In this guide, we go over some of the most common problems that affect Mercedes-Benz vehicles with Active Body Control (ABC) hydraulic suspension.
Mercedes-Benz ABC suspension has been around since the early 2000s and can be found on high-end models such as the S-Class, SL-Class, and CL-Class AMG models.
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Unfortunately, even 20+ years after, very few mechanics know how to work and diagnose problems with the ABC suspension. Taking the time to read this article and learning about what can go wrong with the Mercedes-Benz ABC suspension can save you time, money, and headaches.
What does ABC visit workshop mean on a Mercedes-Benz?
ABC Visit workshop warning message on a Mercedes-Benz means that the Active Body Control (ABC) control module has detected a fault with the hydraulic suspension. If the problem is critical, you will receive a red ABC message, and the vehicle may drop too low or become too bouncy. If the problem is less serious, the warning message will be white, and you should be able to continue driving.
If you receive the ABC warning on your Mercedes-Benz cluster for the first time, pull over where it is safe and restart the vehicle.
Make sure to remove the key for at least one minute. In many cases, a simple engine restart can clear ABC warning messages.
If the ABC warning message comes on again:
Pull over, park in a safe location, then shut off the engine.
If the level is low, add ABC fluid. Low level means that there is a leak or an accumulator has failed. Also, inspect the color of the hydraulic fluid. The new ABC fluid is clear and green in color. Old fluid will be dark or brown.
Drive carefully to an auto repair shop and inspect the ABC system for leaks. Find an auto mechanic who is familiar with the Mercedes-Benz ABC system. Repairing ABC suspension is difficult, and not many mechanics are familiar with this system. It is not uncommon for a mechanic to replace ABC components, and on the other hand, the problem persists.
If you notice an ABC warning message on the dashboard related to the ABC suspension, it means the hydraulic suspension has a problem, and a fault code is stored in the ABC control module. Do not drive with a red ABC warning light on. If you continue to drive, the ABC pump can get damaged, which is a very experience repair.
If the color is white or blue, the warning is not as critical, and in most cases, you can continue driving.
If the ABC warning is red, it indicated a critical problem. The ABC system may turn off, or the vehicle may lower. A red ABC warning on the dashboard means that it is not safe to continue driving your Mercedes-Benz. If the warning is read, the vehicle height drops to an unacceptable level, and the instrument cluster will display a "Stop, Vehicle Too Low" warning. We recommend having the car towed instead of continuing to drive and end up with expensive repairs or cause an accident.
The ABC system may lock the sturts at the common position if there is a critical error. Because there is no longer fluid circulation, the ride becomes bouncy or rough.
A loose connection electrical connection
A failed ABC sensor
ABC pump has failed
ABC system has a leak
ABC fluid level is low.
If the ABC warning message only appears at startup then turns off, the problem most likely was due to a pressure drop. The ABC warning may turn off as soon as pressure builds up and the suspension is working properly. The warning message should turn off as well.
You may not start the redo test if active fault codes on the ABC module point to low pressure. Read the codes to find out what is wrong.
If you have fault code "B4/5 ABC pressure sensor 0 bar" and no action, the ABC pump may be faulty or no hydraulic fluid present due to a large leak in the system.
Can not calibrate ABC suspension
If there are no leaks around the pressure sensor or anywhere else around shock absorbers- the problem is indeed in the supply pump.
Plunger calibration should be successful at idle.
"Drive Carefully" comes up when hitting a speed bump or pothole.
If the Drive Carefully warning comes up when you hit a speed bump, the problem is usually due to a faulty accumulator that no longer provides instant pressures as needed or due to a loose electrical connection.
Start by checking the ABC fluid level. If the level is low and keeps dropping even after you refill it, one of the accumulators may be leaking hydraulic fluid.
You may also notice hydraulic fluid overflowing from the ABC reservoir.
