What exactly is the air suspension supposed to do? Is it for ride comfort, load leveling? I'm not sure the purpose.

Reason I say that is my compressor kicks on for 5 seconds or so shortly after I start the vehicle. After that I don't ever see or hear it do anything. If I jump on the back of the trunk reducing ride height, it still does nothing.

I went on a cross country trip this summer with 5 people and luggage in the car, and the back end sagged considerably due to the weight. The air compressor never kicked in at all.

Oops, didn't realize I was in the performance section. I suppose the mechanical section would be more appropriate for this thread.

It is a load leveling system. When the sensor detects the rear end sitting lower it is suppose to kick the compressor on and attempt to raise the rear with the rear air shocks. What does the compressor sound like when it kicks on? Most likely your compressor is doing what mine did, grinding itself up internally.

I moved your thread to 2000-2005, but I'm not sure if that'* correct.