The crankshaft sensor is an inductive type sensor. It consists of a magnet and a coil wound around the magnet. It is triggered by reference points passing the sensor tip. The reference points can be pins mounted to the flywheel in the form of a trigger disc or relucter. These will divide the 360 degrees rotation of the engine into segments that the engine control can use for the ignition system or for the start of Injection. Each time a reference point passes the tip of the sensor it disturbs the magnetic field creating an AC voltage in the coil.The AC voltage is sent as a signal voltage back to the engine control unit. The crankshaft sensor or engine speed sensor will have 2 or 3 pins. The 3rd pin is used for a screen. The screen suppresses any interference. The engine management light will be triggered if the crankshaft sensor moves outside of normal operating values. A fault with this sensor may inhibit engine startup.