC Mercedes ordeal!

  43. My fiancee has an 06 s55 amg. He woke this morning to find it on the ground! The drive carefully and car too low messages have displayed before, but he has always been able to get the vehicle to raise. Now he cannot get it to raise at all. Does this sound like the pump is gone completely, or the accumulators?

    1. A corner sagging is almost always a failing or failed valve block. The o-ring inside the valve fails, allowing fluid to leave the strut and return back to the reservoir, and the corner sags. It should raise back up, but it is possible your fluid level is low because of this, causing the pump to run dry thus unable to raise the vehicle.

      I would advise checking the fluid levels and add fluid if necessary. If the fluid was extremely low, air may be inside the pump may be preventing it from generating pressure. You will need to prime it by pressurizing the reservoir to force fluid into the pump.

      The car should raise even without accumulators. It will just take longer.

      Assuming you get pressure restored and the vehicle to raise, then your next step would be to replace the valve block or rebuild it. Also check your accumulators and replace as necessary.

    2. Hey Darren,
      How does one "pressurize the reservoir"?

  44. 05 Sl500 The system appears to have overpressurized and ruptured the control valves. Attempting to remove block, it seams that there might be pressure on the lines marked PB1 & PB2 on the block. How to de pressurize or is it normal for those lines to be rigid? I also already have bled the two valves by the front wheels or are there more bleeder valves?

    1. The system should depressurize after the car is shutoff. It may take 5-10 minutes for all the pressure to dissipate. There will still be pressure in the lines between the struts and the valve block though. The pressure is caused by the weight of the vehicle pressing down on the strut. You should put the car on a lift or jack stands before attempting to disconnect any lines.

      Each strut has a bleeder valve in the associated wheel well. Just one bleeder valve per strut. No others I am aware of.

  45. Hi Darren,
    Great write up!
    Can you tell me what is the purpose of the second hydraulic hose that comes from the pump and goes nowhere? It terminates under the engine cradle. Is it a pressure dumper? I think it fills with fluid eventually.

    1. Yes, there is a pressure relief valve that lets off any pressure in excess of 200 bars and that line connects to the return side of the hydraulic system. See 52b on the hydraulic diagram.

  46. Hey Darren,
    Amazing post!
    When talking about replacing a strut can you just explain how to do the extra steps to fill the strut with fluid and bleed the air out of the line after installing the new strut?

  47. Great write up. I have a couple torn boots on my Sl63Amg. Any thoughts on how to repair as I don't think the boot is sold separately?

  48. Hey Darren, I have a 2003 CL55 (with lowering links) I had a leak coming from my pump and I replaced it. Car was fine for a while, but when it got really hot the car lifts itself up an insane amount above the wheels. I noticed the car does sag after sitting even over night. When I start the car and put it in drive, the car lifts itself to my desired height; then everytime i reach 50km/h the ABC visit workshop on and lifts the car up and the car is super bouncy like you mentioned. Sometimes it lifts to stock height( since I lowered it) and sometimes it jacks the car up to an insane height. When I turn the car off and on the message goes away and the car drops back down when put in drive. So confused, the pump is clearly working fine as it makes no noise and lifts and lowers quickly. The other day I had both white and red abc warnings come on when it was extremely hot. What could be causing all 4 corners to lift and look like a monster truck?

  49. Hi Darren,

    Just wanted to say great blog and thanks in advance. I have a 2003 CL55 with 106K and am having ABC issues (obviously). To start off, when the car is parked and not running I have no issues. No sagging, lean, etc. It all starts when I turn the key. At an idle park, either just the front or the passenger side will raise slowly by itself until it reaches peak and then will level off by itself, then repeat the process until the car is put into drive and it will stay at the correct height until you stop at a red light where it will begin this process again until you drive, thus returning it to normal height. When driving, the ride quality doesn't seem floppy or too soft (aka tuna boat). I've also noticed when I use the ride height button to raise and then drop the car back down, I'm getting what sounds like a high-pitch fart coming from the front end of the car which leads me to believe there may be air trapped in the system somewhere. I have had the white "Visit Workshop Message" for a little while. I really don't know where to begin and I figured I'd seek your help before I go throwing my wallet at it. Have you seen this before? Any advice?



  50. Hello, I could not resist the temptation of buying an 03" SL500 for a ridiculously low price with only 20K miles on it. However, I was warned by all the problem areas including vario roof and ABC. After owning the car for a month and after 200 miles on it, smoke and fluid came out of the left front fender while the car was at idle at a fast food drive thru. It imediately lowered on that corner and I drove the car home slowly with the ABC warning light on. What should I do????

  51. My comment is on reading the dipstick. Mine is below the lower mark at idle. From the forums I thought that was a bad main accumulator, with engine off level is at upper full mark. I have no oil blow out but I thought when i checked it in the past oil was on the lower mark exactly. So is being below the lower mark ok or is accumulator bad. No error messages even over speed bumps.

    1. Sounds look your accumulators are in great shape. My car had the rear accumulators replaced 2 years ago and the front ones replaced a few months ago. I have my fluid levels set to be at the upper mark when off, and it reads about another 1/3 BELOW the lower mark when running. I didn't want to assume all cars are the same, so I said to the lower mark was fine to be conservative. When it comes to the fluid drop between off and running, the more the better! So from your description I'd bet all 4 accumulators(I'm counting the pulsation dampener too) are good.

  52. This is awesome document. I'm looking for 01-03 cl500 and after reading this, my knowledge about ABC is totally something else than "pump most likely has to be changed"
    Five stars for you man!

  53. I would like to know, since I have te ABC system in my SL, could it be possible to build a complete ABC system in a custom KIT car, simply using all components ? The pump, pressure block, the struts .. ABC computer ? anybody ?? Im very curious. I like the system

    1. I think it would be possible. The main concern would be getting the ABC control module to operate. The car has a network of computers communicating with each other, and the ABC control module probably needs to communicate with something prior to it starting to operate. If you solve that, the rest of the components could be relocated from a donor car into a kit car.

  54. Hi Darren,

    Thank you for your wonderful write up, as it assisted me greatly. I have an '06 SL65.
    I faced an unusual problem where the "drive carefully" light showed up in white. I immediately pulled over and parked. About a minute or 2 later, fluid started spraying out of the pinholes on the dipstick at a considerably high pressure, and I lost a considerable amount of fluid. When I started the car again, the "drive carefully" warning was red within a few seconds. I immediately turned off the car and had it picked up by a flatbed.
    2 days later without starting, I checked the fluid, and it lost about half a litre of fluid. I topped it up with about a Half a litre of fluid. On starting and running the engine, the warnings were gone, but the fluid level in the reservoir dropped close to empty (No Fluid). I filled it up again with about a litre. And left the car idling for half an hour, in which it maintained the level of fluid(bottom line) while idling. After checking the fluid level another 2 days later without starting the car, the reservoir is completely filled to the lid of the dipstick. I suspect that it is an accumulator that is bad. Is this symptom possible due to a bad accumulator?


    1. Yes, it is a pretty safe bet that you have blown an accumulator. There are two primary accumulators. If it is the first to blow, chances are the ABC system will still operate with only occasional "drive carefully" messages appearing. If the first is already blown and this is the second one to go, then you should be seeing very frequent "drive carefully" messages almost every time you hit a bump.

      I would go ahead and replace all the accumulators (two main accumulators, the return accumulator, and pulsation dampener) in the system as a preventative maintenance item.

    2. Thank you very much for your assistance. I will go ahead and have them replaced. Thank you once again and your help and write up is highly appreciated.

  55. Hi, I work in Formula one on hydraulic systems.Your analysis of ABC is absolutely brilliant well done.

  56. Darren:
    Your information has saved me thousands of dollars. I appreciate it.

  57. i Darren- very informative! I'm looking at purchasing a 2003 SL. Seller replaced wheels with aftermarket (without TPMS sensors)so dash shows "visit workshop". However, this can be cleared. After storing for winter, he had two dead batteries. He did replace both batteries as well as EIS control and SAMS module. What I'm getting at is that the ABC button has no power light (as if non-functional) and car doesn't raise of lower (that button is non-functional as well). Could it need recalibration as well? Any info is much appreciated!

    1. I think your next step is to have the car hooked up to the STAR diagnostic computer and get error codes. And you might be right about the lack of calibration settings, but I would think you would at least get a dash error message.

      ABC is also closely tied to the dynamic stability control system. If that isn't functioning correctly it will disable ABC. Error codes should point you in the right direction.

  58. Hello . Really nice page :) I have got really strange problem my CL600 has somekind of really bad attitude..
    Code says "Rear left Strut control valve" ..Always when i start car abc visit appears , shut off and re start No warnings ...But when i hit in bumb same code..OK nothing strange ? sounds like easy case ?? But Accumounts are changed , Pump is brand new 193bar almost always , REAR valve block changed ..ALWAYS the same trouble left rear. Any good ideas ? wiring? i have checked litle bit of harness and it looks fine.. Thanks and sorry for my poor language ..Im from northern europe

    1. I agree with you that wiring seems the most likely since you rear valve block is new. I have chatted with one owner who found corrosion on the harness connector at the control module. You might try cleaning the connectors at both ends and see if it makes any difference. Or try with the car running and in gear(be safe about how), wiggle the wiring at each end and see if you can reproduce the error.

  59. And the block was 100% working in other car

  60. First off, thank you, thank you, thank you! Ever since I had the classic "burp" with my 03 R230 a few weeks ago, I have been doing research on ABC issues. Five or six years ago, I went through the typical dealer routine of change out components until the problem goes away. They started with the expensive bits first.:)

    I am a novice at best and found your presentation clear, concise, and very easy for a non-technical guy like myself to understand. I actually read much of your stuff on benzworld. Loved the recaps, they helped me fully understand the content.

    Armed with your write up and my STAR clone, I am heading out to the R230 to start with the accumulator test. Pretty sure I have a bad one, just not sure which. Sounds like I should replace all three. A bit of a scary home project for me, but I will use knowledge from this blog to build my confidence.

    thanks again

    Cal1 from MBW and benzworld (seldom on BW though).

  61. Replies
    1. yes, it is best Aktiv Body Control page ever!

  62. Hi Darren. I noticed that your website is I am from South Africa and I'm looking for a left front strut for an '06 SL65. Do you have an idea where I could find one?


  63. Hi darren my name is rigardt strauss i drive a 2004 mercedes cl55 amg my abc hose from the pump leaked at the joint replaced it with new one everytime i drive fast it is leaking again at the joint what can the problem be the pipe is very expensive to purchase because it comes with the damper?

  64. Hi Darren thanks for an excellent document. I drive a 2003 SL500 and the dashboard shows ABC Drive carefully. The next day I observed a lot of fluid on the garage floor coming from the driver side. The vehicle leveling was ok. I drove to my MB mechanic and he originally thought it was the strut but ruled that out and think it is hose related. I will post more once I receive the diagnosis from mechanic.

  65. THIS IS TO DERREN , THANKS! I WAS ABLE TO PUT A NEW PUMP IN BY USING YOUR INFORMATION, EVERYTHYING WENT GREAT 2004 SL500, pump was leaking from bottom so bought a new one and took about 12 hours copleate all new o rings new flush and filters, Wow expensive even to do it yourself, Sounds like abc equipped cars are just time bombs. BUT like everyone says there sure fun for the 200 miles untill they break. Ok so here goes 3 weeks after i noticed a hissing noise from drivers fender, where the pressure accumilater is located, So a few days later the new pump started making a almost belt noise but its inside the pump. I talked to the place i got the pump and there telling me since the pump is working fine and it is... that its the accumilator but i have not seen anything and iv read it all about anyone saying, that when a pump makes noise that its a acumilater or air ball, Or what ever the wourld calls them I am a mechanic by trade so i have alot of experience but i still have a hard time understanding why.. it one of the acumilaters was not working properly that the pump would make a noise and still work fine. Can you Help?

  66. Also, to add when i start the car i can here the noise from the front fender with door open sounds like a hissing or a pressure valve leaking air, Constantly for a long time this is while the car is runnning, Now that is a compleatly different noise than the pump is making now, the pump is also making a noise constant no stop its monotone so its not a clunk or a bang its a squeeky belt noise but its not a belt. its comming from inside the new pump. it worked fine for 3 weeks then all the sudding this noise. So i guess what i am asking is... If the pump is making noise will changing the acumilator under the drivers wheel well fix the pump noise? Again i have not herd or seen anything saying that this is a common problem again i just do not understand how the pump would wine if the pressure valve wasent working properly. But maybe it is, just looking for some clarification i already ordered part #2203270215 to replace 155.00 ebay, And i will replace to see but its a new pump and i thought the front acumilator was suppose to release pressure? So do you get why i am a tadd confused, I hope you can try and explain this, My number is 608-792-two 8 nine six. I owned a shop for 25 years. thanks!

    1. Sorry for the delay in responding.

      I would start with replacing the pulsation dampener in the left front wheel well, on the side of the wheel well closest to the driver. The pump is a radial piston design and without dampening it may make the sound you are describing.

      When the dampener fails, most people describe the sound as hum or whine. The excess vibration in the fluid could also explain the noise coming from the pump.

      Replacing the dampener only costs about $150 and less than an hour of time, so it is best to rule that out before exploring more expensive possibilities like the pump being bad.

  67. Outstanding help and explanations. You are great and are helping so many owners!!! Thanks Rich

  68. Hello I was informed that my abc control module need to be updated because of the codes I'm getting on the suspension what does that consist of...

    1. I've never performed a software update, but unless there is something I am missing, I can't imagine the the software update taking more than a few minutes to should be a simple download from their diagnostic computer to the car's ABC controller. Maybe an hour of shop time at most.

      There is a software update to help with the 1531 error code, but if your car is not showing any symptoms, I wouldn't worry about those codes getting occasionally thrown.

      Without knowing what codes they are seeing in the logs, I can't say if it is necessary or not. Off-hand I can't think of any ABC issues that can be cleared up with a software update.

    2. Is there a way to post attachments in the comments?

  69. Fantastic write up! Thanks for taking the time to put together a very easy to understand and logical explanation of the ABC system.

  70. I have a cl600 2003 v12 bi turbo 5.5 car and it keeps blowing the hydrolic accumulator hose that comes off the abc pump and goes nowhere. each hose last about 2 weeks when I put a new hose on there is no leaks it just blows the joint out between the fitting and hose every time in the same place. any ideas its the hose that bolts to the k member area under the motor with a end in it that looks like a plug

    1. There is a pressure relieve valve that is suppose to bleed off any pressure in excess of 200 bars. It may be possible that it has failed. There is also a throttle valve built into the pump that controls the rate of fluid let into the pump from the reservoir. It might be defective as well. Lastly, if you accumulators have failed there will be no give/take in system pressure and it could cause momentarily extreme spikes in system pressure.

      Also, there is no such thing as a hose in the system that goes nowhere. The hose from the pump should lead to an assembly that contains the pulsation dampener, pressure sensor, and pressure relief valve that routes excess pressure back to the reservoir.

      A bad dampener will cause a wine and will put increased stress on your hoses, but typically it does not cause hose failure as extreme as you are seeing.

      I recommend hooking the car up to SDS and have the car perform a rodeo. Monitor the system pressure during the test to make sure it doesn't exceed 200 bars.

      Also make sure the hose is being mounted correctly to not cause undue stress on the joint.

  71. it also makes a power steering whine if the damper sphere is bad could it make that hose keep blown from to much pressure or what would cause that

  72. put the new hose on filled it up with fluid , now on a star diagnostics I have 0bar of pressure. should I try to pressure the system or is the pump toast .

    1. You may need to prime the pump to get it to pressurize, either by pressurizing the fluid reservoir to force fluid into the pump, or by removing the pulsation dampener and starting the engine briefly to get the air bubble out of the line.

  73. So I pressured the system got the pump to work and light is off suspension is working fine . but I have a rattle humming sound in the car . which accumulator is it the pasengerside or the one in the fender

  74. And thanks for your time and help you have saved me a lot of time and hassles

  75. So I switched 52a under the passenger side the noise is still there . should I switch the pressure accumulator next to the trans

    1. 52a is the pulsation dampener and that is known to hum or whine when it fails. On my SL500 it is located in the drivers side wheel well. Not sure on the CL600 but my guess is near the transmission. Regardless, it should look like the pictures at the beginning of the "Pulsation Dampener / Pressure Limiting Valve / Pressure Sensor Assembly" section.

      I don't believe any of the other accumulators would make any noise, even if they are bad. Maybe a non-abc component is making the noise?

  76. So the car stoped making the noise abc light came back on I believe the pump went out. Are all the pumps the same on the s500 sl500 cl600 and cl500 I have a cl600

  77. Darren, you saved me thousands of dollars the dealership wanted to charge me to repairing my 2006 SL500. They said the pump was bad so I took my car to another independent shop (a guy that used to work for the dealership) and they replaced both accumulators. I did this because of your article saying that short intermittent red warnings (one second in this case when hitting bumps at high speeds) was caused by failing accumuators. I really appreciate you taking the time to help all of us with this post.

  78. Hi Darren great post very helpful.
    I have a strange one on my cl500 w216 in that the left rear strut stays in the high position constantly and the level switch does not work but illuminates.
    I have a star machine and if I rodeo it every strut works except this one it just stays in the high position. Inside the car you can hear this thumping noise when rodeoing like a valve jamming towards the rear you can also feel it through the shock if you touch it but it only happens when it is atempting to move that shock.
    I'm confused........Mercedes have changed the control module 8 times but not sold any in the U.K. In the last 12 months hmmmmm
    My other though would be the rear valve block.
    Your advise would be greatly appreciated.

    1. It could be number of things:

      2) You might also go into the calibration area on STAR and examine the sensor values. If the values are consistent with the other struts even though the strut is higher than normal, then a sensor issue exists. You might also try lowering the strut through commands in STAR as well, and see if it responds. And maybe try recalibrating.

      2) The strut could be physically damaged and won't move. You could try opening the bleed screw to release fluid from that strut. If the strut drops and if the system raises it back up when you start the car and hit the raise button, then the strut is probably fine and would also suggest the valve block is fine and the problem is a sensor issue. If the system fails to raise the strut, then the valve block is probably the problem.

    2. Hi Darren im Erwin from Portugal,I rebuild rear blockvalves unit\all other parts are new:pump and front unit\ after rise the rear with the button and rear stays max up posision\both sides\ contantly. There is no reaction on lower button,looks like presurre go only one direction up and no down mercedes test says:no reaction on rear...
      What to do???

  79. Hi,

    This is a fantastic thread and very helpful in my 4 years of CL500 W215 ownership, but have a problem not listed here..

    Replaced both rear shocks with Arnott remanufactured as 1 was leaking also replaced all 4 accumulators, front and pulsation damper were blown, while at it had the front valve block seal kit fitted, was getting an error code B4/5 prior to this. Started the car and cycled through the raise and lower to purge air but the red warning came back on and noticed air bubbles in the fluid so tested again with SDS and codes 1525-064 & 1526-016 pressure supply, so checked B4/5 with SDS and 170 bar more or less constant.
    Did a Rodeo and took the dipstick out to help purge air but when the rodeo drops the left side front shock the reservoir spurts / overflows frothy white abc fluid... finished/stopped rodeo but red light comes back so ABC is off and pump has no pressure on SDS.... leave car for a couple of hours starts fine rides up and down no errors a little bit of air bubbles in the reservoir, pump pressure 170 bar then red light back and reservoir fluid full of air bubbles and overflowing, codes for pressure supply back on SDS.. Air is getting in somewhere I guess but at a loss as all Hydraulics are fine went back and checked all 4 accumulators in case one had blown but all ok and cant see any leaks or seal where air can be getting in. I have a replacement Tandem Pump but don't think this is the problem so holding back on fitting that until I figure this out or can air get in from a faulty pump?? any help much appreciated, my local garage has an SDS which he lets me borrow.

    1. I think your assumption is correct that there is one or more air pockets in the system. The presence of these air pockets requires more fluid than normal to get the system pressurized, and when the system is shutdown this air expands and pushes fluid to and out of the reservoir.

      One possible location that air can be trapped is the struts. Have you bled the lines between the valve blocks and the struts?

      Also, once the car is running, have you checked how much fluid is remaining in the reservoir? If it is too low the pump will start ingesting air along with the fluid and cause pressure issues.

    2. I would also be curious to know what is the amount of fluid drop in the reservoir when off and then running? If more than 1 and 1/3 the distance between the two notches on the dipstick, that is a good indication there is air trapped in the system.

