Backup Lamps Inoperative (Automatic Transmission)

BACKUP LAMPS INOPERATIVE (AUTOMATIC TRANSMISSION)




DIAGNOSTIC FAULT INFORMATION

IMPORTANT: Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Testing and Inspection

CIRCUIT/SYSTEM DESCRIPTION
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.

CIRCUIT/SYSTEM TESTING
1. Ignition OFF, disconnect the backup lamps relay.
2. Verify that a test lamp illuminates between the backup lamps relay control side voltage supply circuit and ground.
- If the test lamp does not illuminate, repair the reverse lamp relay control side voltage supply circuit for an open/high resistance. If the circuit tests normal, replace the underhood fuse block.
3. Connect a test lamp between the backup lamps relay control side voltage supply circuit and the backup lamp relay control circuit at the backup lamps relay terminals.
4. Ignition ON, apply the brake pedal and place the transmission in the REVERSE position. The test lamp should illuminate as the transmission is placed in REVERSE.
- If the test lamp does not illuminate, repair the reverse lamp relay control circuit for an open/high resistance. If the circuit tests normal, replace the BCM.
5. Ignition OFF, connect a 10-amp fused jumper between the backup lamps relay switch side voltage supply circuit and the backup lamps voltage supply circuit at the backup lamps relay terminals. The backup lamps should illuminate.
- If the backup lamps do not illuminate, repair the backup lamp voltage supply circuit for a short to ground or an open/high resistance.
- If the backup lamp voltage supply circuit tests normal, repair the backup lamps ground circuit for a short to voltage or an open/high resistance.
6. If all circuits test normal, test or replace the backup lamp relay.

REPAIR INSTRUCTIONS

IMPORTANT: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

- Control Module References for BCM replacement, setup, and programming
- Underhood Electrical Center or Junction Block Replacement
- Relay Replacement (Within an Electrical Center) Relay Replacement (Attached to Wire Harness)
- Backup Lamp Bulb Replacement