Backup Lamps Malfunction (Manual Transmission)
Backup Lamps Malfunction (Manual Transmission)
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
When the transmission is placed in the REVERSE position, the backup lamp switch contacts close supplying ground to the engine control module (ECM) via the backup lamp switch signal circuit. The ECM responds by sending a serial data message to the body control module (BCM) indicating the gear selector is in the REVERSE position. The BCM applies voltage to the backup lamp relay control circuit energizing the backup lamp relay coil. With the relay coil energized, the switch contacts close allowing battery voltage to flow through the VEH BCK/UP fuse to the left and right backup lamps. The BCM also supplies voltage the to inside rearview mirror via the backup lamp relay control circuit for automatic day/night operation. Once the driver moves the gear selector out of the REVERSE position, the backup lamp switch contacts open and a serial data message is sent by the ECM requesting the BCM to remove voltage from the backup lamp relay control circuit.
Reference Information
Schematic Reference
Exterior Lights Schematics Exterior Lights Schematics
Connector End View Reference
Component Connector End Views 1-2 Shift Solenoid (SS) Valve (M30)
Description and Operation
Exterior Lighting Systems Description and Operation Exterior Lighting Systems
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
Scan Tool Reference
Control Module References Control Module References for scan tool information
Circuit/System Verification
1. Ignition ON, engine running, observe the Backup Lamps parameter while shifting the vehicle between the park and reverse gears. The reading should change between ON and OFF when changing between the selected states.
• If the reading does not change between the specified values, refer to Backup Lamp Switch Malfunction.
2. Command the Backup Lamps ON and OFF with a scan tool. The backup lamps should turn ON and OFF when changing between the commanded states.
• If the backup lamps do not turn ON and OFF when changing between the commanded states, refer to Backup Lamps Malfunction.
Circuit/System Testing
Backup Lamp Switch Malfunction
1. Ignition OFF, disconnect the harness connector at the backup lamp switch.
2. Ignition OFF, exterior lamps OFF, test for less that 5.0 ohms between the ground circuit terminal A and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
3. Ignition ON, verify the backup lamps are not illuminated.
• If the backup lamps are illuminated, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the ECM.
4. Install a 3 A fused jumper wire between the signal circuit terminal B and ground. Verify the backup lamps are illuminated.
• If the backup lamps do not illuminate, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
5. If all circuits test normal, replace the backup lamp switch.
Backup Lamps Malfunction
1. Ignition OFF, disconnect the X2 harness connector at the underhood fuse block.
2. Ignition OFF, exterior lamps OFF, test for less that 5.0 ohms between the ground circuit terminal N5 and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
3. Connect the X2 harness connector at the underhood fuse block. Disconnect the X5 harness connector at the underhood fuse block.
4. Connect a test lamp between the control circuit terminal H8 and ground.
5. 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.
6. Ignition OFF, connect the X5 harness connector at the underhood fuse block. Disconnect the harness connector at the appropriate backup lamp.
7. Ignition OFF, exterior lamps OFF, test for less than 5.0 ohms between the ground circuit terminal GRD and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
8. Connect a test lamp between the control circuit terminal MJR and ground.
9. 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 and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, replace the underhood fuse block.
• If the test lamp is always OFF and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests normal, replace the underhood fuse block.
10. If all circuits test normal, replace the backup lamp.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
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* Backup Lamp Bulb Replacement (Chevrolet) Service and RepairBackup Lamp Bulb Replacement (GMC) Service and Repair
* Underhood Electrical Center or Junction Block Replacement Underhood Electrical Center or Junction Block Replacement
* Backup Lamp Switch Replacement Service and Repair
* Control Module References Control Module References for BCM or ECM replacement, setup, and programming