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
When the transmission is placed in the REVERSE position, the engine control module (ECM) sends a GMLAN serial data message to the body control module (BCM). The message indicates that the gear selector is in the REVERSE position. The BCM sends battery voltage to the backup lamps and the inside rearview mirror. The backup lamps are permanently grounded at G401 for the left and G402 for the right or G305 depending on the model. Once the driver moves the gear selector out of the REVERSE position, a message is sent by the TCM via serial data requesting the BCM to remove battery voltage from the backup lamp control circuit. Reference Information
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) for scan tool information
Circuit/System Verification
Ignition ON, command the backup lamps ON and OFF with a scan tool. The backup lamps should turn ON and OFF between the commanded states.
If the lamps do not turn ON and OFF between the commanded states, refer to Circuit System Testing.
Circuit/System Testing
1. Ignition OFF, disconnect the harness connector at the inoperative backup lamp.
2. Test for less than 5.0 ohms between the listed ground circuit terminal and ground.
* Backup Lamp terminal E - Chevy, Isuzu
* Backup Lamp terminal G - GMC
• 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.
* Backup Lamp terminal C - Chevy, Isuzu
* Backup Lamp terminal A - GMC
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 BCM.
• 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 BCM.
5. If all circuits test normal, test or replace the inoperative backup lamps.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Backup Lamp Replacement (GMC) (Service and Repair)
* Control Module References (Programming and Relearning) for BCM replacement, programming, and setup.