Brake/Park Brake Indicator: Operation

The brake indicator gives an indication to the vehicle operator when the parking brake is applied, or when there are certain brake hydraulic system malfunctions. The brake indicator can also give an indication when certain faults are detected in the Antilock Brake System (ABS). This indicator is controlled by a transistor on the instrument cluster electronic circuit board based upon cluster programming, and electronic messages received by the cluster from the Central Timer Module (CTM) and the Controller Antilock Brake (CAB) over the Programmable Communications Interface (PCI) data bus. The brake indicator Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit, and that logic will only allow this indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the LED will always be off when the ignition switch is in any position except On or Start. The LED only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster will turn on the brake indicator for the following reasons:

• Bulb Test  - Each time the ignition switch is turned to the On position the brake indicator is illuminated by the instrument cluster for about four seconds as a bulb test.
• Brake Lamp-On Message  - Each time the cluster receives a lamp-on message from the CAB, the brake indicator will be illuminated. The CAB can also send brake lamp-on messages as feedback during ABS diagnostic procedures. The indicator remains illuminated until the cluster receives a lamp-off message from the CAB, or until the ignition switch is turned to the Off position, whichever occurs first.
• Park Brake Applied Message  - Each time the cluster receives a park brake applied message while the ignition switch is in the On position, the brake indicator will be illuminated. If the transmission is in Park or Neutral when the park brake applied message is received, the brake indicator illuminates solid. If the transmission is in any position except Park or Neutral when the park brake applied message is received, the brake indicator flashes on and off. The indicator remains illuminated solid or flashing until the park brake is released, or until the ignition switch is turned to the Off position, whichever occurs first.
• Communication Error  - If the cluster receives no lamp-on or lamp-off messages from the CAB for three consecutive seconds, the brake indicator will be illuminated. The indicator remains illuminated until the cluster receives a single lamp-off message from the CAB.
• Actuator Test  - Each time the instrument cluster is put through the actuator test, the brake indicator will be turned on, then off again during the bulb check portion of the test to confirm the functionality of the LED and the cluster control circuitry.

  The CTM continually monitors the status of the park brake switch on the park brake pedal mechanism. The CTM then sends the proper park brake status messages to the instrument cluster. The CAB continually monitors the ABS system circuits and sensors, including the brake fluid level switch on the brake master cylinder reservoir, to decide whether the system is in good operating condition. The CAB then sends the proper lamp-on or lamp-off messages to the instrument cluster. If the CAB sends a lamp-on message after the bulb test, it indicates that the CAB has detected a brake hydraulic system malfunction and/or that the ABS system has become inoperative. The CAB will store a Diagnostic Trouble Code (DTC) for any malfunction it detects.

  For further diagnosis of the brake indicator or the instrument cluster circuitry that controls the LED, (Refer to INSTRUMENT CLUSTER - DIAGNOSIS AND TESTING  ). For proper diagnosis of the park brake switch, the brake fluid level switch, the ABS, the CTM, the CAB, the PCI data bus, or the electronic message inputs to the instrument cluster that control the brake indicator, a DRBIII(R) scan tool is required. Refer to the appropriate diagnostic information.