Test 28A - Main Relay Shorted (Code 66)

Test 28A: Repairing Code (66) Main Relay Shorted (Part 1 Of 2):

Test 28A: Repairing Code (66) Main Relay Shorted (Part 2 Of 2):

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:
Main Relay shorted

SET CONDITION:
The following criteria must be met for this code to be detected.
1. Ignition key turned on for at least 3.4 seconds.
2. ABS main relay command not given.
3. High feed back voltage from the low side of motor.
4. Low feedback voltage from the low side of all six solenoids
5. Criteria 1 - 3 satisfied for over 30 consecutive Controller Antilock Brakes (CAB) checks spaced 5 ms apart

THEORY OF OPERATION:
Vehicle battery voltage is applied to the high side of the pump/motor in the hydraulic assembly by the ABS main relay whenever antilock activity may be required. The motor is run by turning on a pair of CAB output driver transistors to short the low side of the motor winding to ground. Whenever the main relay command is not given, the feedback voltage should be low. If it is not, and the problem persists long enough to rule out noise input, the (66) Shorted Main Relay code is set.

POSSIBLE CAUSES:
- Anything that keeps motor feedback voltage high when the main relay is supposed to be Open
- Relay contacts stuck shut
- Shorted ABS main relay driver transistor
- Shorted CAB microprocessor relay output
- Short to a high voltage point in the motor feedback to the CAB microprocessor

Schematic:

Front Valve Body (HCU) Connector: