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Home >> GMC >> 2006 >> Sierra 1500 HD, RWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 859 (Lighting Systems) >> Diagnostic Information and Procedures >> High Mount Stop Lamp Malfunction >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  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 connctor 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.
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