Test 27A - Main Relay Open (Code 65)

Test 27A: Repairing Code (65) Main Relay Open (Part 1 Of 2):

Test 27A: Repairing Code (65) Main Relay Open (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 Open

SET CONDITION:
The following criteria must be met for this code to be detected.
1. Ignition turned on for at least 3.4 seconds.
2. ABS main relay command given.
3. None of the six valve (solenoid) commands given.
4. Low feedback voltage from the low side of all six solenoids.
5. Criteria 1 and 2 satisfied for over 400 consecutive Controller Antilock Brake (CAB) checks spaced 5 ms apart.

THEORY OF OPERATION:
Whenever the ABS main relay is closed and no driver transistors are turned on, all valve coil feedback voltages should be high. If they are not, and the problem persists long enough to rule out noise input, the Open relay fail-safe is tripped. Normally a low valve coil feedback voltage during this time is interpreted as an Open valve coil. However, the probability of all six solenoids appearing to be Open at the same time is so low that this event is interpreted as an Open relay rather than as six Open solenoids.

POSSIBLE CAUSES:
- Anything that keeps all six solenoid feedback voltages low when they are expected to be high (no solenoids energized)
- 0pen relay coil or non-closeable relay contacts
- 0pen circuit in series with relay coil or relay contacts
- 0pen circuit or short to ground within CAB
- All six valve driver transistors (CAB outputs) shorted
- All six solenoid feedbacks to the CAB microprocessor shorted to ground

Main Relay Connector (In PDC):

Typical Relay:

ABS Main Relay Output: