Operation

OPERATION

The brake indicator gives an indication to the vehicle operator when there are certain brake hydraulic system malfunctions as indicated by a low brake hydraulic fluid level condition, or when there is a problem in the electronic brake force distribution (also known as EBD or EBV) system. This indicator is controlled by a transistor on the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster over the Controller Area Network (CAN) data bus from the Body Control Module (BCM) (also known as the Signal Acquisition and Actuation Module/SAM) or the Controller Antilock Brake (CAB) (also known as the Antilock Brake System/ABS or Electronic Stability Program/ESP controller).

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 detects that the ignition switch is in the ON position. Therefore, the LED will always be OFF when the ignition switch is in any position except ON. 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 cluster briefly as a bulb test.
- BCM Brake Indicator Lamp-On Message - Each time the cluster receives an electronic brake indicator lamp-ON message from the BCM indicating that the brake hydraulic fluid level is low while the ignition switch is in the ON position, the brake indicator is illuminated. The indicator remains illuminated until the cluster receives a lamp-OFF message from the BCM, or until the ignition switch is turned to the OFF position, whichever occurs first. On vehicles equipped with the optional steering wheel buttons, each time the cluster receives an electronic brake indicator lamp-ON message from the BCM, a Brake fluid - Visit workshop textual message will also appear within the cluster multi-function display.
- CAB Brake Indicator Lamp-On Message - Each time the cluster receives an electronic brake indicator lamp-ON message from the CAB indicating a problem in the electronic brake force distribution system, the indicator will be illuminated. 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. On vehicles equipped with the optional steering wheel buttons, each time the cluster receives an electronic brake indicator lamp-ON message from the CAB, a Brake force distribution or Brake force distribution - Visit workshop textual message will also appear within the cluster multi-function display.

The BCM continually monitors the brake fluid level switch in the brake master cylinder to determine the level of the brake hydraulic fluid, while the CAB continually monitors the electronic brake force distribution system circuits and sensors to decide whether that system is in good operating condition. The BCM and the CAB then send the proper lamp-ON or lamp-OFF messages to the ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN or KOMBI). If the BCM or the CAB sends a lamp-ON message after the bulb test, it indicates that the BCM has detected a low brake hydraulic fluid level or that the CAB has detected that the electronic brake force distribution system has become ineffective. The BCM or the CAB will store a Diagnostic Trouble Code (DTC) for any malfunctions detected.

For proper diagnosis of the brake fluid level switch, the BCM, the electronic brake force distribution system, the CAB, the EMIC or the electronic communication related to brake indicator operation, a diagnostic scan tool is required. Refer to the appropriate diagnostic information.