Rear of Vehicle High

Rear of Vehicle High

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.



Circuit/System Description

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.


Diagnostic Fault Information






Diagnostic Aids

Performing the Suspension Position Calibration (Programming and Relearning) procedure may correct the condition.


Reference Information
Schematic Reference

Automatic Level Control Schematics (Automatic Level Control)
Connector End View Reference

Component Connector End Views
Description and Operation

Automatic Level Control Description and Operation (Automatic Level Control Description and Operation)
Electrical Information Reference

* Circuit Testing
* Connector Repairs
* Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs

Scan Tool Reference

Control Module References for Scan Tool Information


Circuit/System Verification

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


Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the ALC compressor.
2. 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 the exhaust valve control circuit for a short to ground. If all circuits test normal, test or replace the ALC compressor.

3. Connect a test lamp between the ALC compressor connector control circuit terminal C and the B+ circuit terminal B.
4. 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.

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



Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

* Air Compressor Replacement (Service and Repair)
* Suspension Position Calibration (Programming and Relearning)
* Control Module References for electronic suspension control module (ESCM) replacement, programming and setup