CKP TEST 4: Ground Circuit
The ignition control module (ICM) is the one that provides the 3X 18X crankshaft position sensor with a path to Ground internally.
What this means is that you have to very careful not to short this circuit to power as you're testing it, or you could fry the ICM.
NOTE: The ignition module's connector must remain connected to the ignition module for this test to work since it's the ignition module that provides this Ground to the crankshaft position sensor.
This test is done with your multimeter in Volts DC mode too and so, to get this pot of water boiling, this is what you need to do:
Place the multimeter in Volts DC mode.
Connect the black multimeter test lead (using the appropriate tool) to the wire labeled with the letter M in the image above.
Connect the red multimeter test lead to the battery positive (+) terminal.
Turn the key to the ON position but don't start the engine.
Your multimeter should register battery voltage (about 10 to 12.5 Volts).
Now, let's take a look at what those voltage readings (or lack of) mean:
CASE 1: The multimeter showed 10 to 12 Volts when the ignition was turned ON. This test result lets you know that the ignition control module (ICM) is providing a good Ground path for the crankshaft position sensor.
You can correctly conclude that the 3X-18X crankshaft position sensor is bad and needs replacement if you have:
- Confirmed that the CKP is not producing the 3X CKP and/or the 18X CKP signal (TEST 1 and TEST 2).
- Confirmed that the ICM is sending the CKP sensor 10 Volts DC (TEST 3).
- Confirmed, in this test section, that the ICM is providing the CKP sensor with Ground.
If you do need to buy the crankshaft position sensor, take a look at my recommendations here: Where To Buy The Crank Sensor And Save.
CASE 2: The multimeter DID NOT show 10 to 12 Volts when the ignition was turned ON. This result indicates that the ignition module is not supplying a Ground path to the crankshaft position sensor.
This result exonerates the crankshaft position (CKP) sensor as the cause of the cranks but does not start condition you're trying to solve, since without a Ground path, the crankshaft position sensor can't create a signal.
It's very rare that the ignition control module not provide this Ground path. Usually the most common cause of this missing Ground is that the terminal inside the ignition control module connector has worked itself out and is not completely mating with the male spade terminal of the ignition control module (ICM).
Related Test Articles
Testing the 3X 18X crankshaft position sensor can also be done using a simple LED light. You can find this tutorial at easyautodiagnostics.com here: GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
I have written several other 3.8L V6 specific articles that may be of interest to you, you can find them here: GM 3.8L Index Of Articles.
Here's a sample of the tutorials you'll find in the index:
- How To Diagnose Misfire Codes P0300-P0306 (GM 3.8L).
- How To Test Trouble Code P0420 (GM 3.8L).
- How To Clean The MAF Sensor (GM 3.3L 3.8L).
- How To Troubleshoot A No Start (GM 3.8L).
- How To Test The Fuel Injectors (GM 3.8L).
Reader's Real Life Case Studies And Solutions
In this section is input and feedback from all of the folks who have had a similar issue with their vehicle and found a solution. If you're one of them, I want to thank you for sharing your experience with all of us!
If you want to share your repair and/or diagnostic experience, you can use the contact form below.
Real Life Case Study 1
Vehicle: 1995 Buick Park Avenue 3.8L Super Charged
Trouble Codes: None
Complaint: INTERMITTENT NO START “The car would die as I drove it sometimes it would start back up right away and sometimes it wouldnt. Sometimes after parking it and coming back out, it would not start.”
Test Notes: “I had two different shops take a look at it. They replace everything. Computer, fuel pump, ignition module, coilpack, fuel injectors both of them made me spend alot of money on this very old car for nothing because the problem was still there...”
Repair: REPLACED CRANK SENSOR “...I want to tell you that i did not check the crank sensor myself because all i can do is mechanic stuff, this electrical stuff is hard for me, i had a third shop do it. A really young technician had an oscilloscope and he let the car run all day till it all of a sudden died and since he had the oscilloscope connected to the crank sensor, he was able to see that it was bad. The shop replaced the crank sensor and it has been several months now and the problem is solved!!! ...”
“...The reason im sending this to you is to let others know that the crank sensor can start acting up every now and then and you dont talk about this type of problem. I dont have any electronic tools just mechanic tools, but i want to say that finding the right shop with technicians who know 3.8L/how to use an oscilloscope is very important. if they don't have one, DONT GO THERE, they will make you waste time and a lot of money!!! This kid was really smart and finally found the problem, please pass on my advice...”.
Courtesy of: John T.
Real Life Case Study 2
Vehicle: 1990 Buick Lesabre 3.8l
Trouble Codes: None
Complaint: NO START “The car would not restart when hot for at least 4 hours.”
Test Notes: “... no voltage signal (0 v) from either the 3x or 18x wires...”
Repair: REPLACED IGNITION MODULE “This was an awesome write-up on the testing of the CKP sensor. However, I kept on coming up with needing to replace the sensor (was on my 4th already). The car would not restart when hot for at least 4 hours. I would not get any voltage signal (0 v) from either the 3x or 18x wires. When the car finally cooled down enough to restart, I tested these wires again. What I found out is that there was now 7v coming FROM the ICM on both of these wires. I verified this by unplugging the connector at the CKP and was 7v at both terminals there. Therefore, I diagnosed that the ICM was not producing the required 7v on these leads when it was warm. I have since replaced the ICM and has been working great for a week now. You might want to note this as your test kept leading me to the CKP. Thanks for the great write-up!”.
Courtesy of: Jesse (submitted: 09-14-2014)
If this info saved the day, buy me a beer!