The Automatic Level Control (ALC) System uses an exhaust valve to release air from the rear shocks to lower to trim height, or during a compressor head pressure relief cycle. Battery voltage is supplied to the valve. The electronic suspension control module (ESCM) controls the ground to activate the valve and monitors the voltage level to ensure it agrees with its command state.

Inspect for:

Schematic Reference

Automatic Level Control Schematics

Connector End View Reference

Automatic Level Control Connector End Views

Description and Operation

Automatic Level Control Description and Operation

Electrical Information Reference

Scan Tool Reference

Ignition ON, command the exhaust valve solenoid ON and OFF with a scan tool. Listen for the exhaust valve clicking.

1. Ignition OFF, disconnect the harness connector at the ALC compressor.
2. Ignition OFF, test for less than 1  ohm of resistance between the low reference circuit terminal F and ground.

⇒	If greater than 1 ohm, test the low reference circuit for a short to voltage or an open/high resistance. If circuit tests normal, replace the ESC module.

3. Ignition ON, verify that a test lamp illuminates between Ignition circuit terminal B and ground.

⇒	If the test lamp does not illuminate, test the Ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the Ignition circuit fuse is open, replace the air compressor.

4. Connect a test lamp between the control circuit terminal C and the Ignition circuit terminal B.
5. Command the exhaust valve solenoid ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ESCM.

⇒	If the test lamp is always OFF, test the control circuit for an open/high resistance. If the circuit tests normal, replace the ESCM.

6. Ignition ON, test for 4.8-5.2  volts between the 5-volt reference circuit terminal H and ground.

⇒	If less than 4.8  volts, test the 5-volt reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ESC module.

⇒	If greater than 5.2  volts, test the 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the ESC module.

7. Test for 4.8-5.2  volts between the signal circuit terminal A and ground.

⇒	If less than 4.8  volts, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ESC module.

⇒	If greater than 5.2  volts, test the signal circuit for a short to voltage. If the circuit tests normal, replace the ESC module.

8. If all circuits test normal, replace the automatic level control air compressor assembly.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.