No RPM · lost sync · crank sensor
Crankshaft sensor troubleshooting: Hall vs VR signals
No RPM while cranking, a cut-out when hot, a tachometer that jumps — all can be the crank sensor or its wiring. The checks differ for a Hall sensor and a variable-reluctance (VR) sensor, so start by knowing which one you have.
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Hall sensor checks
- Ignition on, sensor plugged in: supply pin shows 5 V or 12 V against ground; ground pin shows under 0.1 V against battery −.
- Signal: turn the engine slowly by hand — the voltage should switch between near 0 V and the high level as teeth pass.
- Signal stuck high: sensor dead or missing pull-up. Stuck at 0 V: signal shorted to ground or sensor dead.
VR sensor checks
- Unplugged, measure across the two pins: a few hundred to about a thousand ohms (check the spec). Open or near 0 Ω — the sensor is faulty.
- Neither pin should have continuity to ground or to the sensor body.
- While cranking, an AC voltmeter across the pins should show from a few hundred millivolts up; too little means too big an air gap or a weak sensor.
- Polarity matters: swapped wires move the edge the ECU triggers on, so timing is off or it never syncs. Swap them and check timing with a lamp.
Symptoms and causes
| Symptom | Likely cause |
|---|---|
| No RPM while cranking | No supply or ground (Hall), open coil or wire, wrong input type set in the ECU |
| Starts, then cuts out when hot | Sensor failing with heat, cracked solder inside, wire touching the exhaust |
| RPM jumps, misfires at high revs | Shield grounded at both ends or not at all, VR polarity, air gap |
| Timing off by a fixed amount | VR wires swapped or wrong trigger settings |
Reproduce it in WireForge
- Open the “ECU fuel injection” demo: the crank sensor has SIG, GND and SHIELD.
- Set Engine speed above 0: the ECU starts firing injectors and coils — that is a good crank signal.
- Remove the crank sensor’s signal wire and simulate again: the ECU reports no crank signal and stops.
- Check that the shield goes to ground at the ECU end only.
Questions
Can a crank sensor fail only when hot?
Yes, it is a classic failure: the internal winding or electronics open up with heat and recover when cold. A heat gun on the sensor while the engine runs can confirm it.
What air gap should a VR sensor have?
Usually around 0.5–1.5 mm; use the engine maker’s value. A bigger gap weakens the signal at cranking speed.
Can I test a Hall sensor with a multimeter?
Yes, at very low speed by turning the engine by hand. At running speed the meter only shows an average; a scope or the ECU’s log is better.