78 Foundation Brake

NAME OF CODE:
78 Foundation Brake


SET CONDITION:
The following criteria must be met for this trouble code to be detected:

1. Vehicle speed above 8 MPH for 2 seconds.
2. Brake system lamp driver command not given.
3. Low feedback voltage from the low side of the brake system warning lamp.
4. Criteria 1-3 satisfied for 255 consecutive controller checks spaced 5 ms apart.


THEORY OF OPERATION:
If the foundation brake pressure differential switch shuttles during antilock mode, front wheel antilock activity is terminated immediately for the remainder of the stop and the rear pressure control circuit enters a controlled pressure re-apply mode to return the rear circuit to conventional braking.

NOTE: Because of special circuitry, if the red brake warning lamp is lit due to the parking brake, no action is taken.


POSSIBLE CAUSES:
- Anything that keeps brake system warning lam~ feedback voltage low when the lamp is not energized by the CAB
- Pressure differential switch in proportioning valve shuttled
- Shorted to ground in the red brake warning lamp feedback path within the CAB
- Open circuit in series with the motor (excluding an open relay)
- Short to ground in the motor feedback to the CAB microprocessor