Test O: The Brake Warning Indicator Is Always On

PINPOINT TEST O: THE BRAKE WARNING INDICATOR IS ALWAYS ON

Normal Operation
The instrument cluster 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 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 exists, the low brake fluid level switch closes to ground through circuit GD120 (BK/GN), pulling signal circuit CMC19 (GY/VT) low. The SJB monitors the parking brake and the brake fluid level inputs sending the instrument cluster a message to turn the brake warning indicator on when the parking brake is applied, when the brake fluid level is low or if the SJB detects an open in the brake fluid level signal or return circuits. 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 to illuminate both the ABS warning indicator and the brake warning indicator.

If the brake fluid level or parking brake status messages are missing for more than 5 seconds, the instrument cluster sets DTC U0140 and defaults the brake warning indicator on. If the instrument cluster does not receive the ABS status message or the electronic brake distribution (EBD) message from the ABS module for more than 5 seconds, the instrument cluster sets DTC U0121 and defaults the ABS warning indicator on. If the brake warning message received from the ABS module is deemed invalid by the instrument cluster for 5 seconds or longer, the instrument cluster sets DTC U2023 and defaults the brake warning indicator on.

- DTC B2479 (Park Brake Switch Circuit Short to Ground) - is an on-demand DTC that sets when the SJB detects a short to ground on the parking brake warning indicator switch input, circuit CMC25 (WH/VT) during the module self-test.
- DTC C1125 (Brake Fluid Level Sensor Input Circuit Failure) - is a continuous and on-demand DTC that sets in the SJB if the SJB detects an open or a short to ground on the brake fluid level input circuit CMC19 (GY/VT).

NOTE:
- If DTC U0140 is set in the instrument cluster, other symptoms may also be present such as tire pressure monitoring system (TPMS), instrument panel backlighting, and door ajar indication.
- Both the ABS warning indicator and the brake warning indicator illuminate if the input to turn on the brake warning indicator is received from the ABS module.

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

O1-O5:

O6-O9:

O9-O11:

O12-O13: