Crankshaft Position Sensor
On a fuel injected engine the crankshaft position sensors input is used by the engine control module for ignition and fuel injector timing. This sensors signal is often used along with the camshaft position sensor to compute ignition timing. Most cars will have either a hall-effect or a magnetic impulse triggering system. The hall-effect sensor is often located in the distributor. The permanent magnet crankshaft position sensor is located behind the harmonic balancer or directly on the engine block. 
Distributorless ignition systems require a crank sensor. They are best diagnosed with a scan tool and an ohmmeter. Expect to see a code with a small description whether there is an intermittent, low, or high input. Always check the vehicles manufacturers specifications before performing any tests on a crankshaft position sensor. These sensors contain a coil and have different values of resistance.
They require a reflector ring located on or around the crankshaft to trigger the signal to the ECM. These rings come in lots of shapes and sizes. The more teeth on the ring the more accurate the signal. If one of these teeth were to become damaged, missing, or bent while installing a timing belt the vehicle will either not start or run rough once started.
These sensors are often located behind the harmonic balancer and can easily become contaminated from hot engine oil or coolant. Check the waveform with a scanner or oscilloscope and make sure amplitude and frequency remain consistent. Use an ohmmeter to check the resistance against specifications. The symptoms of a faulty crankshaft position sensor include no start, intermittent start, and poor engine performance. This is because this sensor effects fuel and ignition timing. A faulty sensor will have a great effect on an engines performance.
