The Automatic Level Control (ALC) system uses a compressor relay to switch 12 volts to the compressor motor. The electronic suspension control module (ESCM) receives inputs from the suspension position sensors and air pressure sensor to determine when to raise the rear of the vehicle by grounding the relay coil circuit, and monitoring to ensure it agrees with the command.

C0712 01

C0712 06

C0712 01

Upon detection of the compressor relay control circuit is shorted to battery.

C0712 06

Upon detection of the compressor relay control circuit shorted to ground, or open.

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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.

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. Install any components or connectors that have been removed or replaced during diagnosis.
2. Perform any adjustment, programming or setup procedures that are required when a component or module is removed or replaced.
3. Clear the DTCs.
4. Use the scan tool Diagnostic Counter Reset output control function to bypass the 3 consecutive ignition cycle requirement for setting a DTC. This will ensure that if a DTC was to be reset during the repair verification process, it will be displayed on the scan tool.
5. Turn OFF the ignition for 60 seconds.
6. Turn ON the ignition.
7. If the repair was related to a DTC, duplicate the Conditions for Running the DTC and use the Freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to the Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.