Automatic Level Control Description and Operation w/o F45

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.

When a load is added to the vehicle, the vehicle body moves down causing the sensor actuating arm to rotate upward. (An arm movement of about 25 mm (1 in) amounts to a height change of about 12 mm (1/2 in) at the rear bumper.) The upward arm movement activates an internal timing circuit in the RIM module and, after an initial fixed delay of 17 to 27 seconds, the RIM module 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 struts through the air tubes.

As the struts inflate, the vehicle body moves upward rotating the actuating arm towards its original position. Once the body reaches its original height, +/- 25 mm (+/- 1 in), the RIM module opens the compressor relay circuit, and the compressor is turned off.

In order to reduce current draw during air compressor starting, the RIM module 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:

When a load is removed from the rear of the vehicle, the body rises, causing the sensor actuating arm to rotate downward. This again activates the internal timing circuit of the RIM module. After the initial fixed delay, the RIM module 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 height, +/- 25 mm (+/- 1 in), the RIM module opens the exhaust solenoid circuit, which closes the exhaust solenoid and prevents air from escaping.

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 of the RIM module is activated. After about 35 to 55 seconds, the compressor is turned on for 3 to 5 seconds. This ensures that the struts 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.

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 behind the right rear wheel well.

The air dryer is attached to the air compressor head assembly at the pressure outlet and provides a dual function:

The exhaust solenoid is a non-serviceable part of the air compressor head and provides a dual function:

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 activating 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.

The RIM module provides supply voltage of 5 volts and ground to the position sensor. The position sensor then supplies an output voltage of 0.35 to 4.75 volts back to the RIM control module. The amount of voltage is dictated by the vertical height of the rear of the vehicle.

The rear air adjustable shock absorbers are constructed with a rubber-like air sleeve attached to the dust tube and reservoir. This sleeve forms a flexible chamber which extends the shock absorber when pressure in the chamber is increased. When air pressure is reduced, the weight of the vehicle collapses the shock absorber. In order to maintain proper operation and reliability of the air adjustable shock absorbers, a system pressure from 48-97 kPa (7-14 psi) must be maintained at all times.

The air tube is a non-serviceable item consisting of the following:

At the rear shock absorbers, the air tube connectors are held on with spring clips which snap into the grooves of the shock absorber air fittings. Air tube connectors are sealed using 2 O-rings.

Although the tubes are flexible, care should be taken not to kink them and to keep them from contacting the exhaust system.