Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairEasy
A technician is diagnosing an intermittent ABS warning light and a C0045 fault code (Right Rear Wheel Speed Sensor Circuit). Visual inspection shows no obvious damage to the sensor or wiring. When performing a resistance test on the two-wire passive wheel speed sensor, the technician measures an open circuit. What is the MOST likely cause?
- AA short to ground within the sensor's internal coil.
- BAn internal open circuit in the sensor's coil winding.
- CAn excessive air gap between the sensor and tone ring.
- DA short to voltage in the sensor's wiring harness.
Show answer & explanationAnswer & explanation
Correct answer: B. An internal open circuit in the sensor's coil winding.
A resistance test on a passive wheel speed sensor measures the continuity of its internal coil. An 'open circuit' reading (infinite resistance) directly indicates a break in the coil winding, preventing signal generation.
Why the other options are wrong
- A. A short to ground would typically result in a very low or zero resistance reading, not an open circuit.
- C. An excessive air gap would cause a weak or erratic signal, but the sensor's coil would still show continuity (correct resistance) if tested.
- D. A short to voltage in the wiring harness would likely cause other electrical issues and potentially damage the ABS module, but the sensor's internal resistance would still be measurable if its coil isn't open.
Passive WSS Open Circuit
An open circuit in a passive wheel speed sensor's internal coil means there's a break in the wire, preventing it from generating an AC voltage signal. This results in an infinite resistance reading during testing.
- Passive WSS has an internal coil.
- Resistance test checks coil integrity.
- Open circuit = infinite resistance = failed sensor.
Memory trick: Resistance Check: Open = Broken Coil!