The Automatic Level Control (ALC) is integrated with the electronic suspension control module (ESCM). Inputs from the suspension position sensors and air pressure sensor is used determine when to raise the rear of the vehicle by grounding the relay and switching 12 volts to the compressor motor to pump air to the rear shocks.

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 compressor relay ON and OFF with a scan tool. Listen for the compressor running or relay clicks.

If the compressor relay clicks go to Automatic Level Control (ALC) Air Compressor Assembly circuit testing. If the compressor relay does not click or the compressor does not run go to compressor relay circuit testing.

1. Ignition OFF, disconnect the harness connector at the compressor relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 86 and ground.

⇒	If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the ignition circuit terminal 85 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, test or replace the compressor relay.

4. Verify that a test lamp illuminates between the ignition circuit terminal 30 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, test the control circuit at terminal 87 for a short to ground.

5. Disconnect the harness connector at the air compressor.
6. Test for less than 1 ohm of resistance between the air compressor ground circuit terminal G and ground.

⇒	If greater than 1 ohm, test the ground circuit for an open/high resistance.

7. Connect the harness connector at the air compressor.
8. Connect a 40-A used jumper wire between the ignition circuit terminal 30 and the control circuit terminal 87. Verify the air compressor is activated.

⇒	If the air compressor does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the air compressor.

9. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
10. Command the compressor relay 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 ESC module.

⇒	If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ESC module.

11. If all circuits test normal, test or replace the compressor relay.

1. Ignition OFF, disconnect the harness connector at the air 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, 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.

4. 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.

5. If all circuits test normal, replace the air pressure sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.