Stop Lamps Malfunction (Cadillac non EXP)

Stop Lamps Malfunction (Cadillac non EXP)

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

When the driver presses the brake pedal, the switch contacts open and battery positive voltage is interrupted to the BCM. The BCM then supplies voltage to both the turn/stop lamp control circuits. The BCM also supplies a control voltage illuminating the CHMSL.

Circuit/System Verification

1. Ignition ON, observe the scan tool Stop Lamp Switch Status parameter while pressing and releasing the brake pedal. The reading should change between 1 count and approximately 1023 counts.

^ If not the specified value, refer to the Stop Lamp Switch Circuit Test.

2. Ignition ON, observe the left, right and center high mount stop lamps while pressing and releasing the brake pedal. The stop lamps should turn on and off.

^ If the left and or right stop lamps are always on or inoperative, refer to the Stop Lamp Circuit Test.

^ 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 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 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.

Stop Lamp Circuit Test
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 affected stop lamp.
5. Test for less than 1 ohm of resistance between the listed ground circuit terminal and ground.

* EXT terminal GRD
* SUV terminal D

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

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

* EXT terminal MJR to terminal GRD
* SUV terminal E to terminal D

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 Circuit Test
1. Ignition OFF, disconnect the harness connector X4 at the BCM.
2. Ignition ON, verify a test lamp illuminates between terminal 4 and ground.

^ 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 center high mounted stop lamp.
5. Test for less than 1 ohm of resistance between the 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 stop lamp control circuit terminal A and ground circuit terminal B.
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 center high mounted stop lamp.

Repair Instructions

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

* Stop Lamp Switch Replacement (Non-Adjustable Pedal) Stop Lamp Switch Replacement (Adjustable Pedal)
* High Mount Stop Lamp Replacement (Escalade) High Mount Stop Lamp Replacement (Yukon) High Mount Stop Lamp Replacement (Avalanche) High Mount Stop Lamp Replacement (GMC HP2)
* Tail Lamp Bulb Replacement (Cadillac) Tail Lamp Bulb Replacement (Yukon) Tail Lamp Bulb Replacement (Tahoe, Avalanche) Tail Lamp Bulb Replacement (GMC HP2)