Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairHard
A technician is performing a post-repair road test on a vehicle after replacing a faulty ABS wheel speed sensor. The ABS warning light is off, but during a controlled stop, the technician notices that the brake pedal pulsates excessively and inconsistently, indicating that the ABS is activating prematurely or erratically. There are no current fault codes. What is the MOST likely cause?
- ATrapped air in the master cylinder.
- BIncorrect tire size on one of the wheels.
- CA malfunctioning hydraulic control unit (HCU) return valve.
- DA faulty brake booster.
Show answer & explanationAnswer & explanation
Correct answer: B. Incorrect tire size on one of the wheels.
Different tire sizes result in different rotational speeds for the same vehicle speed. The ABS module interprets this discrepancy as wheel slip, causing premature and erratic ABS activation even when it shouldn't, as it tries to correct a perceived, but not actual, slip condition.
Why the other options are wrong
- A. Trapped air in the master cylinder would cause a spongy pedal, not erratic ABS activation.
- C. A malfunctioning HCU return valve would typically cause a wheel to lock up or release pressure incorrectly, potentially setting a specific HCU fault code, but less likely to cause erratic, premature ABS activation across the board due to a perceived speed difference.
- D. A faulty brake booster would affect overall pedal feel (hard pedal) and stopping distance, not cause erratic ABS pulsation.
Tire Size Mismatch & ABS
Using tires of different rolling circumferences on a vehicle, particularly on the same axle, can lead to incorrect wheel speed sensor readings. The ABS module may misinterpret these discrepancies as wheel slip, causing premature or erratic ABS activation.
- Causes rotational speed differences.
- Confuses ABS/TCS/ESC modules.
- Can lead to false ABS activations without codes.
Memory trick: ABS Pulse: Check the Tires' Roll!