Test Q: The Brake Warning Indicator Is Never On

PINPOINT TEST Q: THE BRAKE WARNING INDICATOR IS NEVER ON

Normal Operation
The instrument cluster (IC) uses 3 basic messaged inputs to control the brake warning indicator. The first 2 are the parking brake warning indicator switch and brake fluid level switch. The third is for the electronic brake distribution, controlled by the ABS. The parking brake warning indicator switch and the brake fluid level switch status are sent from the smart junction box (SJB) over the medium speed controller area network (MS-CAN) communication bus. The electronic brake distribution message is sent from the ABS module over the HS-CAN communication bus lines. The parking brake warning indicator switch is hardwired to the SJB through circuit CMC25 (WH/VT). The brake fluid level switch is hardwired to the SJB through signal circuit CMC19 (GY/VT) and return circuit RMC19 (YE/GY).

The SJB provides a reference voltage to both the parking brake warning indicator switch and the brake fluid level switch. When the parking brake is applied, the parking brake warning indicator switch closes to ground through circuit GD133 (BK), providing a ground to the SJB and pulling signal circuit CMC25 (WH/VT) low. When a low brake fluid level condition or an open in the signal or return circuits exists, the low brake fluid level switch closes to ground through circuit GD122 (BK), pulling signal circuit CMC19 (GY/VT) low. The SJB monitors the parking brake and the brake fluid level inputs sending the instrument cluster (IC) a message to turn the brake warning indicator on when the parking brake is applied or when the brake fluid level is low. When the ABS module detects a base brake system concern or other ABS related concerns that affect the electronic brake distribution function, the ABS module sends a message to the instrument cluster (IC) to illuminate both the ABS warning indicator and the brake warning indicator.

This pinpoint test is intended to diagnose the following:
- Wiring, terminals or connectors
- Brake fluid level switch (part of the master cylinder)
- Parking brake warning indicator switch
- SJB
- Instrument cluster (IC)

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