Brake Lamp Switch Circuit

BRAKE LAMP SWITCH CIRCUIT





When Monitored and Set Condition
^ When Monitored: With the ignition on.
^ Set Condition: The Controller Antilock Brake (CAB) detects an abnormal condition on the Brake Lamp Switch circuit.





For a complete Brake Assist System (BAS) Circuit Diagram refer to Diagrams; Electrical

Diagnostic Test
1. MEASURE BRAKE LAMP SWITCH VOLTAGE

Note: Inspect Fuse 2 located in the illumination Control Module/IP Fuse Block. If the fuse is open, repair the cause of the open fuse before continuing.

Turn the ignition off.
Disconnect the Brake Lamp Switch harness connector.

Note: Check connectors - Clean/repair as necessary.

Turn the ignition on.





Measure the voltage of the Fused Ignition Switch Output circuit at the Brake Lamp Switch harness connector.
Is the voltage above 10 volts?
Yes >> Go to 2
No >> Repair the Fused Ignition Switch Output circuit for an open. Perform ABS VERIFICATION TEST. ABS Verification Test

2. MEASURE THE RESISTANCE BETWEEN THE BRAKE LAMP SWITCH AND THE BRAKE LAMP RELAY
With the ignition off.
Remove the Brake Lamp Relay.

Note: Check connectors - Clean/repair as necessary.





Measure the resistance of the circuit from the Brake Lamp Switch harness connector B cavity 2 to the Brake Lamp Relay harness connector cavity 8.
Is the resistance below 5.0 ohms?
Yes >> Go to 3
No >> Repair the circuit for an open. Perform ABS VERIFICATION TEST.

3. MEASURE THE RESISTANCE BETWEEN THE BRAKE LAMP SWITCH AND THE CAB
With the ignition off.
Disconnect the CAB harness connector.

Note: Check connectors - Clean/repair as necessary.





Measure the resistance of the Brake Lamp Switch Output circuit from the Brake Lamp Switch harness connector to the CAB harness connector.
Is the resistance below 5.0 ohms?
Yes >> Go to 4
No >> Repair the Brake Lamp Switch Output circuit for an open. Perform ABS VERIFICATION TEST.

4. MEASURE BRAKE LAMP RELAY VOLTAGE
With the ignition off.
Reconnect the Brake Lamp Switch harness connector.
Turn the ignition on.





Have an assistant depress the brake pedal while measuring the voltage at the Brake Lamp Relay harness connector cavity 8.
Is the voltage above 10 volts?
Yes >> Go to 5
No >> Replace the Brake Lamp Switch. Perform ABS VERIFICATION TEST.

5. MEASURE THE RESISTANCE BETWEEN THE BRAKE LAMP RELAY AND THE CAB
Turn the ignition off.





Measure the resistance of the Brake Lamp Relay Control circuit between the Brake Lamp Relay harness connector and the CAB harness connector.
Is the resistance below 5.0 ohms?
Yes >> Go to 6
No >> Repair the Brake Lamp Relay control circuit between the Brake Lamp Relay and the CAB for an open. Perform ABS VERIFICATION TEST.

6. MEASURE BRAKE LAMP RELAY CONTROL CIRCUIT VOLTAGE
Turn the ignition on.





Measure the voltage of the Brake Lamp Relay Control circuit at the CAB harness connector.
Is voltage present?
Yes >> Repair the Brake Lamp Relay Control circuit for a short to voltage. Perform ABS VERIFICATION TEST.
No >> Go to 7

7. MEASURE BRAKE LAMP RELAY CONTROL CIRCUIT VOLTAGE
Turn the ignition off.
Install a jumper wire between cavities 4 and 6 of the Brake Lamp Relay harness connector.
Turn the ignition on.





Measure the voltage of the Brake Lamp Relay Control circuit at the CAB harness connector.
Is the voltage above 10 volts?
Yes >> Go to 8
No >> Repair the Brake Lamp Relay Control circuit for an open. Perform ABS VERIFICATION TEST.

8. MEASURE BRAKE LAMP RELAY OUTPUT CIRCUIT RESISTANCE
With the ignition off and the jumper wire removed from the Brake Lamp Relay harness connector.





Measure the resistance of the Brake Lamp Relay Output circuit from the Brake Lamp Relay harness connector to the CAB harness connector.
Is the resistance below 5.0 ohms?
Yes >> Go to 9
No >> Repair the Brake Lamp Relay Output circuit for an open. Perform ABS VERIFICATION TEST.

9. MEASURE BRAKE LAMP RELAY OUTPUT CIRCUIT VOLTAGE
Turn the ignition on.





Measure the voltage of the Brake Lamp Relay Output circuit at the CAB harness connector.
Is voltage present?
Yes >> Repair the Brake Lamp Relay Output circuit for a short to voltage. Perform ABS VERIFICATION TEST.
No >> Go to 10

10. MEASURE BRAKE LAMP RELAY OUTPUT CIRCUIT VOLTAGE
Turn the ignition off.
Install a jumper wire between cavities 8 and 5 of the Brake Lamp Relay harness connector.
Turn the ignition on.





Have an assistant depress the brake pedal while measuring the voltage of the Brake Lamp Relay Output circuit at the CAB harness connector.
Is the voltage above 10 volts?
Yes >> Replace the Controller Antilock Brake. Perform ABS VERIFICATION TEST.
No >> Replace the Brake Lamp Relay. Perform ABS VERIFICATION TEST.