Stop Lamps Malfunction

Stop Lamps Malfunction

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.

Circuit/System Description

When the driver presses the brake pedal, the switch contacts open and B+ is interrupted to the body control module (BCM). The BCM then supplies voltage to both the turn/stop lamp supply voltage circuit and trailer rear turn/stop lamp supply voltage circuits. The BCM also supplies a control voltage to the center high mount stop lamp (CHMSL) relay energizing the relay and illuminating the CHMSL.

Reference Information
Schematic Reference

Exterior Lights Schematics (Exterior Lights Schematics)
Connector End View Reference

Component Connector End Views (Connector End Views By Name)
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, observe the scan tool Stop Lamp Relay Command parameter while pressing and releasing the brake pedal. The parameter should change between ON and OFF.

• If the parameter does not change between the commanded states, refer to the Stop Lamp Switch Malfunction.

2. Observe the left and right stop lamps while pressing and releasing the brake pedal. The stop lamps should turn ON and OFF.

• If the left or right stop lamps do not change between the selected states, refer to the Stop Lamp Malfunction.

3. Observe the center high mount stop lamp while pressing and releasing the brake pedal. The center high mount stop lamp should turn ON and OFF.

• If the center high mount stop lamp does not change between the selected states, refer to Center High Mount Stop Lamp Malfunction.

Circuit/System Testing

Stop Lamp Switch Malfunction
1. Ignition OFF, disconnect the harness connector at the stop lamp switch.
2. Ignition ON, test for B+ between the 12 V reference circuit terminal C and ground.

• If less than the specified range, test the 12 V reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

3. Verify the scan tool BCM Stop Lamp Switch Status parameter is approximately 1023 counts.

• If not the specified value, test the signal circuit terminal D for a short to voltage. If the circuit tests normal, replace the BCM.

4. Install a 3 A fused jumper wire between the signal circuit terminal D and the 12 V reference circuit terminal C. Verify the scan tool Stop Lamp Switch Status parameter is 1 count.

• If not the specified value, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the BCM.

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

Stop Lamp Malfunction
1. Ignition OFF, disconnect the harness connector X4 at the BCM.
2. Ignition ON, verify a test lamp illuminates between the terminals listed below and ground.

* Terminal 1
* Terminal 2
* Terminal 4

• If the test lamp does not illuminate, test the B+ circuit for an open/high resistance or short to ground.

3. Connect the harness connector X4 at the BCM.
4. Ignition OFF, disconnect the harness connector at the appropriate stop lamp.
5. Test for less than 1 ohm between the ground circuit terminal GRD and ground.

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

6. Connect a test lamp between the stop lamp control circuit terminal MJR and ground.
7. Ignition ON, press and release the brake pedal. The test lamp should turn ON and OFF when changing between the selected 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.

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

Center High Mount Stop Lamp Malfunction
1. Ignition OFF, disconnect the X5 harness connector at the fuse block - underhood.
2. Connect a test lamp between the control circuit terminal J4 and B+.
3. Turn the center high mounted stop lamp ON and OFF by pressing and releasing the brake pedal. The test lamp should turn ON and OFF when changing between the selected states.

• If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the BCM.
• If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. Ignition OFF, connect the X5 harness connector at the fuse block - underhood. Disconnect the harness connector at the center high mount stop lamp.
5. Test for less than 5 ohm between the center high mount stop lamp ground circuit terminal B and ground

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

6. Connect a test lamp between the control circuit terminal A and ground.
7. Turn the center high mounted stop lamp ON and OFF by pressing and releasing 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 circuit tests normal, replace the fuse block - underhood.
• 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, test or replace the fuse block - underhood.
• 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, test or replace the center high mount stop lamp.

8. If all circuits test normal, test or replace the center high mount stop lamp.

Repair Instructions

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

* Stop Lamp Switch Replacement (With Adjustable Pedals) (Stop Lamp Switch Replacement (With Adjustable Pedals))Stop Lamp Switch Replacement (Without Adjustable Pedals) (Stop Lamp Switch Replacement (Without Adjustable Pedals))
* High Mount Stop Lamp Replacement (Extended and Crew Cab) (High Mount Stop Lamp Replacement (Extended and Crew Cab))High Mount Stop Lamp Replacement (Regular Cab) (High Mount Stop Lamp Replacement (Regular Cab))
* Tail Lamp Bulb Replacement (Chevrolet) (Tail Lamp Bulb Replacement (Chevrolet))Tail Lamp Bulb Replacement (GMC) (Tail Lamp Bulb Replacement (GMC))
* Tail Lamp Replacement (Chevrolet) (Tail Lamp Replacement (Chevrolet))Tail Lamp Replacement (GMC) (Tail Lamp Replacement (GMC))Tail Lamp Replacement (HD) (Tail Lamp Replacement (HD))
* Control Module References (Control Module References) for BCM replacement, programming, and setup.