Stop Lamps Malfunction

Stop 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.

Circuit/System Description

The STOP fuse in the engine wiring harness junction block supplies battery positive voltage to the normally open stop lamp switch. When the driver presses the brake pedal, the switch contacts close and battery positive voltage is supplied to both the VEHICLE STOP fuse and the VEHICLE CHMSL fuse. The current flow is now to the stop lamps which are grounded at G401 and G402. The center high mounted stop lamp (CHMSL) is grounded at G401.

Circuit/System Verification

Press and release the brake pedal while observing brake lamp operation.

If all of the brake lamps are always on or inoperative, refer to the stop lamp switch circuit test.

If 1 or 2 of the brake lamps function normally, refer to the stop lamp circuit test.

Circuit/System Testing

Stop Lamp Switch Circuit Test

1. Ignition OFF, disconnect the harness connector at the stop lamp switch.
2. Verify that a test lap illuminates between the battery supply circuit terminal B and ground.

^ If the test lamp does not illuminate, test the battery supply circuit for a short to ground or an open/high resistance. If the circuit tests normal and the battery supply circuit fuse is open, test the stop lamp control circuit terminal A for a short to ground.

3. Verify that the stop lamps are not illuminated.

^ If any of the stop lamps are illuminated, test the stop lamp control circuit for a short to voltage.

4. Install a 25A fused jumper wire between the stop lamp control circuit terminal A and the battery supply circuit terminal B. Verify the stop lamps are illuminated.

^ If the stop lamps are not illuminated test the stop lamp control circuit for an open/high resistance.

5. If all circuits test normal replace the stop lamp switch.

Stop Lamp Circuit Test

1. Ignition OFF, disconnect the harness connector at the affected stop lamp.
2. Test for less than 1 ohm of resistance between the listed ground circuit terminal and ground.

* Center high mounted stop lamp terminal B
* Tail lamp terminal E

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

3. Connect a test lamp between the listed stop lamp control circuit terminal and ground circuit terminal.

* Center high mounted stop lamp terminal A to B
* Tail lamp Chevy, Isuzu terminal D to E
* Tail lamp GMC terminal A to E

4. Press and release the brake pedal. 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 test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance.

5. If all circuits test normal, test or replace the affected stop lamp.

Repair Instructions

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

* Tail Lamp Bulb Replacement
* High Mount Stop Lamp Replacement (Envoy, TrailBlazer, Rainier)
* Stop Lamp Switch Replacement