The 2002-2009 Audi A8 is built with the following suspension components:
2002-2009 Audi A8 Air Struts (x4)- The inner portion of the struts are a cylinder filled with hydraulic fluid under a gas charge, used to dampen vibration and bounciness caused by travel on rough roads. The struts also serve a mounting place for a rubber air spring, managed by a suspension control module. These rubber air springs inflate to support the weight of the vehicle and they are the number one source of leaks in the system -- caused by dry rot and normal wear and tear. The rubber air springs also help maintain the vehicles correct ride height, especially whenever the amount of load put onto the vehicle changes.
2002-2009 Audi A8 Air Lines- Tubes that carry compressed air from an onboard compressor to the rubber bladder portion of the air struts.
2002-2009 Audi A8 Solenoids (x4)- These mostly plastic components work along with the vehicle's control module to regulate air pressure for each air spring. Solenoids start to leak as they wear out causing the vehicle to sag or to lean. This will cause the ride height sensor to send a signal to activate the compressor in an attempt to inflate the air suspension and level out the vehicle. This excessive work load will eventually lead to total failure of the compressor.
2002-2009 Audi A8 Compressor- Creates and sends pressurized air to active suspension components. When the air suspension starts leaking, the compressor/pump starts working harder to try to keep the system inflated. By the time air suspension leaks down completely, the compressor will either have failed or be well on its way to full failure. Buying replacement air struts and taking the time to install them, only to find out the compressor still needs to be replaced, can be very frustrating and expensive.
2002-2009 Audi A8 Dryer Assembly- Used to keep moisture out of air lines and air springs in order to prevent freezing and internal cracking of these parts. Once air suspension starts leaking, moisture will build-up throughout the air system. Moisture damages the air suspension from the inside. The dryer will need to be replaced in order to eliminate the moisture and prevent any further damage.
2002-2009 Audi A8 Suspension Control Module- A computer responsible for the operations of the air suspension system and maintaining the vehicle's correct ride height.
2002-2009 Audi A8 O-rings- Although they are the least expensive parts of the air suspension, they are responsible for maintaining the seal where all of the air lines connect. Be sure to replace the O-rings when repairing any air suspension system.