The Automatic Level Control (ALC) System uses the electronic suspension control module (ESCM), level sensors, compressor, and exhaust valve to increase or decrease air pressure in the system to control trim height. Battery voltage is supplied to the exhaust valve and compressor relay. The ESCM controls the ground circuits that activate the exhaust valve and compressor relay which maintain trim height based on the level sensor input voltage.

Inspect for the following conditions:

Schematic Reference

Automatic Level Control Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Automatic Level Control Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for Scan Tool Information

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 ALC relay.
2. Ignition ON, verify that a test lamp does not illuminate between the relay connector 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 relay connector B+ circuit terminal 85 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ESC/ALC EXH fuse is open, test the relay coil B+ circuit for a short to ground. If all circuits test normal, test or replace the ELC relay.

4. Verify that a test lamp illuminates between the relay connector B+ circuit terminal 30 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ALC/COMP fuse is open, test the compressor control circuit for a short to ground. If all circuits test normal, test or replace the ALC compressor.

5. Disconnect the harness connector at the air compressor.
6. Test for less than 2 ohms between the ALC compressor ground circuit terminal G and ground.

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

7. Connect the harness connector at the ALC compressor.
8. Connect a 40A fused jumper wire between the relay connector B+ circuit terminal 30 and the control circuit terminal 87. Verify the ALC compressor is activated.

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

9. Connect a test lamp between the relay connector B+ circuit terminal 85 and the control circuit terminal 86.
10. Command the ALC 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 ESCM.

⇒	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 ESCM.

11. Test the ELC relay.

⇒	If the ELC relay fails the component test, replace the relay.

12. Ignition OFF, disconnect the harness connector at the ALC compressor.
13. Ignition ON, verify that a test lamp illuminates between the ALC compressor connector B+ circuit terminal B and ground.

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

14. Connect a test lamp between the ALC compressor connector control circuit terminal C and the B+ circuit terminal B.
15. 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.

16. If all circuits test normal, test or replace the ALC compressor.

Relay Test

1. Ignition OFF, disconnect the relay.
2. Test for 50-200 ohms between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 20 amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2.0 ohms between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.