GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • 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

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

TBC Fuse B+

1

1

--

--

Backup Lamp Control

B2550 00

B2550 00

B2550 00

--

Left Backup Lamp Ground

--

1

--

--

Right Backup Lamp Ground

--

1

--

--

1. Backup Lamp Malfunction

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

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References 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.
  3. • 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.
  4. Connect a test lamp between the listed control circuit terminal and ground.
  5. • Backup Lamp terminal C - Chevy, Isuzu
    • Backup Lamp terminal A - GMC
  6. 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.
  7. 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.
  8. If all circuits test normal, test or replace the inoperative backup lamps.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Backup Lamp Replacement
    •  Control Module References for BCM replacement, programming, and setup.