Backup Lamps Malfunction Automatic Transmission

The underhood fuse block supplies battery positive voltage to the coil and switch terminals of the backup lamps relay. When the transmission is placed in the REVERSE position, the transmission control module (TCM) sends a message via GMLAN to the body control module (BCM). The message indicates that the gear selector is in the REVERSE position. The BCM grounds the backup lamp relay control circuit which energizes the backup lamps relay. With the relay energized, the switch side contacts close and provide battery voltage to the backup lamps fuse through to the backup lamps. The backup lamps are permanently grounded at G402. 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 GMLAN serial data requesting the BCM to remove ground from the backup lamp relay control circuit. This opens the backup lamps relay switch contacts turning OFF the backup lamps.

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Lighting Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

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

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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 backup lamp relay.

4. Verify that a test lamp illuminates between the relay switch B+ circuit 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. Test for less than 1.0 ohm 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 10A fused jumper wire between the relay switch B+ circuit and the relay controlled output circuit. 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 and the relay coil control circuit.
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 backup lamp relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Backup Lamps Malfunction Manual Transmission

Battery voltage is applied to the backup lamp switch when the ignition switch is in the RUN or CRANK positions. When the transmission is placed in the REVERSE position, the backup lamp switch provides battery voltage to the backup lamps. The backup lamps are permanently grounded at G402. With both power and ground the backup lamps illuminate. Once the driver moves the gear selector out of the REVERSE position, the backup lamp switch contacts open and the backup lamps turn OFF.

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Lighting Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

1. Disconnect the harness connector at the inoperative backup lamp.
2. Test for less than 1.0 ohm 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.

3. Connect the harness connector at the inoperative backup lamp.
4. Ignition OFF, disconnect the backup lamp switch.
5. Ignition ON, verify that a test lamp illuminates between the backup lamp switch voltage supply circuit terminal A and ground.

⇒	If the test lamp does not illuminate, repair the backup lamp switch voltage supply circuit for a short to ground or an open/high resistance.

6. Verify that a test lamp does not illuminate between the backup lamp voltage supply circuit terminal B and ground.

⇒	If the test lamp illuminates, repair the backup lamp voltage supply circuit for a short to voltage.

7. Connect a 10-amp fused jumper between the backup lamp switch voltage supply circuit terminal A and the backup lamp voltage supply circuit terminal B. Verify the backup lamps are activated.

⇒	If the backup lamps do not activate, repair the voltage supply circuit terminal B for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the inoperative backup lamp.

8. If all circuits test normal, test or replace the backup lamp switch.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.