The Basics Of Crank And Cam Sensors And How To Test Them

  • By: Abraham Torres-Arredondo
  • Created: 01 May 2011
  • Updated: 13 July 2026
  • Articl Id: 20
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What ‘Excites’ The Sensor To Produce Its Signal?

Before leaving the ‘working theory’ alone, I need to talk about one more thing. I'm sure you've noticed the term ‘toothed disc’ thru' out this article. This is some sort of disc with teeth that resembles a gear or a wheel with shutters on it. This disc is what ‘excites’ the sensor into producing its signal. These discs go by names such as: shutter wheel, reluctor wheel, armature, interrupter ring, and the like. The name depends on who built the vehicle and/or the specific type of position sensor being used.

These toothed discs rotate only when the engine is cranking and/or running. They are directly or indirectly connected to the crankshaft or camshaft. Although they all perform the same basic job of exciting the CKP or CMP sensor, they come in all shapes and sizes which are determined by the needs of the software and hardware of the fuel injection system installed in the vehicle.

Well, it doesn't really matter what they look like and it definitely doesn't matter one bit what they're called. The important thing to know is that the sensor works in conjunction with some sort of ‘toothed disc’ to produce its signal.

Where Are The CKP And CMP Sensors Located?

These sensors are located in several different places depending on the year, make and/or model that you're working on. Some of them are in very hard to reach places and this, in my opinion, is the only thing that complicates testing them.

Some of their more common locations are:

  1. In the distributors.
  2. On timing covers.
  3. Behind timing covers.
  4. On the engine blocks themselves. In this type of setup, the sensor goes thru' the block to reach its toothed disc. Examples of this are the GM 3.1 and 3.4L V6 engines.
  5. On transmission bell housings. Chrysler, Dodge, and Jeep cars and trucks are the major ones that use this setup.
  6. Behind the crankshaft pulley.

Now, if you don't know where your particular car's (or truck's) crank or cam sensor is at, this is where owning a good repair manual comes in handy (or Googling it on the internet).

Symptoms Of A Bad Crankshaft Position Sensor

We can take for granted that when a CKP sensor goes bad, your vehicle will not start. It'll crank but not start. But hey, a car (or truck) could not start due to a ton of different reasons like: a bad fuel pump, a bad ignition coil, a bad ignition control module, bad spark plug cables, etc. Therefore, it's not enough to say that your car or truck won't start, what you need to know are some of the measurable/testable effects/symptoms that a bad crankshaft position sensor has on the ignition system.

And so, if the crankshaft position sensor isn't creating a signal, then the measurable/testable effects of this condition are but not limited to:

  1. No fuel injector pulse.
  2. The Triggering device (whether it's the ignition control module or the F.I. Computer) will not produce a Switching signal to the ignition coil.
  3. No spark coming out of the ignition coil or coils.
  4. On some makes, like Chrysler/Dodge/Jeep, the fuel injection computer will not continue to power the fuel pump or the ignition system with 12 Volts after an initial ten seconds or so.

What Tools Do I Need To Test The Crankshaft And Camshaft Positions Sensors?

You don't need expensive tools and/or expensive testing equipment to test these CKP and CMP sensors. Here's what you'll need:

  1. The car or truck's battery must be fully charged.
  2. A digital multimeter that can read Hertz frequency.
  3. A good repair manual. The repair manual will probably be just one of many information resources that you'll use to diagnose the CKP or CMP sensors on your car or truck.
  4. You'll need someone to help you crank the car's or truck's engine while you observe the readings on the multimeter.
  5. A fuel injector Noid light.
  6. You don't need an automotive scan tool (commonly known as a scanner).
  7. You don't need oscilloscope.

Do I Need An Automotive Scan Tool To Test The CKP And CMP Sensors?

I've already covered this Ground a bit, but I'll restate that these sensors are tested without a scan tool. Now before I ruffle some feathers, let me explain that the majority of cars and trucks on the road will not set a crankshaft position sensor trouble code when the sensor goes bad. This is not an absolute truth, mind you. But in my experience, about 95% percent of the bad crankshaft position sensors that I've replaced, the vehicle's onboard self-diagnostics didn't leave any type of CKP sensor code! As you might already know, such a code (or codes) can give you an idea of what is going on and/or where to start the diagnostic process.

