Failsafe is not always failing to safety

By Christopher W. Ferrone

Over the past several months I have had two experiences with anti-lock braking systems (ABS) that pointed to an unsafe condition both for the vehicle and the driver.

From an engineering point of view, any device designed to improve safety should never become a hazard in and of itself. When the ABS detects a malfunction it should simply go to sleep as designed — and not function. The driver should respond as if the vehicle was not equipped with ABS.

However, in my two recent incidents I diagnosed and analyzed two ABS failures that in fact led to a dangerous condition with the vehicle; in this case a body-on-chassis cut-away style bus with hydraulic brakes.

Protect the vehicle

Anytime the ABS fails, the dash light comes on to indicate to the driver a malfunction has occurred. At that point, in theory, the system in its benign state protects the vehicle from any further unsafe or damaging effects.

Generally, the various sensors within the ABS system report constantly to the computer to control its overall performance. A sensor at each wheel end focused on the backside of the hub or tone ring counts teeth to calculate how fast the one wheel end is rotating. If it reports any value for the individual wheel speed lower than the accepted value the computer will react. The ABS automatically assumes that wheel end is skidding and will release or modulate that particular brake in an effort to control or eliminate the skid.

Unfortunately in each of my two situations, the wheel speed sensor failures confused the computer into allowing the ABS to remain awake, which in turn set up a more serious condition.

The computer codes attributed to these failure modes are frequency related. The computer detected and reported them in simple form, wheel speed sensor frequency error, which led to two separate vehicle control problems.

Extended stopping distance

The first was a brake pedal travel symptom, which ultimately caused extended stopping distance on the vehicle. The second was a harsh left turn upon braking. Neither of these two symptoms are safe or acceptable.

We diagnosed the first failure, extended brake pedal travel, and traced it back to a wheel speed sensor that had been physically broken upon installation due to a part incompatibility — a problem in itself. The system should detect this as another reason to default to safe mode, and not cause any type of unwanted vehicle response.

The incompatibility of the part did eventually lead to the sensor becoming broken which precipitated the fault code related to the frequency problem.

We also traced the second failure, harsh left turn upon braking, back to a wheel speed sensor failure. But in this case the sensor was not visibly broken. Its appearance was normal including the associated wiring and plug connectors.

We first diagnosed the harsh left turn condition as a caliper problem. Typically, a failed caliper will create an uneven braking condition and steering bias in one direction.

We tested and changed the caliper and the problem persisted. The vehicle would still turn left each time the driver applied the brakes. The next step was to confirm the adjustments of the wheel bearings and their condition. Still another road test produced the same unwanted result — a harsh left turn.

Root of the system

Why didn’t we just repair the ABS? We didn’t think the ABS was the root of this symptom. Vehicles with steering input during braking typically point to a mechanical problem arising from worn or defective hydraulic, air system or steering components.

The ABS system was falsely detecting a wheel speed differential between the two front wheels and interpreting this as a skid. The system then responded by releasing the front right brake, leaving the left brake applied, consequently creating steering input into the vehicle and causing the left turn.

Initially, we unplugged the ABS computer from its power supply, which defeated the system entirely and resolved the harsh turn symptom. With the symptom removed, we reconnected the computer to its power supply and promptly created the symptom once again.

The next step was to disconnect each of the front wheel speed sensors to isolate the front axle from the ABS system. Of course, this resolved the symptom once again.

When we replaced the two front axle wheel speed sensors to ensure safety, the vehicle was back to normal and functioning safely.

What is so unsettling about this failure condition is both the owner’s manual and maintenance manual describe the ABS as being in safe mode during a detected failure. By design the ABS system should always fail to safety. Until a mechanic can make the necessary repairs to the ABS to prevent such a failure, the system should simply cease to function — just as if no such ABS was ever installed.

This certainly was not the case in my two situations.

What a technician can learn from our experience is when the ABS dash light illuminates the ABS may not always fail to safety. The driver must report the detection of this condition to the company immediately and proceed with caution.