ABC accumulators typically last between ten to twelve years. Once one accumulator fails, it is a matter of time before all the actuators need to be replaced. Replacement accumulators cost between $100 and $200. While not required, it is recommended to replace all at the same time.
ABC Warning Comes Up When Cornering
The ABC When cornering or braking hard, the message may appear briefly, which indicates the system is having trouble keeping up with demand. Common causes include
Low pump pressure
Low hydraulic fluid level
One corner drops when parked
It is not uncommon for one of the corners to lower or sag when the vehicle is parked for a long time.
This is normal if the vehicle is parked for weeks but not if the vehicle is parked for only a couple of days. The vehicle should rise to a normal level after the engine is started.
A common problem that causes a Mercedes-Benz with ABC suspension to drop when parked include:
The valve block is not holding pressure. The valve block O-rings can wear over time which leads to hydraulic fluid flowing out of the strut. It is possible to rebuild the valve block or install a new or used unit.
Struts may have a slow leak.
A dirty filter in the ABC reservoir
"Drive Carefully" or "Visit Workshop" appears when I start the car and stays on.
If one of these warning messages comes up and does not go away, a problem needs to be addressed. Stopping and starting the car sometimes resets ABC warning messages.
This problem may be caused by a:
ABC pump - is not generating enough pressure. In the early stages, the pump may generate enough pressure after a restart, and the message goes away only to come back after driving for a time.
Fluid leak - ABC fluid leaks are usually easy to spot, and one of the most common issues is why the ABC warning comes up or why you are losing fluid. The ABC hoses can rust with time, causing a leak, but leaks can develop at any location.
Air in the system - can also trigger the ABC warning message. This can usually be fixed by pressuring the system and doing a RODEO test.
Level sensor - A travel sensor or ride height sensor may be faulty.
The ABC pump may require priming to operate normally. This is done by pressurizing the reservoir using a Mercedes-Benz scanner to the hydraulic fluid circling again.
Remove the splash shields under the vehicle and inspect the underside for any signs of hydraulic fluid leak.
If the problem is not leak-related, you will need to read the diagnostic trouble codes (DTC) from the ABC Control Unit to determine what the issue is. If the pump is faulty, you will get a code indicating that the ABC pump can not generate enough pressure or pressure maybe 0 if the pump is faulty or a large leak.
Test the ABC pump using a Mercedes-Benz scanner and monitoring pressure as you perform a RODEO test. During the RODEO test, the pressure should stay above 100 PSI. If the pressure drops below 100, the pump may be too weak and needs replacement.
Whine noise from ABC suspension at 2000 RPM
Another common problem we see on Mercedes-Benz with ABC system is a humming noise or winning at RPMS below 2000.
This noise can be caused by a worn-out pulsation dampener. On the SL-Class, the pulsation damper is located behind the front-left wheel well, while on other models such as the S/CL Class, it is attached to the undercarriage by the transmission. On model 2007 and newer, the pulsation dampener is part of the ABC pump.
To find out the noise source, use a stethoscope to listen carefully to the various ABC components.
Similar noise is also caused by a failed torque converter on models equipped with the 722.6 transmissions. If that is the case, you will experience a problem, and the noise is noticeable in Drive or Reverse. When the problem is due to the ABC pulsation damper, the noise is present regardless of the transmission gear.
Hissing or whistling noise coming from the valve blocks
Whistling and hissing noise can come from the valve block if there is air trapped n the hydraulic lines. The valve block is located in various locations depending on the model, but most commonly, you will one at the front left corner behind the bumper or the rear wheel well.
To bleed air out of the system requires running Rodeo Test by going to Control Units > ABC Control Unit > Activations > Rodeo Test.
ABC Suspension Components
Typical components of the ABC suspension.
ABC Control Module - The computer module that monitors and controls the active body suspension.
ABC Pump - Supplies hydraulic pressure for the system
Pulsation Dampener - Smooths out the pressure from the pump
Pressure Limiting Valve - Bleeds off any pressure over 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
Struts - ABC struts control the vehicle height as hydraulic fluid is pumped into the strut or out of it.
