Crankshaft position sensor

Updated October 12, 2017
Crankshaft position sensor
Ford Crankshaft position sensor (CKP)

The crankshaft position sensor is the primary sensor of the electronic fuel injection and ignition systems in any modern car. It measures the rotation speed (RPMs) and the precise position of the engine crankshaft. Without a crankshaft position sensor the engine wouldn't start.

The placement of the crankshaft position sensor varies. 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 wheel attached to the crankshaft pass very close to the sensor tip. The reluctor wheel typically 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 wheel. The photo below shows the actual signal from the crankshaft position sensor with the engine idling. In this vehicle, the reluctor wheel 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 a 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. Most newer cars use the Hall-effect sensors. Typically, 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).

Crankshaft position sensor problems

Problems with a crankshaft position sensor are common, especially in older cars. If the sensor is not working properly, a car may stall or not start. The engine will turn over, but won't fire up.

Sometimes a failing crankshaft position sensor may cause intermittent issues such as long cranking or stalling in certain conditions, for example, when the engine is hot, or in wet weather. Sometimes a car may stall when hot, but restart after cooled down. In many cars, a failed crankshaft position sensor can also cause a Check Engine light to come on.

The most common OBDII code related to the crankshaft position sensor is P0335 - Crankshaft Position Sensor "A" Circuit. In some cars (e.g. Mercedes-Benz, Nissan, Chevy, Hyundai, Kia) this code is often caused by a failed sensor itself, although there could be other reasons, such as wiring or connector issues, damaged reluctor wheel, etc.

In some cars, the intermittent stalling can also be caused by a problem with

You might also be interested: