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Rear of Vehicle Low



Rear of Vehicle Low

Diagnostic Instructions

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



Diagnostic Fault Information






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 Aids

Inspect for:

* Leaking lines or components. Perform the Automatic Level Control System Leak Test (Automatic Level Control System Leak Test) and Air Compressor Leak Test (Air Compressor Leak Test) as necessary.
* 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 (Connector Views)
Description and Operation

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

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

Scan Tool Reference

Control Module References (Programming and Relearning) for Scan Tool Information


Circuit/System Verification

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


Circuit/System Testing

1. Ignition OFF, disconnect the ELC relay from the rear fuse block.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal R8 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 B+ circuit terminal R7 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 B+ circuit fuse is open, test the control circuit terminal R8 for a short to ground or faulty ALC compressor motor. If all circuits test normal, test or replace the ELC relay.

4. Verify that a test lamp illuminates between the relay B+ circuit terminal R6 and ground.

• If the test lamp does not illuminate, test the B+ circuit for an open/high resistance.

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

• If greater than 1 ohm, 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 B+ circuit terminal R7 and the control circuit terminal R8. 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 B+ circuit terminal R6 and the control circuit terminal R10.
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 C 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 ELC fuse is open, test or replace the ALC compressor.

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



Component Testing
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.




Repair Instructions

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

* Relay Replacement (Attached to Wire Harness) (Relay Replacement (Attached to Wire Harness))Relay Replacement (Within an Electrical Center) (Relay Replacement (Within an Electrical Center))
* Air Compressor Replacement (Air Compressor Replacement)
* Suspension Position Calibration (Programming and Relearning)
* Control Module References (Programming and Relearning) for electronic suspension control module (ESCM) replacement, programming and setup


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