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Backup Lamps Malfunction



Backup Lamps Malfunction

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 underhood fuse block supplies battery voltage through the STOP/B/U/ LPS fuse to the coil and switch terminals of the REVERSE LAMP Relay located in the body control module (BCM). When the transmission is placed in the REVERSE position, the transmission control module (TCM) sends a class 2 serial data message to the BCM. The message indicates that the gear selector is in the REVERSE position. The BCM grounds the back up lamp relay control circuit of the reverse lamp relay . This energizes the reverse lamp relay, closing the switch side contacts and provides battery voltage to the REVERSE LAMPS fuse through to the backup lamps. The backup lamps are permanently grounded at G301. With both power and ground the backup lamps illuminate. Once the driver moves the gear selector out of the REVERSE position, a message is sent by the TCM via class 2 serial data requesting the BCM to remove ground from the back up lamp relay control circuit. This opens the REVERSE LAMP Relay switch contacts turning OFF the backup lamps.


Reference Information
Schematic Reference

Exterior Lights Schematics (Exterior Lights Schematics)
Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Exterior Lighting Systems Description and Operation (Exterior Lighting Systems 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)


Circuit/System Testing

1. Ignition OFF, disconnect the reverse lamp relay.
2. Ignition ON, verify that a test lamp does not illuminate between the relay controlled output circuit terminal R8 and ground.

• If the test lamp illuminates, test the relay controlled output circuit for a short to voltage.

3. Verify that a test lamp illuminates between the relay coil B+ circuit terminal S8 and ground.

• If the test lamp does not illuminate, test the relay coil 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 relay controlled output circuit for a short to ground. If the circuit tests normal, test or replace the reverse lamp relay.

4. Verify that a test lamp illuminates between the relay switch B+ circuit terminal S9 and ground.

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

5. Disconnect the harness connector at the inoperative backup lamp.
6. Ignition OFF, exterior lamps OFF, test for less than 5 ohms of resistance between the inoperative backup lamp ground circuit terminal G and ground.

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

7. Connect the harness connector at the inoperative backup lamp.
8. Connect a 15A fused jumper wire between the relay switch B+ circuit terminal S9 and the relay controlled output circuit terminal R8. Verify the backup lamps are activated.

• If the backup lamps do not activate, test the relay controlled output circuit for an open/high resistance. If the circuit tests normal, test or replace the inoperative backup lamp.

9. Connect a test lamp between the relay coil B+ circuit terminal S8 and the relay coil control circuit terminal R9.
10. Command the backup lamps 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 relay coil control circuit for a short to ground. If the circuit tests normal, replace the BCM.


• If the test lamp is always OFF, test the relay coil control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

11. If all circuits test normal, test or replace the reverse lamp 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))
* Backup Lamp Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, setup, and programming


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