GM Service Manual Online
For 1990-2009 cars only

Vehicles Without Road Sensing Suspension

The Automatic Level Control (ALC) system automatically adjusts the rear height of the vehicle in response to changes in vehicle loading.

The ALC system consists of a height sensor, an air compressor assembly, an ALC compressor relay, an intake hose and filter, an air tube, and two rear shock absorbers . The air compressor assembly consists of an air compressor and an air dryer mounted on a bracket. The air compressor head is a replaceable part of the air compressor. The exhaust solenoid is a non-replaceable part of the air compressor head.

The compressor is activated when the ignition is on, and weight is added to the vehicle. The exhaust solenoid is connected directly to the battery (+), enabling the system to exhaust with the ignition on or off when excess weight is removed.

Vehicles With Road Sensing Suspension

The Automatic Level Control (ALC) system automatically adjusts the rear height of the vehicle in response to changes in vehicle loading.

The ALC system consists of the CVRSS control module, two CVRSS position sensors, an air compressor assembly, an ALC compressor relay, an intake hose and filter, an air tube, and two rear shock absorbers. The air compressor assembly consists of an air compressor and an air dryer mounted on a bracket. The air compressor head is a replaceable part of the air compressor. The exhaust solenoid is a non-replaceable part of the air compressor head.

The vehicles rear vertical height is measured by the two CVRSS position sensors. These two position sensors convert this rear height measurement into an analog voltage (0 to 5 volts DC) which is read by the CVRSS control module. The control module then determines what action (exhaust, compress, or no action) shall take place. To compress, the CVRSS control module switches the low-side of the ELC compressor relay to ground.

The air compressor is enabled (switched to battery only when the ignition is on. The air compressor is activated when a sufficient amount of weight has been added to the vehicle.

The exhaust solenoid is enabled at all times. The exhaust solenoid is activated when weight is removed from the vehicle.