Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is using an oscilloscope to analyze the signal from an inductive wheel speed sensor. The vehicle exhibits an intermittent ABS light. The oscilloscope trace shows a sinusoidal waveform that occasionally drops to zero volts for short periods. What does this waveform pattern MOST likely indicate?
- AA faulty ABS control module.
- BA mechanical issue with the wheel bearing.
- CA short to ground in the wheel speed sensor wiring.
- DAn intermittently open circuit in the wheel speed sensor wiring.
Show answer & explanationAnswer & explanation
Correct answer: D. An intermittently open circuit in the wheel speed sensor wiring.
An inductive wheel speed sensor produces an AC sinusoidal waveform. If this waveform intermittently drops to zero volts, it strongly suggests an open circuit somewhere in the sensor's wiring or the sensor itself. This could be due to a loose connection, a broken wire that makes intermittent contact, or internal sensor damage, causing the signal to disappear temporarily.
Why the other options are wrong
- A. A faulty ABS module would likely affect multiple sensors or cause no signal at all, not an intermittent zero-volt drop on a single sensor.
- B. A mechanical issue with the wheel bearing (e.g., play) might cause signal amplitude variations or noise, but not typically a complete intermittent drop to zero volts.
- C. A short to ground would typically cause the signal to be consistently low or flat-line at zero, not intermittently drop and return.
Inductive Sensor Open Circuit
An open circuit in an inductive sensor's wiring or internal coil will cause its output AC signal to intermittently or constantly drop to zero volts.
- Inductive sensors produce AC voltage from magnetic field changes.
- An open circuit breaks the path for current flow.
- Oscilloscope shows signal loss (zero volts) during the open.
Memory trick: Sine wave goes flat, open circuit's impact, no more signal, that's a fact.