Intermittent Fault Diagnosis

As with almost any electronic system, intermittent faults in the Anti-lock Brake System (ABS) may be difficult to diagnose accurately. In order to set a Fault Code, the failure must occur during the ignition cycle in which diagnosis is being attempted, as the Fault Code remains set only until the ignition is turned off. The Anti-lock Brake Control Module (ABM) tests for different failures under different vehicle conditions. For this reason, it is often necessary to test drive the vehicle and repeat the failure to set a Fault Code.

If the system malfunction is not duplicated during the test drive, vehicle behavior may be helpful in locating a "most likely" faulty component or circuit. The Symptom Diagnosis Chart may be useful in isolating the failure.

Most intermittent faults are caused by faulty electrical connections or wiring. When an intermittent fault is encountered, check suspect circuit for:
1. Terminals not fully seated in the connector body or poor mating of connector halves.
2. Poor terminal to wire connection. This requires removal of the terminal from the connector body to inspect.
3. Damaged or improperly formed terminals. Increase contact tension by carefully reforming all connector terminals.

Most ABS system failures will disable the anti-lock function for the entire ignition cycle even if the fault clears before key-off.

There are a few fault conditions, however, which will allow ABS operation to resume during the ignition cycle, if the fault condition disappears. Intermittent illumination of the BRAKE warning and/or ANTI-LOCK warning lamps may be the result of the following conditions and should be investigated if a complaint of intermittent ABS warning system operation is encountered.
1. Low brake fluid. A low brake fluid condition will cause the BRAKE warning lamp to illuminate. The BRAKE warning lamp will go out when the fluid sensor again indicates an acceptable fluid level.
2. Low system voltage. Detection of low system voltage by the ABCM will turn on the ANTI-LOCK warning lamp until normal system voltage is achieved. Normal operation resumes once normal voltage is seen at the Anti-lock Brake Control Module (ABM).
3. Low Accumulator Pressure. Low Accumulator Pressure will cause both the BRAKE warning and ANTI-LOCK warning lamps to illuminate. Once normal operating pressure is achieved, the lamps will extinguish and the system will return to normal operation.

Additionally, any condition which results in interruption of power to the ABCM or hydraulic assembly may cause the BRAKE warning and ANTI-LOCK warning lamps to illuminate intermittently. These circuits include the Over Voltage Protection Relay (OVPR) and related wiring and fusible links.