Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairEasy
A technician is diagnosing an ABS warning light and a C0035 fault code: 'Left Front Wheel Speed Sensor Circuit Range/Performance'. The technician measures the resistance of the wheel speed sensor and finds it to be 1.2 kΩ. The manufacturer's specification for a good passive wheel speed sensor is between 800 Ω and 1500 Ω. Which of the following is the MOST likely cause of the fault?
- AIncorrect air gap between the sensor and tone ring.
- BAn intermittent open circuit in the wheel speed sensor.
- CA short circuit in the wheel speed sensor wiring.
- DA faulty ABS control module.
Show answer & explanationAnswer & explanation
Correct answer: A. Incorrect air gap between the sensor and tone ring.
The measured resistance of 1.2 kΩ is within the manufacturer's specified range, indicating the sensor itself and its basic wiring integrity are likely good. A 'Range/Performance' fault often points to issues affecting the sensor's signal generation, such as an incorrect air gap, rather than an electrical circuit fault within the sensor or wiring.
Why the other options are wrong
- B. An intermittent open circuit would result in an infinite resistance reading when the open circuit occurs, or a fluctuating reading outside the specified range, which is not indicated by the 1.2 kΩ measurement.
- C. A short circuit would typically result in a very low resistance reading, often near 0 Ω, which is not the case here.
- D. While an ABS control module can be faulty, the provided information points to a specific wheel speed sensor circuit issue. It's less likely to be the module if the sensor's electrical integrity (resistance) is within spec and the fault is 'Range/Performance' related to signal generation.
Wheel Speed Sensor Air Gap
The precise distance between a passive wheel speed sensor and its tone ring, critical for generating a strong and accurate AC voltage signal.
- Too large an air gap reduces signal strength.
- Too small an air gap can cause physical contact and damage.
- Incorrect air gap often results in 'Range/Performance' fault codes.
Memory trick: Air gaps can make a sensor 'act' faulty, even if its wires are fine.