Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairHard
An active wheel speed sensor provides a digital square wave signal to the ABS control module. If the sensor's internal circuitry fails, leading to a constant high voltage output (stuck high), what would the ABS control module MOST likely interpret this as?
- AA continuous, very low wheel speed.
- BAn implausible or faulty signal.
- CA complete loss of signal (0 MPH).
- DA continuous, very high wheel speed.
Show answer & explanationAnswer & explanation
Correct answer: B. An implausible or faulty signal.
Active wheel speed sensors produce a digital square wave signal, where the frequency represents wheel speed. A constant high voltage (stuck high) means there are no rising and falling edges, and thus no frequency to interpret. The ABS module is programmed to recognize valid signal patterns. A constant high signal is not a valid square wave and doesn't correspond to any plausible wheel speed, leading the module to flag it as an implausible or faulty signal, typically setting a DTC.
Why the other options are wrong
- A. A low wheel speed would still be a square wave, just with a lower frequency. A constant high voltage doesn't represent a low frequency square wave.
- C. A complete loss of signal is typically 'stuck low' (0V) or an open circuit. 'Stuck high' is a different failure mode.
- D. A continuous high voltage isn't a high frequency; it's a lack of frequency, so it wouldn't be interpreted as high speed.
Active WSS Stuck High
A failure mode in an active wheel speed sensor where its digital output signal remains at a constant high voltage level.
- Prevents the ABS module from detecting a valid frequency (wheel speed).
- Interpreted as an implausible or faulty signal by the module.
- Often triggers specific diagnostic trouble codes for sensor malfunction.
Memory trick: Stuck high, no square wave to tell, the module knows something's not well!