      If less than the distance between the two dipsticks, it suggests a bad accumulator but i would be surprised if this is the case since your accumulators are all new.

  80. Hi & Thanks,

    Only front was bled & no air, rear is very corroded and mechanic did not want to do that one, will get it back on the lift over the weekend and bleed all.

    The reservoir stays perfect once topped up, engine off at top mark and engine running middle mark, today I drove the car for 2 hours with three 10 minute stops before Red ABC came on but White ABC comes on after 10 minutes.

    Just started car after an hour and tried lift button 10 times, car is lifting very slowly and no warning but some air bubbles in ABC fluid... When Red ABC on with the engine running the Reservoir is at the top level on the dipstick and fluid is clear of air but guess that is because the pump is not running or system pressurised.
    Only accumulator not rechecked is the inline pressure near the fuel pump, this was replaced 12 months ago so going to check that next and bleed and 2 new/remanufactured front shocks next week.

    1. "car lifting very slowly" is interesting. On my SL500, if I press the ride height switch twice, the car will lift solely on the pressure stored in the accumulators. You will hear a drop in engine RPM afterwards which is the pump kicking in to top off the accumulators. Yet fluid levels are good and SDS reports good pump pressure. Odd.

      I have to admit I'm stumped on this one. I would recommend posting your problem to the R230 forum on the site. There are some knowledgeable owners on that board, especially "bayhas". Getting a few more opinions may help. I go by "benz_SL500_newbie" on that site.

  81. Hello
    First of all I want to thank you Darren for the quality of your blog and especially the precision of the information.
    I want to know if it is possible that you help me because I have a problem where I can’t find a solution.
    My car is a SL55 2003
    I explain to you :

    When my car was powered off, it fell to the right front and right rear within 5 minutes after the engine stopped.
    And at the same time turning off the engine the left rear rises slightly.
    After several consultations from several sources I realized that it was the valve blocks that stopped holding the oil at the stop, which caused the collapse of the car.
    So I decided to buy two block valves reconditioned that I installed this weekend.
    I have changed the oil and I also purged the shock absorbers on each wheel.
    I also installed a magnetic filter and changed the 4 pressure accumulators as a precaution.

    All is well spent and the improvements are:

    - The car at the left rear does not go up when I turn off the engine.

    The following day the following problems occurred:

    - The left side and the right side and the right rear of the car fall within 5 minutes after the engine stops.

    I do not see any leakage of the blocks or the suspensions.
    So what is the problem? I am lost…
    an idea? Is it possible that the new blocks have a problem? Personally I only see that.
    Thank you in advance for your answer.

    1. I agree the reconditioned valve blocks aren't holding the fluid in the struts as they should. There is only one explanation for corners sagging while parked and it is the valve blocks. EIther the internal seals aren't holding (most likely) or maybe the solenoids that close the shutoff valves aren't functioning property

      Hopefully they came with a warranty or are returnable.

    2. Thank you Darren for your quick answer.
      I agree with you. I ordered these valves block with a deposit principle.
      I get my deposit back when the company receives my old blocks.
      So I took the opportunity to open my old blocks and see if the joints were defective or if they were dirty, but they look clean and flawless, is it possible to send you photos to have your opinion? Inside the shut-off valves there is a cylinder with a spring. Can this spring be relaxed over time? Can the blue ring of this cilynder be defective even if visually everything is ok?
      I'am agree for the solenoids, can i check them by myself?
      A lot of question which I thank you for, again in advance.

  82. Been reading this article and comments for over an hour. What a guru. Thank you for the in-depth info and proper explanation of this ABC system. I just posted in the merc forum that i could not find any proper info on the ABC to come to any conclusions. I am now however, perplexed. My car seems unique. 2003 SL that works fine, no ABC errors or displays on the dashboard. When ticking over at a standstill after a few minutes, the drivers side shock rapidly lowers. Raising the car re-sets it until the next time. I was thinking it was the valve block leaking but you state that it only does this when parked and turned off. How can this happen Darren and throw no errors and what do you think the cause is. It has had 2 new height sensors, new fluid and filter. Driving is no problem at all.. it only happens if left sat on the drive ticking over or if in traffic stationary. Proper stumped now I have read this. Mike

    1. sorry for the late response.

      The valve block has two valves for each corner of the car. The shutoff valve holds fluid in the strut when the car is off. The control valve adds and removes fluid. If the o-rings are getting worn, fluid sneaks by and fills the strut. Eventually the controller detects the problem, and releases the fluid and returns the corner back to its target level.

      During normal driving, the control module is active enough to correct for the leak and it isn't noticeable. When stationary for long periods the leak becomes noticeable.

      So while not an urgent problem, it does indicate you have a valve block rebuild in your future.

