High Mount Stop Lamp Malfunction

High Mount Stop Lamp 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

Ground is applied at all times to the CHMSL relay coil. Upon brake activation, the body control module (BCM) applies voltage to the CHMSL relay control circuit to the coil side of the relay, energizing the relay. Battery voltage is then applied through the switch side of the relay, the CHMSL fuse, and the CHMSL control circuit to the center high mounted stop lamp (CHMSL). The CHMSL is grounded at G 200.

Circuit/System Testing

1. Ignition OFF, disconnect the X5 harness connector at the underhood fuse block.
2. Connect a test lamp between the CHMSL relay control circuit terminal K6 and ground at the harness connector.
3. Apply the brake pedal ON and OFF. 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.

4. Ignition OFF, reconnect the X5 harness connector to the underhood fuse block. Disconnect the harness connector at the center high mounted stop lamp (CHMSL).
5. Test for less than 1.0 ohm of resistance between the ground circuit terminal B and ground.

• If greater than the specified range, test the ground circuit for and open/high resistance.

6. Connect a test lamp between the CHMSL control circuit terminal A and ground.
7. Apply the brake pedal ON and OFF. 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 an short to ground. If the circuit tests normal, replace the CHMSL.

8. If all circuits test normal, test or replace the CHMSL.

Component Testing

Center High Mounted Stop Lamp (CHMSL)

1. Ignition OFF, disconnect the harness connector at the CHMSL.
2. Install a 15A fused jumper wire between the control circuit terminal A and 12 volts. Install a jumper wire between the ground circuit terminal B and ground. Verify that the CHMSL illuminates.

• If the CHMSL does not completely illuminate, test or replace the inoperative bulb or CHMSL.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

* High Mount Stop Lamp Replacement (Extended and Crew Cab) High Mount Stop Lamp Replacement (Regular Cab)
* High Mount Stop Lamp Bulb Replacement (GMC Denali)
* Underhood Electrical Center or Junction Block Replacement
* Control Module References for BCM replacement, setup and programming Programming and Relearning