Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairMedium
A vehicle's ABS system uses a magnetic wheel speed sensor (passive type) and a toothed tone ring. If the air gap between the sensor and the tone ring is excessively large, what effect will this MOST likely have on the sensor's signal?
- ADecreased signal voltage.
- BNo effect on the signal, only on reliability.
- CIncreased signal voltage.
- DDecreased signal frequency.
Show answer & explanationAnswer & explanation
Correct answer: A. Decreased signal voltage.
Passive magnetic wheel speed sensors generate an analog AC voltage signal as the tone ring's teeth pass by. The strength of this magnetic field, and thus the voltage generated, is inversely proportional to the air gap. An excessively large air gap weakens the magnetic field, resulting in a decreased signal voltage, potentially too low for the ABS module to accurately read.
Why the other options are wrong
- B. An excessively large air gap will significantly decrease the signal strength, directly affecting the signal and its readability, not just reliability.
- C. An increased air gap weakens the magnetic field, which would decrease, not increase, the signal voltage.
- D. Signal frequency is determined by wheel speed and the number of teeth on the tone ring, not the air gap.
WSS Air Gap Effect (Passive Sensor)
The impact of the distance between a passive magnetic wheel speed sensor and its tone ring on the generated signal.
- An inverse relationship exists: larger gap = weaker magnetic field.
- Weaker magnetic field results in lower induced AC voltage.
- Excessive gaps can lead to intermittent or no signal, triggering DTCs.
Memory trick: Gap too big, signal too small, that's the magnetic sensor's fall!