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.