Return Accumulator - Evens out the pressure on the return side of the hydraulic system (once the fluid leaves the struts)
Return Pressure Check Valve - 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
Reservoir - Stores extra fluid to raise the ride height, pressurize the accumulators, and replace 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)
Based on Mercedes-Benz we have worked on here are the most common problems with ABC suspension:
ABC Pump Failure
This is the TANDEM pump. It operates both the power steering and ABC suspension. Even though it is one pump for both systems, each system is separated in its own chamber.
The tandem pump has two separate chambers for each system:
The hydraulic suspension chamber can fail while the power steering works fine. The pump has to be replaced as one unit, even if only one system fails.
The ABC chamber of the TANDEM pump may fail. It cannot generate pressure— which means you will have under 180 bar of pressure. ABC pump rings wear and cannot have enough pressure.
Keep in mind that low pressure can also be caused by:
And not TANDEM pump failure.
When the pump is weak, you will get pressure but will get intermittent ABC "Drive Carefully" and "Visit Workshop" warnings. Initially, you may get random ABC wite warning messages. Eventually, the pump can not generate much pressure, which leads to more frequent error messages.
If the pump is noisy at idle but gets quiet when pressing the ride height button, this can be a sign of a worn TANDEM pump. Performing a test for the ABC pump can confirm if the pump is weak. The ABC pump is fine if the car can get through the rodeo successfully or generate at least 180 bar pressure.
ABC high-pressure line
Metal and high-pressure hoses connect all the parts of the ABC system. ABC hoses can leak typically due to corrosion or an accident. A hose leak repair can cost between $200 - $650.
These lines are common in trucks, which means repairing the existing line by visiting a hydraulic shop that works on trucks. A repair shop can replace the bad section of the line by fitting new compression joints.
How much does it cost to replace the TANDEM ABC pump on a Mercedes-Benz?
At the dealership, a replacement pump costs between $1200 and $1500. Replacement costs can range between $800-$1200 at independent repair shops.
DIY cost to replace the ABC pump yourself ranges between $450-$600. It is possible to purchase rebuilt ABC pumps. ABC rebuilt kits are available as well if you decide to repair your ABC pump yourself.
If you are not familiar with ABC suspension on a Mercedes-Benz, don't rush to conclude that the TANDEM pump is bad because you have low pressure.
Several owners have reported that they replaced the TANDEM pump (an expensive repair), but the ABC light was still on, and the vehicle didn't rise.
The pulsation dampener is the black spherical ball. The pulsation damper serves the function of a reservoir and provides the required fluid as demanded by the shocks.
This is the first component immediately after the pump.
Each pulsation dampener has a membrane and an air chamber. The pulsation dampener also smoothes the pulsations and vibrations that the ABC pump causes when it pumps hydraulic fluid.
It is common for the pulsation dampener membrane to fail, which causes loss of the dampening ability. Failed pulsation dampener in ABC suspension creates a humming or winning sound and ABC fluid pulsations in the reservoir. The ABC control module may not show a fault code if the pulsation damper is faulty.
Pulsation dampeners also have built-in release valves designed to release high pressure, for example, if it goes over 200 PSI.
Pressures at pulsation dampeners are monitored via the pressure sensors (B4/5).
Around 2007, Mercedes-Benz updated the ABC design by integrating the pulsation dampener into the ABC pump.
The valve blocks control hydraulic fluid that goes to each of the struts. Typically on a Mercedes-Benz with ABC suspension, you will find two valve blocks:
- one for the front struts and one for the rear struts.
The valve block often fails due to the age of the ABC fluid. Old fluid can cause the O-rings to fail and dirt to build up inside the valve block.
If an o-ring in the valve block is worn, the car may lower at one corner. Keep in mind that if the car lowers at one corner, the problem could also be a bad strut or leak in one of the hydraulic lines.
Similar to standard sturts, the ABC strut contains a spring and a shock absorber.
The ABC strut adds a hydraulic fluid chamber to adjust the height as fluid is added or removed from the strut. The ABC control module or driver can adjust the height at each corner of the car at a rate of 1/10 of a second.
Replacing a faulty strut at the Mercedes-Benz dealership costs between $1500-$2000. A cheaper alternative is installing aftermarket ABC struts which cost around $500 per strut.
The cheapest solution is to install a used strut. The drawback of installing used ABC struts is that there is no guarantee on how long they will last.
Just because the car sags at one corner, it doesn't mean that the problem is the strut. In many cases, the issue can be the valve block. Inspect the strut for leaks. If no leaks are present, the strut may normally be operating, and the problem can be at the valve block.
Replacing a bad strut is fairly straightforward. There is a connection and a sensor connection; the rest of the steps to remove an ABC sturt are the same as in other struts.
After replacing a bad strut, you will need to fill the strut with fluid then bleed the air out of the system. An advanced Mercedes-Benz scanner is required to recalibrate the ride height.
Wheel alignment is recommended after replacing a strut.
Stores hydraulic fluid that the TANDEM pump can use on-demand.
The dipstick has two marks.
The lower marks are used when is when the engine is running
The higher one is for when the system is shut off.
Leaks can develop at the reservoir itself. Check the hoses that go to the reservoir and inspect for leaks.
If the system keeps losing fluid, it indicates a leak in the system. The leak can be due to a rusted line or a blown accumulator.
ABC fluid that has never been flushed can cause microscopic debris to wear o-rings, seals, hoses, accumulator membranes, struts, and rubber components.
Old ABC fluid shortens the life of the ABC components in the system, such as the valve block. Microscopic metal shavings can accumulate in the fluid, acting as corrosive material which damages the pump bearings and valve block o-rings.
Keep contaminants and dirt out of the fluid when checking the ABC level. Wipe the area around the dipstick clean. Contaminants that get into the reservoir will enter the ABC system before encountering the filter.
The control module is what controls the operation of the ABC suspension and monitors all the sensors.
It controls the amount of fluid that should go to each strut based on current vehicle height and road conditions.
It controls the sport and comfort mode by making the suggestion more aggressive.
It also controls vehicle height and raises the vehicle when the switch to raise the vehicle is pressed.
If it detects a malfunction with one of the sensors or the hydraulic pressure is too low, it triggers the ABC warning message by sending a signal to the Instrument Cluster (IC). In addition, a fault code gets stored in the ABC control module for further troubleshooting.
Accumulator (Nitrogen ball)
The accumulator is located near the struts and stores pressure for immediate use. The function of the accumulator is to have pressure ready for instant use when the hydraulic fluid required is beyond what the ABC pump can provide.
In other words, accumulators store pressurized hydraulic fluid so that when the valve blocks need fluid to feed to a strut, there's a ready supply next to the valve block.
When you hit a speed bump, and ABC fluid is required, but the pump can't provide it fast enough, the required fluid is drawn from the accumulator.
If the accumulator has a failed diaphragm and/or lost its gas pressure, there is very little reserve fluid in the accumulator, and when a valve tries to draw fluid, the pressure will drop.
When the ABC has a fault and goes into limp mode, the car bounces because the ABC locks all the valve blocks to seal fluid into the four struts.
How to Check ABC Fluid Level
Check the ABC fluid level and make sure the level is not empty or low.
Park on level ground.
Turn off the engine and wait for ten minutes.
Pull the hood release and open the hood.
Locate the ABC reservoir on the driver's side of the engine.
Remove the dipstick, clean it and reinsert to measure the current ABC fluid level. The ABC reservoir dipstick has two marks. The upper mark is when the engine is off—the lower mark is when the engine is running. Wait ten minutes for the system to depressurize and the fluid to return to the reservoir.
