BMW 750i Active Suspension System Height Sensor Replacement 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. The height sensor is perhaps the most crucial sensor in the active suspension system due the high congruence of information it provides.

As the vehicle goes down the road, turns, parks, accelerates, or slows, the height sensors on each corner of the vehicle are always measuring the ride height of each corner. This information is relayed to the active suspension system control module at a rate of over 100 times per second in many systems. Since these sensors are monitoring all four corners of the vehicle, the control module can easily compare the ride height information for each wheel, and use air pressure or relief to adjust the suspension settings to provide an appropriate and desirable driving experience.

A failed or failing height sensor will alert the driver in one of two ways. Either the sensor will allow one corner of the vehicle to sit much higher or lower than the other corners of the vehicle, or a warning light related to the active suspension system will illuminate. In the event that the ride height is affected, it may not be evident there is an issue at all. If the vehicle is typically driven and parked on smooth, flat surfaces, the suspension may only call for minute adjustments that the driver never notices. However, if the vehicle is parked on a hill, the driver may notice the vehicle inexplicably sitting in a very awkward angle. This would be due to the fact that one corner of the vehicle would either adjust at a much different rate, or not adjust at all.

If the height sensor causes any handling or driveability concerns at all, it should be repaired immediately. Suspension that is unpredictable or very uneven can be hazardous at worst, but will wear tires prematurely at best. In the event the vehicle only displays a warning message, but no drivability issues are noticed, take the vehicle for service at the next convenience. If conditions worsen, expedite this visit.

Height sensors, their linkages, and their electrical circuits reside underneath the vehicle, are subjected to road debris and grime, and are typical points of failure for many active suspension vehicles. Due the fact they constantly articulate with every minute imperfection in the road, they will still wear even in the cleanest and debris-free environment. Understanding when a height sensor will fail is impossible, however, it should be a given that most vehicles with active suspension will have one fail at some point.

Diagnosing a height sensor can be easily completed with a diagnostic scan tool, but should be followed up with a hands on inspection with a multimeter to ensure the control module or circuit is not at fault. With a quality scan tool, a technician can observe the signal voltage from the height sensor, and determine if it is returning an incorrect signal voltage for the ride height at that corner of the vehicle. If the ride height and voltage shows discrepancy, the technician will disconnect the linkage for the height sensor, and move the sensor through it's entire range while checking with a multimeter. This will determine if the height sensor is receiving proper voltage, and sends proper voltage for the position at which it currently sits. If the range of the sensor is incorrect, or the sensor does not communicate in a closed loop with the control module, the sensor will be replaced.

Replacing height sensors is typically a matter of removing linkage from the sensor or chassis, and unbolting the unit from the vehicle. Finally, there will be an electrical connector that must be removed, cleaned, and placed on the new sensor. After installation is complete, the system will be retested to ensure the complaint does not return.

Checking the operation of the height sensor is imperative to determining if failure has occurred, but don't forget that the sensor, and circuit, reside underneath the vehicle. This means the circuit is prone to damage from road debris, and should be inspected any time there is an issue with the active suspension system.

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.

The height sensors can be diagnosed by the DIYer with a multimeter and the service manual showing specifications for the circuit. However, it should be noted that inexperience with a multimeter and diagnosing computerized electrical systems can lead to incorrect diagnosis. In general, if reading the reference and signal voltage can be done correctly, the sensor can be diagnosed and replaced at home.