Backup Lamps Always On (Automatic Transmission)

BACKUP LAMPS ALWAYS ON (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 ON, verify that the vehicle transmission is in the PARK position.
2. Install a scan tool and observe the Backup Lamp Relay Cmd. parameter in the BCM lighting data list. The scan tool should indicate the Backup Lamp Relay Cmd. parameter is OFF.
- If the scan tool displays ON, replace the BCM.
3. Ignition OFF, disconnect the backup lamp relay.
4. Ignition ON, observe the backup lamps. The backup lamps should be OFF.
- If the backup lamps remain illuminated, repair the backup lamp voltage supply circuit for a short to battery voltage.
5. Ignition OFF, disconnect the C2 harness connector at the underhood fuse block.
6. Verify that a test lamp does not illuminate between battery voltage and the reverse lamp relay control circuit at the underhood fuse block C2 harness connector.
- If the test lamp illuminates, repair the reverse lamp relay control circuit for a short to ground. If the circuit tests normal, replace the BCM.
7. 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)