OK, even if you tried using a scan tool, most makes and models will not let you have access to the Live Data (to read the RPMs) that the scan tool provides while you're cranking the car or truck. So if you have no live Data, you won't know/see if there is an RPM signal on the scan tool's display screen (in case you didn't know, the scan tool displays the RPMs from info from the CKP sensor). Therefore, knowing how to test them with a multimeter (or an LED or an oscilloscope or whatever) independent of a scan tool becomes very important.

Now, when it comes to camshaft position sensors, a scan tool does come in handy since a bad CMP sensor does register a diagnostic code. This code usually lights up your check engine light on your instrument cluster. But testing them requires a method that is independent of the scan tool, and well, as I've mentioned before, the test steps that apply to a CKP sensor also apply to a CMP sensor.

The other thing that really sucks, when you're trying to diagnose a CKP or CMP sensor, is that most of the service literature does not have very specific test information. After all, these service manuals take for granted that the person reading them are professional service technicians that already know the basic working theory and/or tests.

Alright, let's jump into the next part where I get into some testing specifics.

You've covered a lot of information so far, in this section, I'll get into the basic flow of tests that are part of diagnosing the crankshaft position sensor (and camshaft position sensor).

Do's And Don'ts When Testing CKP And CMP Sensors

Testing the crankshaft position sensors or camshaft position sensors requires that you test them in action, that is with the engine cranking. So it goes without saying that you have to be very careful and use tons of common sense so that you won't get hurt.

One piece of advice that I have always followed religiously (and that you should too) has been to have my helper wait outside of the car or truck, I'm testing, till I need him or her to crank the engine up for me. This way, I can and have avoided losing a finger or getting hurt in case my helper thought he or she heard me say "crank it" and cranks the engine while I still have my hands in or around the engine.

When piercing the signal wire(s) of the CKP or CMP sensor, you need to use wire-piercing probes. Why? Because using a wire-piercing probe is probably the safest way to keep from shorting out any of the wires that you're testing. Also, the wire-piercing probe will always leave a small puncture wound in the wire's insulation.

When performing the spark test, always use a dedicated spark tester. The only one that I recommend you use is the HEI spark tester.

Everything always boils down to being alert and taking all necessary safety precautions!

What Does Each Wire (Circuit) In The Connector Do?

OK, now to get into the ‘meat and potatoes’ of testing these CKP and CMP sensors, you need to know what each wire does in the connector that attaches to the crank or cam sensor. Since you're dealing with two types of sensors, it's logical to conclude that each circuit will provide a different type of signal to or from their respective sensors and fuel injection computer or ignition control module.

In this primer, I can only go as far as to give you a basic general idea of what each circuit does. To find out what each circuit does (wire) in the CKP or CMP sensor's connector of your specific car or truck, you'll need to look at a wiring diagram of the ignition system in a professional service manual. The next best place, of course is to Google it on the Internet.

Basic Circuit Description Of A Three Wire Sensor

On this type of sensor, each of the three wires has a specific job to do. Here's the breakdown:

  1. One wire is the power source and it normally provides 12 Volts although some provide 9 Volts.
    • You'll test for this voltage with your multimeter in DC Volts mode.
  2. One wire is the Ground path for the above 9 or 12 Volts. This Ground is generally provided inside the fuel injection computer or the ignition control module, but not always.
    • You'll test for this Ground with your multimeter in DC Volts mode.
  3. The third wire is the Triggering signal wire. It's thru' this wire that the crank (or cam) sensor sends the signal it produces to the fuel injection computer or ignition control module.
    • It's on this wire that you'll connect/attach the red lead of your multimeter to test for the signal.
    • The black lead you'll connect to Ground.
    • The multimeter will have to be either in Volts DC mode or frequency (Hz) mode to verify the signal.
    • The rule of thumb, if you're using Volts DC mode, is that this signal should output the amount of voltage that come's into the sensor on the power circuit. So, when you crank the engine, you should see anywhere between 9 to 12 Volts.
    • If the CKP or CMP sensor is bad, you'll get no reading.

