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3800 gm use labscope to check if timing chain has jumped

  • craig.phillips
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24 Apr 2019 08:45 #28962 by craig.phillips
How can I use my labscope to check timing chain on a 01 buick lesarbe with a 3800 motor. I think I need to check the 3x and cam sensor at the same time just not sure what to look for.

thanks

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  • Tyler
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24 Apr 2019 11:32 #28964 by Tyler
For that engine, you'd want to check the relationship between the 3X CKP and the CMP. You can tap into both at the Ignition Control Module connector. Pin J brown/white for the CMP, and pin H (light blue/white) or pin G (yellow) for the CKP. I don't recall which CKP pin is which, 3X or 18X. :silly:

You'll also need a known good to compare to. I don't have one on hand, but I remember that the CMP pulse centers over every other 'medium' pulse of the 3X sensor.

If you haven't seen it already, this video will help with CKP identification:

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24 Apr 2019 12:52 #28966 by Chad
From the Pico Library:


"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."
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24 Apr 2019 12:53 #28967 by craig.phillips
My cam sensor is right in between the medium 3x signal. so i have backed out of thinking it is out of time. I also put my scope in the number 1 injector and the cam signal wire and the injector on/off square wave is stacked on top of my cam sensor square wave. So now I have started looking at other reasons for a no start condition. I have noticed I never get a cam sync at my scanner. but i have the cam square wave at pcm. I am leaning towards a pcm issue at this moment..

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25 Apr 2019 10:16 #28984 by Tyler
So you do have injector pulse, then? That's good news for the ECM. :cheer:

Are you missing anything else? Spark, fuel? Will it run with an external fuel source?

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16 May 2019 14:55 #29720 by GypsyR
I wouldn't even do any of that stuff. For a quick rough idea I would do like a relative compression test. Disable fuel. Ignition signal #1. Amp clamp on the battery cable. Crank and observe how close #1 fires to your compression peak. You want to see something like 10 degrees BTDC so just to the left of the peak. This isn't precision setup, so don't seat the actual degrees. If it looks like it's firing way early, right at TDC , or way after, there you go.

First time doing this you probably want to try the setup on another car or two (whatever kind of car is handy) that are in good shape so you know what it's supposed to look like.

There are more precision ways to do this where you can be sure it has jumped and even figure out by how much. THis is just a quickie "Is it broken?" check.

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