Automatic Level Control

Automatic Level Control Description and Operation

The automatic level control (ALC) system maintains a desired rear suspension position under all types of towing, hauling, and loading conditions.

The system uses the electronic suspension control module (ESCM), ELC relay, ALC compressor assembly, air dryer, air lines, shock absorbers with integral air chambers, suspension position sensors, exhaust valve, driver information center (DIC), and the serial data circuit to perform the system functions.

When the vehicle is unloaded, the rear suspension is at a desired position, which is monitored by the ESCM using the position sensor signal voltage inputs. As weight is added to the rear of the vehicle, the position sensor's signal voltage inputs change. When the ESCM detects a steady substantial change in the position sensor signal voltages for at least 10 seconds, it responds by commanding ON the ELC relay, which activates the compressor causing the air pressure in the shock absorber chambers to inflate. This inflation raises the rear of the vehicle to regain the desired suspension position. When weight is removed from the vehicle, the ESCM responds by commanding ON the exhaust valve causing the air pressure in the shock absorber chambers to deflate. This deflation lowers the vehicle to regain the desired suspension position.

The compressor is a positive displacement piston air pump driven by a 12-volt DC permanent magnet motor. The compressor draws filtered air through an intake line attached to an under body rail. The compressed air is then run through a dryer containing a moisture-absorbing chemical that dries the compressed air before it is sent to the shock absorber chambers. Each time the compressor is activated, the ESCM commands ON the exhaust valve for 1.5 seconds to release air from the compression chamber in the compressor's cylinder head ensuring low motor current draw upon compressor activation. The ESCM also limits the amount of compressor run time to 255 seconds to protect the compressor components from thermal damage.

The system also performs a self test on each ignition ON cycle in which the ESCM commands the ELC relay ON, activating the compressor for 4 seconds then monitors the position sensor's signal voltage inputs to verify the compressor is functioning and the system is holding air pressure.

The ESCM has the ability to detect malfunctions within the ALC system. Any malfunctions detected will cause the DIC to display the SERVICE SUSPENSION SYSTEM message. For more information on other functions of the ESCM, refer to Electronic Suspension Control Description and Operation Electronic Suspension Control.