A technician is using an oscilloscope to analyze the signal from an anti-lock braking system (ABS) wheel speed sensor. The vehicle is lifted, and the wheel is spun by hand. The oscilloscope displays an AC sine wave with an inconsistent amplitude and occasional dropouts at higher spin speeds. Which of the following is the MOST likely cause of this waveform?
- AExcessive resistance in the sensor's wiring.
- BA damaged or dirty tone ring.
- CA short to voltage in the sensor circuit.
- DA faulty Hall-effect wheel speed sensor.
Show answer & explanationAnswer & explanation
Correct answer: B. A damaged or dirty tone ring.
An AC sine wave with inconsistent amplitude and dropouts is characteristic of an inductive wheel speed sensor interacting with a damaged or dirty tone ring. Inductive sensors generate voltage based on changes in magnetic flux from the tone ring. Damage (bent teeth, missing teeth) or debris (metal shavings, rust) on the tone ring will cause inconsistent magnetic field changes, leading to an erratic or inconsistent waveform. Hall-effect sensors produce square waves, and wiring/short issues typically cause different waveform anomalies.
Why the other options are wrong
- A. Excessive resistance in wiring would typically reduce the overall amplitude of the signal, but not necessarily cause inconsistent amplitude or dropouts in this specific manner.
- C. A short to voltage would likely introduce a constant high voltage offset or damage the sensor/module, not produce an inconsistent sine wave with dropouts.
- D. Hall-effect sensors produce square waves, not sine waves. This indicates an inductive sensor.
Inductive Wheel Speed Sensor Waveform
An inductive wheel speed sensor produces an AC sine wave, whose amplitude and frequency increase with wheel speed. Inconsistencies often point to tone ring issues.
- Outputs an AC sine wave.
- Amplitude and frequency are proportional to wheel speed.
- Tone ring integrity is critical for a clean signal.
Memory trick: Sine Wave's Jitters: Tone Ring's Trouble, Not the Sensor's Core.