1992 Mercedes-Benz 300D Active Suspension System Diagnosis & Testing Cost

Active suspension systems on modern vehicles use hydraulic or pneumatic pressure to change the ride height, dampening characteristics, and rigidity of the suspension according the the road surface any precise moment. They will typically use a pump or compressor to send pressurized fluid or air to the suspension on each corner of the vehicle, which will raise or lower the vehicle, or increase and decrease stiffness. In order to control this action, a central computer, the active suspension system control module, will use feedback from pressure sensors, yaw sensors, and ride height sensors to determine when and how the vehicle should react. Issues with these systems is likely during the life of the vehicle, and when those issues arise, a technician will follow a diagnostic strategy specific to the active suspension system.

The active suspension system functions by comparing sensor readings from each corner of the vehicle, speed, throttle application, and a variety of other vehicle sensor readings to determine the best possible position for the suspension is for that moment, and use the compressed air or pressurized fluid to make adjustments. The result is a vehicle that stays level when parked, rolls less during turns, and conforms to the road manners the driver desires.

Any time the active suspension system functions abnormally, diagnosis will be required. This may be a warning lamp for the suspension system, a noise when hitting bumps, or incorrect ride height on one or more corners of the vehicle to name a few. Other symptoms are possible, but it will always be obvious they are problems with the active suspension system due to the nature of adjustable suspensions.

The urgency of the repair is based on the type and severity of the issues resulting from failures in the suspension system. If the ABS and traction control systems are turned off, the vehicle should be repaired immediately since these are safety systems that the driver is likely accustomed to using. When the symptoms have a noticeable effect on handling and driveability, again, this should be repaired before the situation turns hazardous. However, if the suspension system gives no reason for handling, drivability, or safety concerns, the repair can likely wait until the earliest convenience.

There is no trend regarding active suspension component failure, though age will play a factor with air springs. It is most important to understand that proper maintenance and inspections of the system will catch most concerns before they turn into problems.

The diagnostic strategy for the active suspension system will vary with the issue presented to the technician. For instance, if one air spring fails to inflate, the technician would likely begin by checking the hardware at that air spring or inspecting for leaks. However, if one air spring at a time randomly failed to inflate, the technician would likely begin inspection at the control module. It is less important to understand the diagnostic process for active suspension systems than it is to understand that the process differs greatly depending on the fault exhibited and other clues which may be present. Since modern vehicles use sensors from the engine and transmission control system to adapt to driver inputs, it is also important to note that diagnosing suspension system faults is secondary to diagnosing any other driveability concerns, check engine or transmission warning lights, and anything else that could affect the operation of the active suspension system.

Any time the air suspension system is opened, it should be purged of air before reclosing the system. This will help rid the system of moisture caused by humidity and condensation. Also, this will allow the technician to monitor for debris, and take action if needed.

When diagnosing any active suspension system, the suspension must be deactivated before raising the vehicle with a jack. When the vehicle is lifted, the active suspension will attempt to level itself if left on, and this may result in damage to the active suspension system, or unstable lifting with the jack. If deactivating the system is not possible, disable the compressor.

Diagnosing active suspension systems is nearly always outside of the scope of the DIY mechanic. This is typically due to two factors, availability of high quality and high functioning scan tools, and inexperience servicing computerized automotive systems. The cause of the issue may not be immediately understood as part of the system, and this can lead to misdiagnosis of correctly functioning components due to the inability to detect the real issue. If the fault is not abundantly clear, leave diagnosis of the active suspension system to a trained professional. This will ensure the issue is diagnosed and repaired properly.