Test M: The Brake Warning Indicator Is Always On

PINPOINT TEST M: THE BRAKE WARNING INDICATOR IS ALWAYS ON

M1-M7:

M7-M9:

M9-M11:

M12-M13:

Normal Operation
The instrument cluster uses 3 basic messaged inputs to control the brake warning indicator. The first 2 are the parking brake switch and brake fluid level switch. The third is for the electronic brake distribution, controlled by the anti-lock brake system (ABS). The parking brake 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 switch is hardwired to the SJB through circuit CCB01 (VT/BN). The brake fluid level switch is hardwired to the SJB through signal circuit CMC19 (GY/VT) and return circuit RMC19 (YE/GY). When the parking brake is applied, the parking brake switch closes to ground through circuit GD143 (BK/VT), providing a ground to the SJB and pulling circuit CCB01 (VT/BN) low. When a low brake fluid level condition exists, the low brake fluid level switch closes to ground through circuit GD120 (BK/GN), pulling circuit CMC19 (GY/VT) low. The SJB monitors the parking brake and low brake fluid level inputs and sends the instrument cluster a message to turn on the brake warning indicator when the parking brake is applied or the brake fluid 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 to illuminate both the ABS warning indicator and the brake warning indicator.

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

DTC B2479 - sets on-demand if the SJB detects a short to ground on the parking brake input circuit CCB01 (VT/BN) during the module self-test.

DTC C1125 - is a continuous and on-demand DTC that sets in the SJB if an open is detected on the brake fluid level input circuit CMC19 (GY/VT) or return circuit RMC19 (YE/GY) or if a short to ground is detected on the 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 anti-lock brake system (ABS) module.

Possible Causes
- Circuit CCB01 (VT/BN) open or short to ground
- Circuit CMC19 (GY/VT) open or short to ground
- Circuit RMC19 (YE/GY) open
- Parking brake switch
- Master cylinder (brake fluid level switch)
- ABS module
- SJB
- Instrument cluster