how to crankshaft position sensor is work

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Crankshaft position sensor
Ford Crankshaft position sensor (CKP)
The crankshaft position sensor measures the rotation speed (RPMs) and the precise position of the engine crankshaft. Without a crankshaft position sensor the engine wouldn't start. 

In some cars, the sensor is installed close to the main pulley (harmonic balancer) like in this Ford in the photo. In other cars, the sensor could be installed at the transmission bell housing, or in the engine cylinder block, as in the photo below. In the technical literature, the crankshaft position sensor is abbreviated to CKP.

How the crankshaft position sensor works

Crankshaft position sensor
In this GM engine, the crankshaft position 
sensor is installed at the cylinder block
The crankshaft position sensor is positioned so that teeth on the reluctor ring attached to the crankshaft pass close to the sensor tip. The reluctor ring has one or more teeth missing to provide the engine computer (PCM) with the reference point to the crankshaft position.

As the crankshaft rotates, the sensor produces a pulsed voltage signal, where each pulse corresponds to the tooth on the reluctor ring. The photo below shows the actual signal from the crankshaft position sensor with the engine idling. In this vehicle, the reluctor ring is made with two missing teeth, as you can notice on the graph.

The PCM uses the signal from the crankshaft position sensor to determine at what time to produce the spark and in which cylinder. The signal from the crankshaft position is also used to monitor if any of the cylinders misfires. 
Crankshaft position sensor signal
Crankshaft position sensor signal on the oscilloscope screen.

If the signal from the sensor is missing, there will be no spark and fuel injectors won't operate. 

The two most common types are the magnetic sensors with a pick-up coil that produce A/C voltage and the Hall-effect sensors that produce a digital square wave signal as in the photo above. Modern cars use the Hall-effect sensors. A pick-up coil type sensor has a two-pin connector. The Hall-effect sensor has a three-pin connector (reference voltage, ground and signal).

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