2004 BMW 745Li Active Suspension System Air Compressor Replacement Cost

Active suspension systems come in a few forms, the most common of which are air suspension and hydraulic suspension systems. Air suspension, otherwise known as air spring suspension or pneumatic suspension, use a compressor to pump high pressure air to the air springs to inflate them when needed. High pressure is always maintained in the system so the air springs can adjust at will, and the compressor will run any time it is needed to replenish this pressure.

The air compressor for pneumatic active suspension systems functions by high amperage switched power, which is turned on and off by a control module or relay. Any time the height sensors, pressure sensors, control module, or suspension mode selector switch request an increase of pressurized air, the compressor is activated, and pumps that pressurized air to the springs where it is needed.

Air compressor failure is evident in that the entire system will either cease to function, or the compressor will constantly run with or without achieving its target pressure in the system. This can happen even when the vehicle is off, and battery drain may occur until there is insufficient power to activate the air compressor relay. In nearly all cases, all four corners of the vehicle will sag to the extent that the vehicle ride height is much lower than desired. Also, the ride will be extremely uncomfortable. In some cases, the suspension may not sag for quite some time, but ride quality will diminish as will the ability to adjust the suspension with the mode selector. Finally, a hissing may be heard from the compressor in the event of an air leak from the compressor or manifold.

Just like coil spring systems, a failed airbag suspension system should be serviced right away. Not only is driving with deflated airbags difficult or dangerous, but there is a risk of harming other components - the shocks at the very least. Also, if the vehicle is burdened by any kind of a load, bumps will likely cause the vehicle to lose control at even moderate speeds.

Active air suspension compressors fail sporadically, but generally provide years of useful service to the vehicle. Especially when the suspension system is kept in good repair and free of leaks, the compressor should last long enough to forget it is there. Vehicles subjected to poor maintenance standards for the charging system and active suspension system are likely to suffer failures more frequently, as well as vehicles with water intrusion issues. The best way to ensure the compressor lasts as long as possible is to prevent leaks in the suspension system, and ensure that water is kept out of the pneumatic lines.

If the air suspension is not receiving sufficient pressurized air, the air compressor will be tested for failure. First, the technician will test the air compressor electronically. They must understand if the compressor is receiving switched power, has ground, and whether the compressor motor is functional. If these are all found to be true, the compressor will need to be tested to find how much pressure it is creating. It is entirely possible for a compressor to run normally without creating air pressure. If the compressor does not function electronically or pneumatically when power, signal, and ground are present, it will be replaced.

Active suspension air compressors are connected to the vehicle both electrically and pneumatically. This means they must be disconnected from battery power, switched power, and air lines. Installation is the reverse of removal, and instruction should be followed to determine how to seal the air lines and if oil should be added to the compressor. It is important to note that some compressors are driven by the serpentine belt from the engine, but this is rare for most passenger vehicles. Also, in many cases, the compressor may be tucked away underneath the seat, trunk lining, or somewhere in the engine compartment, so referring to the service guide is necessary to locating the compressor for testing.

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.

Air compressors are not typically difficult to diagnose or replace, and the average DIY mechanic can handle the repair with the use of the service manual for the vehicle. It is important to understand how to properly seal and service pneumatic systems, and whether the compressor for that vehicle requires lubricating oil to be added. If any portion of the repair is unclear, or diagnosis is not completely certain, it is best left to a technician due to the high cost of parts.