Basic Circuit Description Of A Two Wire Sensor

Since this type of sensor only has two wires and no power supply, testing them is not that hard:

  1. One of the two wires is the signal wire that sends the signal to the fuel injection computer of ignition module.
  2. The other wire acts as a Ground return. This Ground is always provided by the fuel injection computer or the ignition control module.
  3. On this type of sensor, you'll connect both multimeter leads to both wires. That is the red lead can be connected to either of the two. The black lead is connected to the remaining one. It doesn't matter which lead goes where, since the polarity does not matter.
  4. Your multimeter has to be in Volts AC mode to see this signal.
  5. When your helper cranks the engine, the multimeter will display about 1 Volt AC. Usually, this AC voltage will move between 0.3 Volts AC to 1 Volt AC the whole time the engine is cranking, this is normal. If the sensor is bad, the multimeter will not display any AC voltage.
    • This voltage increases with engine RPMs. So the faster the engine cranks, the higher the AC voltage.

What Are The Actual Testing Steps

Testing a CKP sensor isn't hard and the diagnostic flow is pretty straightforward. The following testing path applies to a 'cranks but does not start' condition. Although the following tests only apply to a crankshaft position sensor that has failed completely, with some modification you can also follow the same diagnostic path in diagnosing a CMP sensor.

  1. Step One.
    • Make certain that the battery is in a fully charged condition.
    • Test for spark.
      • You'll need to test for spark at all cylinders to ascertain that there's no spark present at all.
      • If spark is present, the crankshaft position sensor is working properly.
    • Test for fuel injector pulse, although depending on the fuel system design this is not always possible.
      • If the fuel injector pulse is present, the crankshaft position sensor is working properly.
  1. Step two.
    • Find the location of the crankshaft position sensor.
    • Determine type of CKP sensor (either a two or three wire type).
  2. Step three.
    • On three wire type CKP sensors:
      • Determine which wire is the power circuit.
      • Determine which wire is the Ground circuit.
      • Determine which wire is the signal wire.
    • On two wire type CKP sensors:
      • You don't have to determine which wire is which since you don't have to test for a power supply. Also, the multimeter's leads are hooked up to both wires at the same time to read the signal the sensor produces.
  3. Step four.
    • On three wire type CKP sensors:
      • Probe the power circuit to verify the presence of the specified voltage. This voltage is usually verified with the key on or engine cranking.
      • Probe the Ground circuit to verify that Ground does exist. This Ground is usually verified with the key on or engine cranking.
      • Probe the signal wire. The presence of this signal can only be verified with the engine cranking and with the multimeter in Hertz (Hz) frequency mode or in Volts DC mode.
    • On two wire type CKP sensors:
      • Probe both wires coming out of the sensor with both leads of your multimeter. The polarity of the leads doesn't matter. In other words, the red and black lead can go to any of the two wires.
      • The presence of this signal can only be verified with the engine cranking and with the multimeter in Volts AC.
  4. Step Five.
    • If no signal is present:
      • The crankshaft position sensor is bad, replace it.
    • If a signal is present:
      • The crankshaft position sensor is good.

As always, things on paper always seem easier than in their actual application and this is sometimes true when you're testing CKP and CMP sensors. For example, in some cars and trucks, it's next to impossible to verify the fuel injector pulses with a fuel injector Noid light. Why? Well because the engineers have located them inside or underneath the intake manifold plenum. Don't let this deter you, it's just a matter of skipping this test and going on with the rest.

Once you overcome these obstacles, you'll find that testing them is not that hard. The one thing I recommend you do, is to practice on a good working car to see how all of this works. Well, in closing, if you found this article helpful -please tell a friend!!!

Related Crankshaft Position Test Articles

Below you'll find a list of test tutorials that will show you how to test the crankshaft position sensor for several different types of makes:

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