  83. Hi Darren took the car up the drag strip after its runs came back found the car was up tilting to the left so pressed the ride hight button it lowered it self but have ABC light on with one malfunction. The car will not raise or lower it self any more ? So I took car to friend he put his laptop on and have this code C11128 come up so I brought a rear ride height sensor but before I was about to fit it the suspension started working going up and down again but the ABC light was still on ?? Now I have fitted the new Sensor rear left car wont go up or down ABC light on and one Malfunction and this C11128 code will Not Clear it self Please help its doing my nut in as I love this car and this is the only thing that has gone wrong she is a 2004 Sl55 AMG with 64.500 miles on her THANKS DARREN

  84. Hi Darren forgot to tell you car now sits half a inch lower on the rear right. And on the drag strip I did let the rear tyres down on the car. This has No tyre sensors fitted. As I said before I fitted this new travel sensor the car was showing on the dashboard ABC white light on But had a NO Malfunction. Should of I just left it before I jumped into fitting the new travel sensor drr me ? As now I have this One Malfunction and ABC light still on

    1. Hi Graham. Sorry for the late response. I hope you successfully sorted out your issue.

      Code C1128 refers to the left rear strut travel sensor. This sensor is deep inside the strut and the wire lead comes out the top of the strut.

      It is different from the ride height sensor.

      I recommend double checking the wiring, and if all is good you are probably looking at replacing your left rear strut. The travel sensor is deep inside it and it not feasible to replace the sensor only. The work to disassemble a strut to replace the sensor costs more than the strut.

      Arnot sells remanufactured struts with a lifetime warranty for around $500 or so.

  85. Have a 2008 cl600 that have owned since new. In the morning it makes a loud bang about two seconds after the car starts. It will only make the noise once. I can start it all day after that and it is fine. When I let it sit overnight the bang noise happens again. It is very loud and vibrates through the steering wheel and brake pedal. The Mercedes dealer has had it for 4 weeks now. The problem is, they only get one shot at it per day because the noise only happens onc0 in the morning. If they cant figure where its coming from, they need to wait to the next day for the noise to happen again. The Mercedes dealer and Mercedes technical support are leaning towards the ARS system. But what could create such a loud bang noise in the system? Mercedes technical support and the dealer seem at a loss as to what's causing it. They have replaced a strut (I didn't think that was going to help) and the noise reoccurred the next morning. They are now talking about the pulsation dampener but could that cause such a loud bang or thunk. The article above doesn't mention a loud bang noise but could it come from the accumulators or pulsation dampner or pressure valve??? Thanks for the help.

    1. hi Cory, sorry for the late response. I hope you got the problem solved.

      I have never run across a discussion or have heard of a situation where ABC can make a loud bang or clunk. I can't think what could theoretically cause this sound other than a hose or line blowing, but that should have very obvious symptoms.

  86. What a fantastic article. I recently purchased a 2001 CL500. I just got the dreaded red "Drive Carefully" message as I pulled out of my driveway, so I have had it parked ever since. I looked in the street where I back out of my driveway, and I see a couple puddles of stains that are most likely hydraulic leaks. Also, I see a small puddle below the right-front strut. Checking the dipstick, it is dry.

    My plan is simply have it towed to a repair shop (the shop has serviced this car from the previous owner for many years) with my observations, and from what I can deduce, it is likely a blown accumulator? I was getting the 'quick' drive carefully signal when I hit big bumps for about a month.

    My question is this: how do I manage if the shop will go through the same process of elimination to save on cost rather than simply starting at replacing the pump?

    1. sorry for not responding sooner! I hope you got things resolved successfully. For the benefit of others reading the blog. I am posting a response. If you are still monitoring this thread, I would be happy to hear what the final solution was.

      Sounds like you have too issues. Puddle and low fluid level clearly indicates a leak somewhere. Either a bad line, connection, or strut. That should be fairly straight forward to diagnose.

      The "drive carefully" when hitting pumps is almost always due to one or more accumulators having failed, but if the reservoir level was low it could also cause problems like this

      So I would recommend fixing your leak, getting fluids topped off. It might require priming the pump to get pressure back. Then see if still getting "drive carefully" messages, and if so, look into replacing the accumulators. The section on accumulators in this blog can go into more detail on that.

  87. I recently purchased a SL 500 2003 model. Rear height seems not normal although the height adjustment button is working ok in both rear and front.
    I took the car to local garage and they told me the rear valve block is faulty. Does faulty valve block cause the height to drop or to raise?
    Should I take it to Mercedes for another check?

    1. I need a little more detail on "not normal". It is consistently at the incorrect height? If so might be a calibration issue. if the height slowly raises while stationary and suddenly drops back to normal, then yes you have a worn valve block.

      If the valve block is faulty, fluid can escape past the o-ring and get into the strut and fill it when the car is standing still. Eventually the control module catches on that this has occurred and opens a valve to release fluid from the strut and return the corner back to the target level.

      When driving, the control module is more actively managing strut corner height, and small leaks on the valves aren't noticeable.

  88. Hello, it's a perfect article !

    I can learn a lot of information concerning ABC system.
    But I'am a problem on my 600 SL 2008, it's a V12 biturbo.

    I hear a noise as the power steering pump had a resistance and lot of vibration on resented in the steering wheel and the pedals.

    I change the pump by a used model, I change the accumulator on the front block valve and the problem it's the same.
    I change the accumulator on the pump by a used model but no amelioration.

    The vibration comes from the pipe coming out of the part ABC of the pump, the pipe becomes rigid and then enters the valve block, it vibrates the valve block and transmits the vibration to the body.

    Thanks in advance for your help and Still bravo for your article Darren

    1. I got same problem and check other small accumulator on the front\right side of the car,when I change it problem wad goone...

