89 ABS Warning Lamp Circuit

NAME OF CODE:
89 ABS Warning Lamp Circuit


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

1. Microprocessor control software not generating pulses for the external watchdog.
2. High feedback voltage from the low side of the ABS warning lamp.
3. Criteria 1 and 2 satisfied for 30 consecutive checks spaced 5 ms apart.


THEORY OF OPERATION:
When ever the CAB's external watchdog circuit is not functioning, the ABS main relay should be de-energized and the pump/motor feedback voltage should be low. If it is not and the problem persists long enough to rule out noise in the pump/motor feedback and to account for response delay in the external watchdog circuit, the shorted indicator lamp (yellow) code is set.

POSSIBLE CAUSES:
- Anything that keeps ABS warning lamp feedback voltage high when the yellow ABS warning lamp is supposed to be turned on
- Short to vehicle battery voltage in the wiring between the ABS Warning lamp and the CAB
- Open ABS warning lamp control circuit to relay
- Normally closed ABS warning lamp relay contacts open
- Open ABS warning lamp relay ground circuit
- Open ABS warning lamp relay driver transistor in the CAB
- Short to high voltage in the lamp feedback path within the CAB