🧲Crankshaft position sensor (TDC sensor)

How it works

The crankshaft position sensor, also called the TDC or engine speed sensor, is usually a Hall-effect or inductive pickup reading the teeth of a trigger wheel that turns with the crankshaft. From that signal the ECU works out crank angle and engine speed, the baseline information without which it can neither time the spark nor fire the injectors. That makes it a vital component: if the signal disappears or turns erratic, the engine cuts out on the spot, hard and completely, or refuses to start at all. Typical failures are a worn sensor or one shorted internally, an air gap (the clearance between sensor and reluctor wheel) that has drifted out of spec, a wheel fouled with metal filings and, very often, a corroded connector or damaged wiring that mimics a dead sensor perfectly. A tired sensor tends to drop out once everything is hot. The fault normally lights the check-engine light (MIL) and stores a code. Diagnosis, meaning reading the codes, checking the signal on a diagnostic tool or a scope, measuring air gap and resistance and inspecting the harness, is engine-electronics work for a properly equipped shop: the connector and the wiring are always ruled out before the sensor is condemned.

Maintenance tips

• ✓If a check-engine light comes with the engine cutting out, get the fault codes read straight away: the crankshaft position sensor is the prime suspect behind a sudden stall.
• ✓Before replacing the sensor, which can be pricey, have its connector and wiring checked: corrosion or a broken wire behaves exactly like a dead sensor.
• ✓Keep the sensor and trigger wheel area clean: metal filings pulled in by the magnet are enough to upset the signal.