    2. Congratulations on solving the problem!

      This is the first I've heard of a failed accumulator causing vibration issues, but it does make some sense. There is no difference between the pressure dampener on the pump and the front main accumulator, other than size and location. So a decrease in fluid vibration dampening would be expected if the accumulator failed.

      The pulsation dampened may be sized to only handle the additional dampening needed assuming the front accumulator is functioning normally.

    3. Hello Erwin and Darren,

      Thank you for your reply but I have a SL 600 facelift and I have 3 accumulateur.

      The configuration is different in my car, there is the pump with an accumulator, a valvesblock with an accumulator on the front left and an accumulator for the rear suspension.

      After change pump and accumulator on the valvesblock, I still have vibrations but less important.

      Do you think the accumulator on the pump can cause this vibrations ?

      Think in advance,

  89. Hi Darren,im from Portugal with cl500 rebuild pump,front valvesblock and car driving well. Then I give rear block for rebuilding. After instalation rear struts rise to max and never go down again\no reaction on rise button\ when connect diagnostic by mercedes- no reaction from rear of any commands,what to do as nobody whant help.before I give rear blick for rebuilding car was doing fine with abc

    1. I would think the diagnostic computer would be reporting error codes.

      I agree the most likely explanation is that the valve block was not rebuilt successfully (the seals aren't holding causing fluid to leak into the struts.

      Also, since you had new struts installed, make sure they didn't forget to hook up the sensor wire at the top of the strut. There is a sensor inside the strut that reports how far extended it is to the ABC control module. I would think error codes would be thrown for sure, but I am not sure how the ABC system would behave under that circumstance.

      Lastly, maybe the valve block was not installed correctly? If some hydraulic lines got crossed or wiring got crossed, a problem like that might happen.

    2. Thank you Darren,what about bleding the lines between block and struts of air-is this nesecery?

  90. Hi Darren, thanks for this great article - I have an SL55 2004 with 69000km's what I thought was an ABC pump problem. The ABC works fine when started from cold - go for a 10min drive - stop - start again and the white ABC light comes on or sometimes goes straight to red warning and locks in the tuna boat position.

    I decided to check the pressure reading from the ABC pressure sensor via the OBDC port using a code reader whilst driving. It turned out the pump was producing 170-192bar from cold and during the 10min drive. On stopping the pressure gradually declined as it does - i let it get to about 98bar. On restart it continued to decline and at about 96bar the white ABC warning came up. It declined further until about 2 mins into the drive home - then all of a sudden the pressure shot up rapidly back to 175bar. The fluid reservoir is full when the car is off so would seem the pressure accumulators are ok i.e. they are not pressurising during the 2 min period.

    Given this additional info. I'm inclined to believe the pump is in good health. Has anyone else observed this? Why would the pump behave so erratically? Could there be an issue with engine temp affecting the ABC suction restriction valve - which might explain why the pressure continued to decline even though the car was turned on?

    Thanks for insight...

    1. Some more info. With the engine running, I checked the fluid level in the ABC oil reservoir when the pressure dropped. The level was way past the lower indicator and oil on the dipstick was full of air bubbles. Reading Pols post my symptoms seem similar. It seems that the oil is being drained out of the reservoir to the point where air is getting sucked into the system. When this happens the pressure drops to 15bar and the red warning light comes on. So it seems as though the pump and intake regulator are ok - just the oil is being pumped out somewhere. It would also seem that the only place for the extra oil to go given no external oil leaks, is a broken dampener or accumulator. Only thing that doesn't add up is that the oil reservoir fills back up completely - if a accu/damper membrane had failed the oil should stay in place.?? Puzzling...

    2. Good job on solving the problem! Yes, when the fluid levels are too low the pump will ingest air causing pressure problems.

      It is normal behavior for the reservoir level to drop the top notch on the dipstick to the lower notch on the dipstick. What is happening is that the accumulators contain air trapped behind a rubber membrane, and the when car is started the pump builds up pressure and the air pocket in each accumulator shrinks, building up a supply of pressure. When the engine is shut off, the accumulators push the fluid back to the reservoir.

      These accumulators are important is these pressure reservoirs are needed for sudden demands for fluid to fill the struts. The pump's job is to keep the accumulators topped off.

  91. This blog is terrific, I have 2007 CL600 5.5 v12 bi-turbo, 57k miles,with a power steering whine at or below 2000rpm. Then after car is driven and shutoff the power steering resivoir has power steering fluid overflowing from under the filler cap.I read your blog... is this more systematic of a occilation dampner or a accumulator? Thank you for help with this issue.

    1. The reservoir overflowing is a classic sign of accumulator failure. Air from a recently failed accumulator is still trapped in the system and working its way out. The air pocket expands when you shut the car off and pushes fluid to and out the reservoir. It will eventually stop and you will start getting "drive carefully" messages on the dash when hitting pumps.

      The wine is also indicative of the pulsation dampener having failed. But the volume of the pulsation dampener is not enough to cause overflow problems. If you are replacing your accumulators, now would be a good time to replace the pulsation dampener too.

      Lastly, there is a return side accumulator next to the rear main accumulator, and it would be advisable to replace that as well. My recommendation is to replace all 4 (2 main accumulators, pulsation dampener, and return accumulator). The life expectancy is about 10 years for them.

