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How does the CKP work2017-11-06T12:29:42+01:00

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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:

  1. The Ignition System could produce spark.
  2. 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.

CKP sensor #1

CKP sensor #1

CKP electrical schematic in RQB engine

CKP electrical schematic in RQB engine

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.

RQB engine CKP inductive sensor (reference part: Ford 948F 6C 31 SAA)

RQB engine CKP inductive sensor (reference part: Ford 948F 6C 31 SAA)

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.

CKP inductive sensor signal example

CKP inductive sensor signal example

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.

CKP Sensor in RQB engine #1

CKP Sensor in RQB engine #1

If the CKP isn’t creating a voltage signal (for whatever reason), then the main measurable effect of this condition is (but not limited to) is the fuel pump isn’t work at all, because if the ECM/ECU can’t detect the crankshaft position sensor voltage signal it will not send any voltage to the fuel pump, so the fuel can’t be pumped to the engine, and, of course, it won’t start.

In our case, this has already happened to us in our Lotus Seven VM77. CKP issues in this RQB engine are a well known matter, so we could diagnose the problem quickly and solve it replacing this part, but, in other cases, the diagnosis could be harder than this so, our advice is you have to discard all the possible causes if your engine doesn’t start properly.

Here you have a video about the functioning of CKP performed by our colleagues from ADPTraining, enjoy it!

We hope our first technical article has been useful for you 🙂

Thank you PetrolHeads!

Comment anything you want! 😉

How does the CKP work
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