How does the CKP work?
This article will help you to learn and understand the essentials and basic working theory of the CKP Sensor, which is a basic part of the engine. The examples we’ll show you are from the Ford RQB engine (4 cylinders/1.8 petrol injection/96 kw) mounted in our Lotus Seven VM77, but the concept could be applied to almost all engines.
A Crankshaft Position Sensor (CKP) is a magnetic type sensor that generates voltage using a sensor and a target wheel mounted on the crankshaft, that tells the Fuel Injection Computer or the Ignition Control Module the exact position of the cylinder pistons as they come up or go down in the engine cycle. The ECM (Engine Control Module)/ECU (Engine Control Unit) calculates engine RPM by using the sensor’s signal and controls the injection duration and the ignition timing. Using the signal differences sent by CKP, the ECM/ECU identifies which cylinder is at top dead center.
The CKP’s job is to help when:
- The Ignition System could produce spark.
- the Fuel System could start injecting fuel into the cylinders.
In general, there are 58 slots in the target wheel where one is longer than the others. When the slot in the wheel aligns with the sensor, the sensor voltage outputs low. When the metal (tooth) in the wheel aligns with the sensor, the sensor voltage outputs high. During one crankshaft rotation there are 58 rectangular signals and one longer signal.
Basically, there are two different types of CKP, inductive sensors (RQB engine case, with two wires) and optical sensors (also called «Hall effect», with three wires), each one produces a different type of signals (analog and square analog signals respectively). There is a third type of CKP, but, for not making this article too long we will focus on the inductive sensor with analog signals.
The inductive sensor is excited by the target wheel in close proximity with, and CKP starts to produce an analog voltage signal. In this kind of inductive sensor this voltage signal is created without the help of an external power source, it’s created only by the inductive effect caused by magnetism between the rotary target wheel and the sensor itself. As long as the engine is turning, the signal is being produced and sent to ECM/ECU, but when the engine is off, so is the CKP.
What are the symptoms of a bad CKP?
In most cases, if you have a bad CKP your vehicle won’t start. It’ll crank but not start. But hey, a car could not start due to so many different reasons like: a bad fuel pump, a bad ignition coil, a bad ignition control module, bad spark plugs & ignition cables, bad ECM/ECU etc. So, it’s not enough to say that your car won’t start, what you need to know are some of the symptoms that can be caused by a bad crankshaft position sensor.