Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is diagnosing an intermittent issue with a vehicle's ABS system. The scan tool shows a C0035 (Left Front Wheel Speed Sensor) code. The technician uses an oscilloscope to capture the waveform from the left front wheel speed sensor while rotating the wheel by hand. The waveform shows a consistent, clean square wave with appropriate voltage swings, but sometimes it briefly disappears. What is the MOST likely cause?
- AA damaged ABS reluctor ring
- BA faulty wheel speed sensor
- CAn internal fault within the ABS control module
- DAn intermittent open or poor connection in the wheel speed sensor wiring
Show answer & explanationAnswer & explanation
Correct answer: D. An intermittent open or poor connection in the wheel speed sensor wiring
A clean square wave indicates the sensor itself is generating a good signal and the reluctor ring is intact. If the signal intermittently disappears, it points to an issue in the signal's transmission path, such as an intermittent open circuit or a poor connection in the wiring harness between the sensor and the ABS module.
Why the other options are wrong
- A. A damaged reluctor ring would typically cause an inconsistent or distorted waveform (missing teeth, irregular amplitude), not a complete, brief disappearance of an otherwise good signal.
- B. A faulty sensor would typically produce a consistently bad signal (no signal, distorted, intermittent amplitude), not a perfectly good signal that completely drops out intermittently.
- C. An internal ABS module fault would usually manifest differently, perhaps with no signal at all, or a module-specific code, rather than an intermittent loss of a good sensor signal.
Intermittent Open Circuit
A fault in an electrical circuit where the connection is temporarily broken, often due to loose terminals, damaged wiring, or corroded connectors, causing intermittent component operation.
- Signal disappears completely
- Often related to movement or vibration
- Difficult to diagnose without dynamic testing
Memory trick: Wave's a ghost, where'd it go? Wiring's toast, or connection's slow.