Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairEasy
A technician is diagnosing an ABS warning light on a vehicle. The scan tool reports a C0035 fault code: 'Left Front Wheel Speed Sensor Circuit Range/Performance'. Live data shows the left front wheel speed sensor signal fluctuating erratically at low speeds and dropping to zero intermittently, while other wheel speed sensors show consistent readings. What is the MOST likely cause of this issue?
- AA faulty ABS control module
- BLow brake fluid level in the master cylinder reservoir
- CA malfunctioning hydraulic control unit (HCU) solenoid for the left front wheel
- DExcessive air gap between the left front wheel speed sensor and its tone ring
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive air gap between the left front wheel speed sensor and its tone ring
An erratic or intermittent signal from a wheel speed sensor, especially at low speeds, often indicates a problem with the sensor's ability to accurately read the tone ring. An excessive air gap directly affects the sensor's signal strength and consistency, leading to performance issues.
Why the other options are wrong
- A. A faulty ABS control module would likely affect multiple sensors or system functions, not just one specific wheel's sensor signal in this manner.
- B. Low brake fluid level would typically trigger a brake warning light and affect pedal feel, not specifically cause a wheel speed sensor circuit performance fault.
- C. A malfunctioning HCU solenoid would affect braking performance and trigger different fault codes related to hydraulic control, not wheel speed sensor signal issues.
WSS Air Gap Impact
The air gap between a wheel speed sensor (WSS) and its tone ring is critical for accurate signal generation. An incorrect or excessive air gap can lead to erratic or absent speed readings.
- Too large an air gap weakens the signal.
- Too small an air gap can cause physical damage.
- Affects passive and active WSS differently but both require proper spacing.
Memory trick: Sensor's Gap: Too Far, No Star!