Suspension Control ( Automatic - Electronic ): Description and Operation

Automatic Level Control General Description
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 Rear Integration Control Module (RIM), height sensor, an air compressor assembly, an ALC compressor relay, an intake hose and filter, an air tube, 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.

Automatic Level Control System Components Description
Air Compressor
The air compressor is a positive displacement air pump, powered by a 12V DC permanent-magnet motor. The air compressor head contains intake and exhaust ports plus a solenoid-operated exhaust valve that releases air from the ALC system when energized. Air compressor intake air is drawn through an intake air hose and filter, mounted in the body rail. The air compressor is a serviceable part and is mounted on a bracket. The bracket is mounted to the underbody, right side, in front of the rear wheel suspension.

Air Dryer
The air dryer is attached to the air compressor head assembly at the pressure outlet and provides a dual function:
- It contains a moisture-absorbing chemical (desiccant) that absorbs moisture from the compressed air before it is delivered to the rear shocks. Moisture is removed from the air dryer and returned to the atmosphere when air is exhausted out of the struts during lowering.
- It contains valving that maintains a system pressure (residual pressure) of 48 - 97 kPa (7 - 14 psi). This system pressure is maintained for improved ride characteristics and improved air sleeve reliability.

Exhaust Solenoid
The exhaust solenoid is a non-serviceable part of the air compressor head and provides a dual function:
- When energized, it exhausts air from the rear shocks.
- It acts as a relief valve to limit the maximum air pressure output of the compressor.

Automatic Level Control System Operation
Raising the Vehicle
When a load is added to the vehicle, the vehicle body moves down causing the sensor actuating arm to rotate upward. The upward arm movement activates an internal timing circuit in the RIM once the doors and rear deck lid are closed. After the initial fixed delay, the RIM provides a ground to complete the compressor relay circuit. The 12V (+) circuit to the compressor is then complete and the compressor runs, sending pressurized air to the shock absorbers through the air tubes.
As the shock absorbers inflate, the vehicle body moves upward rotating the actuating arm towards its original position. Once the body reaches its original trim height, the compressor relay circuit is opened and the compressor is turned off.

Air Compressor Head Relief Sequence
In order to reduce current draw during air compressor starting, the RIM performs an air compressor head relief sequence before air compressor operation. This sequence reduces the air pressure in the air compressor cylinder during start-up. The air compressor head relief sequence occurs as follows:
1. Exhaust solenoid is energized.
2. Air compressor is activated 1.3 seconds after the exhaust solenoid is energized.
3. Exhaust solenoid is de-energized 0.5 seconds after the air compressor is activated.

Lowering the Vehicle
When a load is removed from the rear of the vehicle, the body rises, causing the sensor actuating arm to rotate downward. Once the doors and the rear deck lids are closed, this again activates the internal timing circuit in the RIM. After the initial fixed delay, the RIM provides a ground to complete the exhaust solenoid circuit, energizing the solenoid. Now, air starts exhausting out of the struts, back through the air dryer and exhaust solenoid valve, and into the atmosphere.
As the vehicle body lowers, the actuating arm rotates to its original position. When the vehicle body reaches its original trim height, the exhaust solenoid circuit is opened, which closes the exhaust solenoid and prevents air from escaping.

Air Replenishment Cycle
The sensor actuating arm position is checked when the ignition is turned on. If the sensor indicates that no height adjustment is needed, an internal timer circuit is activated in the RIM. After about 30 seconds, the compressor is turned on for 3 to 5 seconds. This ensures that the shocks are filled with the proper residual pressure. If weight is added to or removed from the vehicle during the time delay, the air replenishment cycle is overridden, and the vehicle rises or lowers after the normal delay.

Automatic Level Control Sensor Description
The suspension position sensor is a potentiometer, which detects height changes at the rear of the vehicle and, in turn, relays these changes to the RIM, which controls the functions of the compressor and exhaust solenoid. To prevent energizing the compressor relay or exhaust solenoid during normal ride motions, the RIM provides a calibrated delay before ground is completed to either circuit.
The sensor is mounted to the left rear side of the rear suspension support assembly. The actuation arm is attached to the left rear control arm by a short link.