How The Crankshaft Position Sensor Works
The crankshaft position sensor, on your GM 4.3L, 5.0L, or 5.7L Vortec engine is a Hall Effect type sensor, which in plain English means that it needs a power and a Ground source to produce a signal.
Once the crank sensor produces this signal, the signal is an on/off voltage signal (in DC Volts).
Below, in a nutshell, is how the crankshaft position sensor works in your 4.3L, 5.0L, and 5.7L Chevrolet or GMC (or Olds, Cadillac) Pick Up or SUV:
- When you turn the key and start cranking the engine the crank sensor gets 12 Volts on Circuit A and Ground on Circuit B.
- A round ‘target wheel’ attached to the front of the crankshaft and positioned so that it rotates above and in close proximity to the crank sensor, starts to rotate.
- As the ‘teeth’ on this target wheel pass by the crank sensor, they start to activate the sensor into creating an ON/OFF Pulse.
- ON is when a tooth on the target wheel passes directly over the sensor, which induces the sensor to create and send a 5 Volt DC pulse.
- OFF is when that same tooth passes completely away from the crank sensor, which causes the sensor to drop the voltage down to 0 Volts.
- The PCM uses these ON/OFF 5 Volt pulses to start activating the ignition control module and a host of other things to get your vehicle started.
The most important thing to know about the crank sensor is that if it goes bad, your GM Pick Up or SUV will 'crank but not start' due to a lack of spark and fuel injection.
In my crankshaft position sensor test instructions, I'll ask you to manually rotate the crankshaft pulley by hand because this is the only way that your multimeter will be able to see these ON/OFF voltage pulses. Using the starter motor will make the multimeter average out these pulses and this type of test result is not as accurate.
TEST 1: Testing The CKP Signal With A Multimeter
The crankshaft position sensor is located on the bottom (passenger-side) of the engine timing chain cover. The very first thing you'll need to do is to verify if the crank sensor is producing a crank signal.
This is a very easy, but accurate test, that will verify the performance of the CKP sensor. But remember (and I know that by now I'm starting to sound like a broken record), this accuracy is accomplished by measuring the crank sensor signal with a multimeter and manually turning the engine by hand, since cranking the engine with the starter motor would not accomplish the same result (unless you use an oscilloscope).
NOTE: The crankshaft position sensor must remain connected to its electrical connector to read the CKP signal. You'll need to use a back probe on the connector or a wire piercing probe on the wire. You can see an example of this tool here: Wire Piercing Probe.
OK, here's the test:
Raise the front of the pick up or SUV and then lower it on jack stands. This will also allow you to comfortably crank the engine manually using a 1/2 ratchet and the appropriate socket.
Disable the fuel system. One way of doing this by disconnecting the Spider fuel injector connector.
This is important! By disabling the fuel injectors, you'll keep the vehicle from possibly starting as you perform the crank sensor test.
Disable the ignition system. You can easily accomplish this by simply disconnecting the ignition coil or the ignition control module. This will prevent the ignition system from creating spark.
Connect the red multimeter test lead to the wire labeled with the letter C in the photo above (with an appropriate tool, like a wire-piercing probe).
The wire (circuit) labeled with the letter C is the one that transmits the CKP sensor signal to the PCM.
Connect the black multimeter test lead to the battery negative (-) terminal.
If you decide to connect the black multimeter test lead to a Ground point somewhere underneath the vehicle, make sure it's a nice clean and rust-free spot.
Turn the Key to the On (RUN) position (but don't crank the engine).
Place your multimeter in Volts DC mode.
Manually turn the engine by hand using a 1/2 inch ratchet and a socket on the crankshaft pulley (turn it clock-wise).
Your multimeter will register an ON/OFF voltage signal as you turn the crankshaft pulley.
ON is when the multimeter displays 5 Volts DC and OFF is when it displays 0 Volts (0.1 Volt equals 0 Volts).
Let's interpret the results of your Crank Sensor Signal Test:
CASE 1: The multimeter registered the ON/OFF voltage as you turned the engine with the ratchet. This is the correct test result and it confirmes that the crank sensor is working properly and creating a CKP signal.
No further testing is required since the other two tests in this article check that the crankshaft position (CKP) sensor is getting power and Ground. Since your test confirms that the CKP sensor is creating a signal, then it also proves that it's getting power and Ground.
If you need help diagnosing the no start condition on your vehicle, the following tutorial may be of help:
CASE 2: The multimeter DID NOT register the ON/OFF voltage signal as you turned the engine with the ratchet. This test result usually confirms that the crankshaft position sensor is bad and needs to be replaced. About 90% of the time this is hitting the nail on the head, but I recommend that you check that the sensor is also getting power and Ground too.
By testing and confirming that the CKP sensor is getting power and Ground, you'll be able to decisively confirm it is bad or that maybe another issue exists, go to: TEST 2: Making Sure The CKP Sensor Is Getting Power.