Engine-Speed Measurement

Crankshaft Sensor

The crankshaft sensor registers the position of the crankshaft with the aid of a sheet metal wheel with various apertures screwed to the crankshaft. It operates in accordance with the Hall effect principle.

It is necessary for the fully sequential injection system (fuel injection takes place individually for each cylinder at the specific firing point).

Function
A permanent magnet in the sensor produces a magnetic field. A conductor with current flowing through it is located in this magnetic field. An electric voltage dependent on the strength of the magnetic field is produced in this conductor perpendicular to the current and perpendicular to the magnetic field. The magnetic field in the sensor is varied by the crankshaft sensor wheel and its apertures rotating passed the sensor. The teeth and gaps on the sensor wheel can be recognized by means of the voltage.

Sensor wheel
The circumference of the sensor wheel is divided in 60 teeth of equal size. The shorter the signal impulses it creates in the crankshaft sensor, the higher the speed that the engine control detects.

The precise position of the pistons must be known for the synchronization of injection and ignition. To this end, there is a large gap of two teeth lengths on the sensor wheel.

Diagnosis
The number of teeth between the occurrence of two gaps is monitored continuously. The signals of the camshaft sensor are also compared with the signal of the crankshaft sensor and they must lie within specified limits.