A crank sensor is an electronic device on most automobiles that detects engine speed by monitoring the rotation of the crankshaft. Most newer vehicles us a crank sensor in conjunction with camshaft position sensor to provide data that the automobile's computer uses to coordinate the engine ignition sequence.
The computer depends on this information to calculate the appropriate amount of fuel to inject and the timing of the spark plug sequence.
Crank sensors consist of two parts. One is a wheel or disk with teeth or slots mounted on the camshaft that rotates with the shaft. The other part is the sensor, which can be mechanically operated by physical detection from contact with the rotating teeth or electronic detection that uses an LED light to read the passing slots.