GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2008 🢒 Sierra - 2WD 🢒 Body Systems 🢒 Lighting 🢒 Stop Lamps Malfunction

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.

Circuit/System Description

When the driver presses the brake pedal, the switch contacts open and B+ is interrupted to the 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 CHMSL relay energizing the relay and illuminating the CHMSL.

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
  1. Ignition ON, observe the scan tool BCM Stop Lamp Switch Status parameter while pressing and releasing the brake pedal. The reading should change between 1 count and approximately 1023 counts.
    2. ⇒If not the specified value, refer to the Stop Lamp Switch Circuit Test.
  2. Ignition ON, observe the scan tool TCM Brake Switch parameter while pressing and releasing the brake pedal. The reading should change between Pressed and Released.
    3. ⇒If not the specified value, refer to the Brake Switch Circuit Test.
  3. Ignition ON, observe the left, and right stop lamps while pressing and releasing the brake pedal. The stop lamps should turn on and off.
    4. ⇒If the left and or right stop lamps are always on or inoperative, refer to the Stop Lamp Circuit Test.
  4. Connect a test lamp between the relay side of the CHMSL fuse and ground.
  5. Verify that the test lamp turns on and off while pressing and releasing the brake pedal.
    6. ⇒If the test lamp does not turn on and off while changing between the commanded states, refer to the CHMSL Relay Control Circuit Test.
  6. If the center high mount stop lamp is always on or inoperative refer to the CHMSL Circuit Test.

Circuit/System Testing

Stop Lamp Switch Circuit Test

  1. Ignition OFF, disconnect the harness connector at the stop lamp switch.
  2. Ignition ON, test for B+ between the 12-volt reference circuit terminal C and ground.
    3. ⇒If less than the specified range, test the 12-volt 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.
    4. ⇒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 3A fused jumper wire between the signal circuit terminal D and the 12-volt reference circuit terminal C. Verify the scan tool Stop Lamp Switch Status parameter is 1 count.
    5. ⇒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.

Brake Switch Circuit Test

  1. Ignition OFF, disconnect the harness connector at the stop lamp switch.
  2. Ignition ON, test for B+ between the ignition supply circuit terminal B and ground.
    3. ⇒If less than the specified range, test the ignition supply circuit for a short to ground or an open/high resistance.
  3. Verify the scan tool TCM Brake Switch parameter is Released.
    4. ⇒If not the specified value, test the signal circuit terminal A for a short to voltage.
  4. Install a 3A fused jumper wire between the signal circuit terminal B and the ignition supply circuit terminal A. Verify the scan tool Brake Switch parameter is Applied.
    5. ⇒If not the specified value, test the signal circuit for an open/high resistance.
  5. If all circuits test normal, test or replace the stop lamp switch.

Stop Lamp Circuit Test

  1. Ignition OFF, disconnect the harnes connector X4 at the BCM.
  2. Ignition ON, verify a test lamp illuminates between the terminals listed below and ground.
    3. • 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.
    6. ⇒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 commanded states.
    8. ⇒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 appropriate stop lamp.

CHMSL Relay Control Circuit Test

  1. Ignition OFF, disconnect the harness connector X5 at the BCM.
  2. Ignition ON, verify the center high mount stop lamp is not on.
    3. ⇒If the center high mount stop lamp is on, test the CHMSL relay control circuit terminal 5 for a short to voltage. If the circuit tests normal replace the underhood fuse block.
  3. Install a 10A fused jumper wire between the CHMSL relay control circuit terminal 5 and B+. Verify the center high mount stop lamp is on.
    4. ⇒If the center high mount stop lamp is not on or the fuse in the jumper wire opens, test the CHMSL relay control circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the underhood fuse block.
  4. If all circuits test normal, replace the BCM.

CHMSL Circuit Test

  1. Ignition OFF, disconnect the harnes connector at the center high mount stop lamp.
  2. Test for less than 1 ohm between the ground circuit terminal 1 and ground.
    3. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  3. Connect a test lamp between the control circuit terminal A and the ground circuit terminal B.
  4. Ignition ON, press and release the brake pedal. The test lamp should turn ON and OFF when changing between the commanded states.
    5. ⇒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 center high mounted stop lamp.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Stop Lamp Switch Replacement
    •  High Mount Stop Lamp Replacement
    •  Tail Lamp Bulb Replacement
    •  Control Module References for BCM replacement, programming, and setup.