There is a button in the trunk to turn on the compressor if you want to inflate something with the hose. Check to see if that button has been jammed in the depressed state. Remove the plastic trim piece the button is installed to and remove the wire from the button. See if that fixes the problem.
The components of the ALC (automatic level control) system are:
- rear integration module (RIM)
- height sensor
- air compressor assembly
- intake hose and filter
- air tube
- two rear shock absorbers
- accessory inflator switch
I still don't know what year your car is so I am going to assume it is 96 or newer.
The only way to know if there is a problem with the RIM is to get a tech 2 scan (at the dealership).
The height sensor mechanism is just inboard and forward of the rear left tire. Remove the left rear tire and look for a mechanism with an arm (height sensor arm) coming out to the rear, and a rod (sensor link)attached to the arm. Look at the components to see if anything looks damaged, disconnected or out of place.
Accessory inflator button, RIM and height sensor mechanism are the most likely culprits.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)