The ALC system controls the ride height of the rear of the vehicle via a compressor with an integral exhaust solenoid valve. The system consists of the following components:

The level sensor senses the height of the rear of the vehicle. The level sensor then, compares it with a desired trim height programmed into the level sensor's memory. The exhaust valve energizes if the vehicle is higher than desired. The compressor relay energizes in order to activate the compressor assembly if the vehicle is lower than desired.

The leveling indicator light performs a four second bulb test when the ignition is switched to the ON position.

The leveling indicator light illuminates when an extreme load condition is present. When an extreme load condition is present a potential rear suspension clearance problem may exist. The light goes out when the leveling is complete.

The leveling indicator light does not illuminate during normal leveling cycles.

B+ is applied at all times to the rear suspension leveling air compressor and the ALC sensor. This allows the ALC system to vent after the load is removed from the vehicle, even if the ignition is in the OFF position. Approximately 30 seconds after the ignition is turned to the ON position, the compressor will run for 4 seconds to ensure that the shock absorbers will be filled with adequate residual pressure.

The ALC sensor must detect an out of trim state for 3 seconds on a static vehicle (20 seconds for a vehicle moving faster than 1 km/h (0.62 mph) before activating the intake or exhaust output. This action prevents operation during normal riding motions. In addition, the sensor limits the compressor run time or the exhaust solenoid valve energized time to a maximum of 2.5 minutes. This time limit is necessary to prevent continuous compressor operation in case of a severe system leak or continuous vent. Cycling the ignition resets the 2.5 minute run timer.

The compressor is a positive displacement rocking piston air pump powered by a 12-volt direct current permanent magnet motor. The compressor head casting contains the piston intake valves and contains the piston exhaust valves. The compressor is mounted to the right front of the body by a bracket.

The ALC sensor is an electronic device. The following is controlled by the ALC sensor:

The sensor circuitry provides a 25 second delay before either circuit can be completed to prevent falsely actuating the compressor or to prevent falsely actuating the solenoid valve circuits during normal ride motions.

The sensor also limits the compressor run time or the solenoid valve energized time to a maximum of 2.5 minutes. This time limit function is necessary to prevent continuous compressor operation in the case of a severe system leak or in the case of a continuous exhaust valve operation. Turning the ignition off and on resets the electronic timer to renew the 2.5 minute maximum run time. The sensor mounts on the rear axle cradle. The actuator arm of the sensor attaches to the right rear lower control arm by a link.

The shock absorber is essentially a conventional shock absorber enclosed in an air chamber. The shock absorbers are constructed with a rubber-like sleeve attached to the dust tube and attached to the shock reservoir. This makes a flexible chamber which extends the shock when the air pressure in the chamber is increased. The weight of the vehicle collapses the shock absorber when the air pressure is reduced. To maintain proper operation and to maintain reliability of the shock absorber a minimum residual air pressure of 55 kPa (8 psi) is maintained at all times by the air dryer.

The snap-on connectors attach the tubing to the rear shock absorbers. The connector housings contain retainer clips. When a tubing is attached to a shock fitting, the retainer clip snaps into a groove in the shock fitting, locking the tubing in position. All tubing fittings are sealed with two O-rings.

The air enters the ALC system through the filter mounted to the air compressor bracket. The filter housing contains a foam filter which prevents contaminants from entering the ALC system.

The vehicle body is forced downward when a load is added to the vehicle. This load causes the sensor actuating arm to rotate upward. The upward movement of the sensor actuating arm activates the internal timing circuit. The sensor completes the compressor circuit to ground after an initial delay of 25 seconds. With the compressor circuit complete, the compressor runs, taking in air through the air compressor filter and sending the air though the tubing to the shock absorbers. As the shock absorbers inflate, the rear of the vehicle moves upward, rotating the sensor actuating arm back toward its original position prior to the load addition. The sensor opens the compressor circuit and shuts off the compressor once the body reaches its original height +/- 19 mm (+/- 0.75 in).

The body is forced upward when an excess load is removed from the rear of the vehicle. This causes the sensor actuating arm to rotate downward. The downward rotation of the actuating arm activates the internal timing circuit. The sensor completes the solenoid valve circuit to ground after an initial delay of 25 seconds. With the solenoid valve energized, the air begins to exhaust out of the shock absorbers back through the air dryer, the solenoid valve, the air compressor filter and into the atmosphere. The sensor actuating arm rotates upward toward its original position as the rear of the vehicle lowers. When the vehicle body reaches its original height +/- 19 mm (+/- 0.75 in), the sensor opens the solenoid circuit, de-energizing the solenoid valve, which prevents any further air from escaping.

The air replenishment cycle serves to ensure that the shock absorbers are filled with at least minimum residual pressure. The sensor commands an air replenishment cycle each time the ignition switch is turned on. Sensor position is checked when the ignition switch is turned on. If the sensor indicates that it is not necessary to raise or lower the vehicle, an internal timer circuit is activated. After a 35 to 40 second delay, the compressor is turned on for approximately 4 seconds. This ensures that the shock absorbers are filled with adequate residual pressure.

If the weight is added to, or removed from, the vehicle during the initial 35 to 40 second delay, the air replenishment cycle will be overridden and the vehicle will raise or lower after the normal delay.

A 1.5 second actuation of the exhaust solenoid valve occurs each time the compressor is to start to ensure the low compressor motor starting current. The 1.5 second actuation of the exhaust solenoid valve reduces the air pressure in the compressor head cavity.

The ALC system uses vehicle speed input from the Electronic Brake Traction Control Module/Brake Pressure Modulator Valve (EBTCM/BPMV) assembly to change vehicle leveling response time.

The response time of the intake or the exhaust control switch is 3 seconds when the vehicle is stationary. The response time of the intake or the exhaust control switch is 25 seconds when the vehicle is moving faster than 1 km/h (0.62 mph).

The calibration of the ALC sensor can be done with the scan tool. Select RECALIBRATE LEVELING SENSOR from the BODY menu on the scan tool. If the ALC sensor has been replaced, the sensor needs to be calibrated. The scan tool will continue with the calibration and then informs you when it is successful.