Automatic Level Control Circuit Description Programming Procedures

The calibration of the Automatic Level Control (ALC) sensor can be checked with the scan tool. Select CHECK SOFTWARE/CALIBRATION VERSION from the SPECIAL FUNCTIONS menu on the scan tool.

If the ALC sensor has been replaced, the trim setting needs to be calibrated. You will be instructed to raise the vehicle on a hoist and allow the rear wheels to hang. The scan tool continues with the calibration and then informs you when it is successful.

Automatic Level Control Circuit Description Raising and Lowering Vehicle

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 (.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.

When a load is applied to the vehicle, the vehicle body is moved downward. When the sensor has detected an out of trim state for 3 seconds on a static vehicle (20 seconds for a vehicle moving faster than 1 km (.62 mph), the intake control switch is closed to ground. With circuit B25 grounded, the coil in the rear suspension leveling air compressor relay K19 is energized and the relay contacts close. B+ is applied to the compressor and air is pumped into the adjustable shock absorbers. As the shocks extend, the rear of the vehicle raises. When the vehicle body reaches its original trim height (+25 mm [+1 inch]), the sensor opens the intake control switch, removing the ground from the relay and stopping the compressor.

When a load is removed from the vehicle, the vehicle body moves upward. When the sensor has detected an out of trim state for 3 seconds on a static vehicle (20 seconds for a vehicle moving faster than 1 km (.62 mph), the exhaust control switch closes contact with ground. With circuit M101 grounded, the exhaust solenoid is energized allowing the exhaust valve to open. The air from the shock absorbers vents into the atmosphere, lowering the rear of the vehicle. When the vehicle body reaches its original height (+25 mm [+1 inch]), the sensor opens the exhaust control switch, de-energizing the solenoid and closing the valve.