Circuit/System Description
The brake switch signal circuit is a high voltage input to the body control module (BCM), engine control module (ECM), transmission control module (TCM), Power Take Off (PTO) module (if equipped) and Trailer Brake controller (if equipped). The BCM monitors the stop lamp switch circuit to detect when the brake pedal has been applied. The ECM monitors the brake switch signal to determine when the brake pedal is applied. When the brake pedal is applied, the BCM sends a GMLAN serial data message to the ECM indicating that the brake pedal has been applied. The ECM senses a high voltage signal on the brake switch signal circuit when the stop lamp switch is closed.
Conditions for Running the DTC
The ignition is ON.
The engine is running.
Battery voltage is greater than 11.5 volts.
Conditions for Setting the DTC
The ECM detects a low voltage signal on the brake switch signal circuit when the serial data message from the BCM indicates the brakes are applied.
Action Taken When the DTC Sets
The cruise control system becomes inoperative.
The ECM stores the DTC to memory.
The ECM will record the operating conditions at the time the diagnostic failed. The ECM stores this information in Failure Records.
Conditions for Clearing the DTC
A last test failed, or the current DTC, clears when the diagnostic runs and does not fail.
A history DTC clears after 40 consecutive warm-up cycles, if failures are not reported by this or any other emission related diagnostic.
Diagnostic Aids
Ensure all the stop lamps operate properly. Refer to Exterior Lighting Systems Description and Operation to avoid misdiagnosis. If the vehicle stop lamps operate improperly, refer to Symptoms - Lighting. See: Lighting and Horns\Testing and Inspection\Symptom Related Diagnostic Procedures\- Symptoms - Lighting
Circuit/System Testing
Ignition OFF, disconnect the harness connector of the stop lamp switch.
Ignition ON, verify that a test lamp illuminates between the stop lamp switch voltage circuit terminal B and ground.
¤ If the test lamp does not illuminate, test the stop lamp switch voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the RUN/CRANK Relay.
Ignition OFF, disconnect the harness connectors of the BCM X5, ECM X1, TCM (if equipped), Power Take Off (PTO) module (if equipped) and Trailer Brake controller (if equipped).
Ignition OFF, verify that a test lamp does not illuminate between the stop lamp switch harness terminal A and B+.
¤ If the test lamp illuminates, test the brake switch signal circuit for a short to ground.
Test for less than 1.0 ohm resistance between the stop lamp switch harness terminal A and one of the following:
ECM connector X1 terminal 83 for the 6.6L engine.
ECM connector X1 terminal 55 for the 4.3L engine.
ECM connector X1 terminal 9 for the 4.8L, 5.3L, 6.0L, 6.2L or 7.0L engines.
¤ If resistance is not as specified, test the stop lamp switch circuit for an open/high resistance.
Ignition ON, verify that a test lamp does not illuminate between the stop lamp switch TCC brake signal circuit terminal A and ground.
¤ If the test lamp illuminates, test the brake switch signal circuit for a short to voltage.
If all circuits test normal, test or replace the stop lamp switch. If the component and all circuit tests are normal, replace the ECM.
Component Testing
Relay Test
Ignition OFF, disconnect the RUN/CRANK relay.
Test for 60-180 ohms of resistance between terminals 85 and 86.
¤ If the resistance is not within the specified range, replace the relay.
Test for infinite resistance between the following terminals:
30 and 86
30 and 87
30 and 85
85 and 87
¤ If less than infinite resistance, replace the RUN/CRANK relay.
Install a 20-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2.0 ohms of resistance between terminals 30 and 87.
¤ If greater than 2.0 ohms, replace the RUN/CRANK relay.
Switch Test
Ignition OFF, disconnect the stop lamp switch.
With the brake pedal released, measure for infinite resistance between terminals A and B of the switch.
¤ If resistance is measured, replace the switch.
With the brake pedal released, measure for less than 1.0 ohm resistance between terminals C and D of the switch.
¤ If resistance is not as specified, replace the switch.
With the brake pedal applied, measure for less than 1.0 ohm resistance between terminals A and B of the switch.
¤ If resistance is not as specified, replace the switch.
With the brake pedal applied, measure for infinite resistance between terminals C and D of the switch.
¤ If resistance is measured, replace the switch.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. See: Verification Tests
Stop Lamp Switch Replacement (Non-Adjustable Pedal) Stop Lamp Switch Replacement (Adjustable Pedal)
