Backup Lamps Malfunction
Backup Lamps Malfunction
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
When the PRNDL is placed in the REVERSE position, a signal is sent to the body control module (BCM). The BCM then applies voltage through the backup lamps relay control circuit to the BACK/UP lamp relay in the underhood fuse block, and the inside rearview mirror. The BACK/UP lamp relay then illuminates the backup lamps.
Circuit/System Verification
1. Connect a test lamp between the relay side of the backup lamp fuse and ground.
2. Ignition ON, verify that the test lamp turns on and off while commanding the backup lamps on and off with a scan tool.
^ If the test lamp does not turn on and off while changing between the commanded states, refer to the Backup Lamp Relay Control Circuit Test.
3. If 1 or both of the backup lamps do not turn on and off, refer to the Backup Lamp Circuit Test.
Circuit/System Testing
Backup Lamp Relay Control Circuit Test
1. Ignition OFF, disconnect the harness connector X2 at the BCM.
2. Ignition ON, vehicle in park, verify the backup lamps are not on.
^ If the backup lamps are on, test the backup lamp relay control circuit terminal 22 for a short to voltage. If the circuit tests normal replace the underhood fuse block.
3. Install a 10A fused jumper wire between the relay control circuit terminal 22 and battery voltage. Verify the backup lamps are on.
^ If the backup lamps are not on, test the backup lamp relay control circuit for an open/high resistance or short to gerund. If the circuit tests normal replace the underhood fuse block.
4. If all circuits test normal, replace the BCM.
Backup Lamp Circuit Test
1. Ignition OFF, disconnect the harness connector at the affected backup lamp.
2. Test for less than 1 ohm between the listed ground circuit terminal and ground.
* Cadillac SUV terminal B
* Cadillac EXT and Chevy terminal GRD
* GMC terminal 1
^ If greater than the specified range, test the ground circuit for an open/high resistance.
3. Connect a test lamp between the listed control circuit terminal and ground circuit terminal.
* Cadillac SUV terminal A to B
* Cadillac EXT and Chevy terminal MJR to GRD
* GMC terminal 3 to 1
4. 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 control circuit for a short to voltage. If the circuit tests normal, replace the underhood fuse block.
^ If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the underhood fuse block.
5. If all circuits test normal, test or replace the affected backup lamp.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests
* Tail Lamp Bulb Replacement (Cadillac) Tail Lamp Bulb Replacement (Yukon) Tail Lamp Bulb Replacement (Tahoe, Avalanche) Tail Lamp Bulb Replacement (GMC HP2)
* Underhood Electrical Center or Junction Block Replacement