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Home >> GMC >> 2006 >> Sierra 1500 HD, 4WD >> Repair and Diagnosis >> External Pages >> Different car >> Section 981 (Lighting Systems) >> Diagnostic Information and Procedures >> Stop Lamps Malfunction >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

Stop Lamp Switch Circuit Test

  1. Ignition OFF, disconnect the harness connector at the stop lamp switch.
  2. Ignition ON, test for battery voltage between the 12-volt reference circuit terminal C and ground.
    • 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.
    • 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.
    • 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 battery voltage between the ignition supply circuit terminal B and ground.
    • 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.
    • 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.
    • 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.
    • 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 harnes connector X4 at the BCM.
  4. Ignition OFF, disconnect the harness connector at the affected stop lamp.
  5. Test for less than 1 ohm of resistance 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 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.
  8. If all circuits test normal, test or replace the affected 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.
    • 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 battery voltage. Verify the center high mount stop lamp is on.
    • 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 of resistance between the ground circuit terminal 1 and ground.
    • 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.
    • 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.
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