  92. Cl500looking for a schematic that shows three 10mm bolts holding abc reservoir in place.. Great blog

  93. Hoping someone can help with this...

    I have an '03 SL55 AMG, approx 90k miles.

    ABC has been working fine with the front driver side wheel sagging ocassionally when the car would sit.

    Having unrelated work done on the car, the suspension needed to be lowered and now the ABC will not raise at all. The car has been sitting for a considerable time due to other things being worked on (tranny, pulleys, etc.)

    The lines were flushed and bled several times.

    The pump pressurized from 80 up to about 170.

    Trouble Codes:

    C1525-79 ?? refer to technical manual

    C1526-016 Malfunction in pressure supply

    Any ideas as to why we cannot get the car to raise up at all?



    1. I'm going to state what might be the obvious just in case...if you were flushing and bleeding the ABC lines it is possible air got into the pump and it needs to be primed to get working again. The pressure supply error code seems to confirm this, but it sounds like you have checked the pressure and it is fine. So I am perplexed.

      Also check and make sure the ABC fluid level are at the lowest of the two notches on the dipstick when the engine is running. If too low air can get ingested into the pump preventing it from getting up to pressure.

      If the pump is not getting up to pressure, you should get an message on the ABC error message on the dash about 10-15 seconds after starting, which would point to a pump pressure issue.

      Is there an error message on the dash when you are trying to raise the car?

      If the pump was fine and this problem came on due to the flushing/bleeding, then priming the pump makes the most sense.

      Another thing to consider is that pump is going bad. When not under load it can achieve pressure but as soon as there is a demand for fluid, the pressure drops quickly because it can't keep up.

      I'm inclined to believe the pump is the must likely culprit and I would double/triple check those pressure readings.

      Very, very remote possibilities are the pressure sensor and control module. If you can access the control module with SDS to get pressure readings that would suggest both are fine.

      So in short I don't have a good answer to you. Symptoms are error codes are pointing to a pump issue.

    2. It appears I had the wrong information. The pump only maxed out at 80psi. Gonna try swapping out the pump. I guess maybe it was strong enough to maintain pressure when the system was fully charged but doesn't have enough power to pressurize the system from being fully bled?



  94. Hi Darren,

    Firstly, can I just say, fantastic blog. The amount of detail of knowledge of the ABC system staggers me. Just wish I understood it!

    I was hoping you may be able to share your thoughts on the possible cause of my problem. I have a 2003 CL55 AMG Kompressor. I have owned it for a year now and in that year, very occasionally (like maybe 3 times in the entire year), the blue ABC warning light would come on. Nothing would seem to be affected and when I next stopped, turned the car off and back on again, it would be gone.

    About a week ago I went on a fairly long drive (approximately 1000 miles) over a couple of days. Towards the last few miles of the journey the blue ABC warning light came on. I thought nothing of it, arrived home, parked up and went in for the night. In the morning I noticed the front right corner was sitting slightly lower than the others (front right approximately 68cms from floor to wheel arch while the other 3 were approximately 71cms). I checked the fluid in the reservoir which was slightly low but only VERY, VERY slightly. I topped it up and pressed the ride height button. All four corners raised but the front right was still about 3cms lower than the others. When I lowered the car the front right lowered a lot quicker than the others but still only to 3cms below the others. I haven't driven the car since for fear of damaging something.

    In a week the car has not got any lower (the front right still sits at approximately 68cms) and if I raise the car to the two height levels all 4 corners raise but the front right is always approcimately 3cms lower than the others. I plugged a diagnostic into the car and it came up with the error: C1531-002 Right front suspension strut moves although locking valve is closed.

    Any help or advice would be greatly appreciated and if you require any more information to help you better understand my situation, please do not hesitate to ask.

    Thanks in advance for your time and any help you may offer.


  95. Hi Darren,

    Just a quick update in case it helps...I cleared the fault codes yesterday. The front right corner is still 3cms lower than the others but hasn't got any lower. I then took the car for a short drive. Within two roads the blue ABC visit workshop had flashed up on the dashboard. I returned home and plugged it in and had one new error code. This time however it is: C1133 - right front - Fault in component - level sensor. That is the only fault code I now have.

    If you could offer any help what so ever, I would be very grateful.

    Thanks in advance.


    1. I would check the level sensor and look for damage. Make sure the linkages are tight and wiring connector is in good shape. It should be accessible by removing the wheel. See the section on the level sensor in this guide for what it looks like.

      You could try swapping the sensor with the one from the other wheel well (i think the part numbers are the same), and see if the problem moves to that side. Or just replace the level sensor.

    2. In this guide I refer to it as the "ride height sensor"

    3. Hi Darren, thanks for your replies, I really appreciate your time. Unfortunately I have been unwell since posting and haven't been able to do anything to the car but am hoping to get on it this weekend now I'm feeling better. Will let you know what I find.

      Thanks once again.


  96. I have a 02 cl500 giving codes c1120 and c1142 for b24/15 Lateral sensor can someone tell me where this sensor is located on my car,

    1. Hi,

      I'm not 100% certain (and you may want to wait for Darren to confirm) but a quick Google search suggests that it is located under the left rear seat.

      Hope this helps.


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