TEST 3: Making Sure The Crank Sensor Is Getting Power
Now that you have verified the basics, in this test you're gonna' verify that the ignition control module is supplying 12 Volts to the camshaft and crankshaft position sensors.
They're both fed power from the ICM thru' the same wire. This is the wire labeled with the letter N in the illustration above.
IMPORTANT: The ignition module must be connected to its electrical connector to be able to read the voltage the ICM is sending the crankshaft position sensor. I suggest using a wire piercing probe to check for this voltage.
These are the steps:
With the red multimeter test lead and a wire piercing probe, probe the WHITE with BLACK stripe wire of the connector shown in the figure above.
This is the wire labeled with the letter N in the photo above.
With the black multimeter test lead probe the battery negative (-) terminal.
Turn the key to the ON position but don't crank or start the engine. You should see 10 to 12 Volts on the multimeter.
Let's take a look at what your test results mean:
CASE 1: Your multimeter registered 10 to 12 Volts. This tells you that the module is supplying power to the crank sensor (and cam sensor if equipped with one).
The next step is to verify that the crank sensor is producing the CRANK 18X signal. For this go to: TEST 4: Verifying The 18X CKP Sensor Signal.
CASE 2: Your multimeter DID NOT register 10 to 12 Volts. This result confirms that the module is fried and needs to be replaced.
Here's why: The ignition module is the only source of power for the crank (and cam) sensor, so if you have no power at all in this wire, the module is defective.
TEST 4: Verifying The 18X CKP Sensor Signal
Now we're gonna' test the CRANK 18X CKP signal from crankshaft position sensor. The wire that carries the CRANK 18X signal is the one labeled with the letter G in the illustration above.
We can check for the presence of this crank signal by using an LED light test tool. To see an example of this tool, check out this link: The LED Light Test Tool and How To Make One.
NOTE: I've received a ton of feedback (these past two years, since I wrote this article) on this specific LED test, and it all has been 99.9% good. This LED light test works! But, in an effort to give my readers another choice, I've written a tutorial on how to to accomplish this same test using a multimeter, instead of the LED light. You can find it here: Multimeter Test of the 3X 18X GM 3.8L Crank Sensor (this article is found at troubleshootmyvehicle.com).
IMPORTANT: The ignition module must be connected to its electrical connector to be able to test the crankshaft position sensor CRANK 18X signal. I suggest using a wire piercing probe to check for this signal.
OK, here are the test steps:
With a suitable tool, pierce the YELLOW wire of the ignition control module (ICM) connector. Pierce as far back as possible from the connector itself.
This is the wire labeled with the letter G in the photo above.
Connect RED wire of LED to the tool that is piercing thru' the wire.
Connect BLACK wire of the LED to the battery negative (-) terminal.
Have an assistant crank the engine while you observe the LED
When your assistant cranks the engine, the LED will stay lit continuously till the moment he lets go of the key and the engines starts to slow down to a stop. Only then will you be able to see the LED blinking ON and OFF. This is normal. You may have to repeat this step as many times as necessary to ensure you do see the LED blinking ON and OFF (when your assistant lets go of the key).
Oscilloscope users see bottom image for full size picture of scope trace.
Lets take a look at what your test results mean:
CASE 1: The LED light blinked ON and OFF. This means that the crank sensor is creating the 18X CKP signal.
Now you need to test that the 3X CKP signal is also present. Go to: TEST 5: Verifying The 3X CKP Sensor Signal.
CASE 2: The LED light DID NOT blink ON and OFF. This tells you that the crankshaft position sensor is not creating the CRANK 18X signal.
Without this signal the ignition control module (ICM) will not spark the ignition coils. The crankshaft position sensor is defective and needs to be replaced.