Test 31A - Foundation Brake (Code 78)

Test 31A: Repairing Code (78) Foundation Brake:

NOTE: When connecting or disconnecting connectors and inspecting connectors clean and repair as necessary.

NOTE: After completion of a repair or test then perform Verification Test. Verification Test VER-1A - ABS

Name of code:
Foundation Brake

SET CONDITION:
The following criteria must be met for this code to be detected.
1. Vehicle speed above 8 mph for 2 seconds.
2. Brake system lamp driver command not given.
3. Low feed back 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 rear pressure control circuit enters a controlled pressure reapply 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 lamp feedback voltage low when the lamp is not energized by the Controller Antilock Brake (CAB)
- Pressure differential switch in proportioning valve shuttled
- Short to ground in the red brake warning lams feedback path within the CAB

Schematic:

Ignition Switch 7-Way Connector:

Controller-Antilock Brake Connector: