The crankshaft sensor sits near a tone wheel (often on the flywheel or crank). As the engine turns, it reads the passing teeth and sends a signal to the ECU showing:
- Engine speed (RPM)
- Crank position (where the pistons are in the cycle)
- A reference point (often a missing tooth) so the ECU knows “this is home”
The ECU uses that to time fuel injection precisely (pilot injection, main injection, post injection) and to control things like idle stability and starting.
Most diesels use either:
- Magnetic/inductive (2-wire VR sensor): makes its own signal from the moving teeth
- Hall effect (3-wire): powered sensor that outputs a digital on/off signal
Why metal filings cause problems (especially on diesel)
Metal filings are a headache for two main reasons:
1) They build up on the sensor tip and change the air gap
Many crank sensors have a magnetic core, so they attract filings. That buildup effectively increases the distance between sensor and tone wheel, and the signal becomes weak—especially during cranking.
2) They distort the signal the ECU is trying to read
The filings mess with the magnetic field and the “clean edges” of each tooth passing. The ECU can get:
- dropouts (missing pulses)
- false pulses
- inconsistent tooth counting
What that does to a diesel engine
On a diesel, if the ECU can’t confidently read crank position and speed, it can’t time injection properly. That shows up as:
- Hard start / long crank (ECU won’t inject properly until it syncs)
- Intermittent no-start
- Rough idle or uneven running
- Engine light with crank sensor / sync faults
- Sometimes stalling if the signal drops out while running
