Rear of Vehicle High (W/O F55)

Rear of Vehicle High (W/O F55)

Diagnostic Instructions

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

Diagnostic Fault Information





Circuit/System Description

The automatic level control (ALC) system uses a combination level sensor/module, 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 ALC module controls the ground circuits that activate the exhaust valve and compressor relay which maintain trim height based on the level sensor input voltage.

Diagnostic Aids

Performing the Suspension Position Calibration (Suspension Position Calibration - Trimset) procedure may correct the condition.

Reference Information
Schematic Reference

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

Component Connector End Views (Connector Views)
Description and Operation

Automatic Level Control Description and Operation (With F55) (Automatic Level Control Description and Operation (With F55))Automatic Level Control Description and Operation (W/O F55) (Automatic Level Control Description and Operation (W/O F55))
Electrical Information Reference

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

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

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

Circuit/System Testing

Important: Whenever repairing and/or replacing ALC system components, perform the Suspension Position Calibration procedure.


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

• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

3. Connect a test lamp between the control circuit terminal A and the B+ circuit terminal C.
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 ALC sensor/module.
• If the test lamp is always OFF, test the control circuit for an open/high resistance. If the circuit tests normal, replace the ALC sensor/module.

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

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Air Compressor Replacement (Air Compressor Replacement)
* Suspension Position Calibration (Suspension Position Calibration - Trimset)
* Control Module References (Programming and Relearning) for automatic level control sensor/module replacement, setup, and programming