If the level is low, add Pentosin CHF 11S High-Performance Synthetic Hydraulic Fluid. Do not start the car without correcting the fluid level. This is the recommended fluid for ABC suspension as per the Mercedes-Benz owner's manual. It can be purchased from the dealer online. When purchased from the dealer, the container is blue but online. It is green. It's still the same fluid.
Start the engine and monitor the dashboard to make sure that the ABC dashboard warning is cleared.
Do not drive or start a Mercedes-Benz without ABC fluid. Driving with no fluid can destroy the ABC pump, which can be a very experienced repair. In case of emergency, when Pentosin CHF 11S is not available, use a power steering fluid. Once you reach your destination, we recommend flushing the system with the correct fluid type.
Do not drive the car if the ABC fluid level is low. The ABC pump can get damaged and send metal shavings into the ABC components if it runs dry. This will create a more experience problem for years to come.
Frequently Asked Questions
What's the difference between ABC vs. AirMatic suspension?
The main difference between the ABC and Airmatic suspension is that ABC suspension uses hydraulic fluid to operate and fill the struts, which Airmatic uses compressed air.
Mercedes-Benz ABC suspension is a more complicated system than the AirMatic (air suspension). ABC suspension while it provides superior handing compared to the airmatic suspension. ABC suspension is predisposed to develop hose leaks, worn o-rings in the valve block, and pumps that wear out.
ABC suspension is most common on high-end Mercedes-Benz vehicles such as SL500, SL55 AMG, S550, S600, CLS55 AMG, S65 AMG, S63 AMG.
If you own a Mercedes-Benz with a problem with the ABC suspension, it is essential to learn about this system even if you don't plan on performing the repair yourself.
Do Mercedes-Benz cars with ABC suspension require maintenance in the winter?
Drive the vehicle at least once a month. Inactivity is hard on hydraulic systems, even more so than daily use.
What color is ABC fluid on a Mercedes-Benz?
New ABC fluid has a green tint. Old ABC fluid has brown or black color, which means it is time to flush it.
Why does my Mercedes with ABC suspension lower when parked overnight?
It is not uncommon for a Mercedes-Benz with ABC suspension to lower when parked for several weeks. Those cases are acceptable, and even a Mercedes-Benz would not perform any repairs. If the vehicle lowers when parked overnight or parked only a couple of days, a problem needs to be addressed.
Do you need to flush Mercedes-Benz ABC fluid regularly?
Mercedes-Benz dealerships and the owner's manual state that you do not need to flush the ABC fluid regularly, but considering the problems caused by old ABC fluid, it is better to flush the system at least every 50,000 miles. Old ABC fluid can cause the valve block seals to degrade over time, leading to car sagging when parked. The life of the ABC pump can be shortened by old hydraulic fluid.
ABC reservoir overflows after a drive.
There are two possible issues:
The reservoir is overfilled by accident. Do not fill to the top marking while the engine is running.
You may have a faulty accumulator. One of the membranes inside at least one of the accumulators may be blown. Fluid replaces the air in the accumulator. The reservoir overflowing fluid out the dipstick cap is a pretty clear sign an accumulator has failed. If there is a need to keep ABC fluid levels below normal to keep the reservoir from overflowing, it means the accumulator has failed. Eventually, you will notice a drop in the ABC fluid level at the reservoir as fluid fills all of the accumulators.
Suspension Fault Codes
B4/5 pressure sensor shows 192 bar at idle.
Rodeo test, pressure is usually around 115-150 bar.
C1525-001 Critical vehicle level front left
C1525-002 Critical vehicle level front right
C1525-032 Calibration of the plunger travel sensors has not been successfully performed
C1525-064 System pressure too low
C1526-016 Malfunction in pressure supply
C1526-064 Load adjustment not successfully performed
The ABC suspension can be found in many Mercedes-Benz models including:
Keep in mind that ABC suspension was an available upgrade and does not mean it is found on all the models. Most of these models had AirMatic suspension as standard, with the ABC suspension available on higher-level trims and AMG versions.