Relative compression

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03 Feb 2021 21:14 #46501 by AJeep18
Replied by AJeep18 on topic Relative compression
Are you having a problem with your truck or just want to see what kind of health it is in?

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03 Feb 2021 21:40 #46503 by Mike T
Replied by Mike T on topic Relative compression
That is good information! Thanks for it but this is an older model there is no wires going to the injectors the injectors are fired off of pressure.

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03 Feb 2021 21:41 #46504 by Mike T
Replied by Mike T on topic Relative compression
Just checking it’s Health and playing with my scope trying to get it all figured out

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03 Feb 2021 21:58 #46505 by AJeep18
Replied by AJeep18 on topic Relative compression

That is good information! Thanks for it but this is an older model there is no wires going to the injectors the injectors are fired off of pressure.


Oh yeah right second gen... SMH

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03 Feb 2021 22:19 #46506 by AJeep18
Replied by AJeep18 on topic Relative compression
You could scope the cam and/ or crank sensor signals, then you would have a known good for the future. I'm not sure how much more there is to look at on those older engines.

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04 Feb 2021 12:01 #46514 by Mike T
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This keeps telling me error

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04 Feb 2021 12:05 #46515 by Mike T
Replied by Mike T on topic Relative compression
This is a screen shot of one of my other dodges it’s a 2002 Dodge Ram 3500. 5.9l 24 valve second gen and it’s not inverted either but now I’m confused. What way is up?
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04 Feb 2021 12:09 #46516 by Mike T
Replied by Mike T on topic Relative compression
I know I might be driving you crazy with this lol but if that one is upside down then it’s almost perfectly matched on the compression

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04 Feb 2021 12:18 #46517 by Mike T
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here is one of my 2011 Ford F-150 5.0l
It all look right but I could have filtered it and inverted my coil but it’s right. Right? Lol I have done a few different tests this was my first one with amp clamp and and it was on backwards I thought. And I didn’t ac couple this one. So should I ac couple? Sorry For so many questions

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04 Feb 2021 12:22 - 04 Feb 2021 13:43 #46518 by Chad
Replied by Chad on topic Relative compression

not that it is really any easier than using the invert function, but could one just switch the test leads around also?


Yes, you can reverse the leads to invert the waveform. That is how I do it with the Pico. But, know your scope. If your scope shares a common ground with all channels, (and Snap-on does), then you may run into some unexpected results. :woohoo: With Snap-on, I would recommend using the Invert function.

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

I have learned more by being wrong, than I have by being right. :)
Last edit: 04 Feb 2021 13:43 by Chad.
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04 Feb 2021 14:01 #46521 by AJeep18
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Are you using the guided component testing feature?

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04 Feb 2021 14:07 #46522 by AJeep18
Replied by AJeep18 on topic Relative compression
These are the settings I used and the capture I got on a 6.7 Cummins with a bad cylinder, black lead on ground yellow lead on the positive battery cable
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04 Feb 2021 14:08 #46523 by AJeep18
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I do not use an amp clamp. Just use Volts with AC coupling on

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04 Feb 2021 17:38 #46526 by Mike T
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i dont really use the guided one. well i do sometimes but alot of time i just try to figure out the right settings so im use to it if there isnt a guided test. but i still use it to try and get the right idea the first time i test something. i guess i should use it more so i dont have to bug you so much lol

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04 Feb 2021 17:42 #46527 by Mike T
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Did you see that picture i put of my 2002 dodge the compression looks so close that it looks right side up and its not inverted the spikes across the bottom of the screen witch should have been the top are straight across... lol

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06 Feb 2021 23:41 #46566 by DougS
Replied by DougS on topic Relative compression
Hi Mike,
It looks as if you did use a voltage scale on your 2002 Dodge and that is how it should look normally. As the starter is cranked, the voltage load on the battery drops as a cylinder hits it's compression stroke and then comes back up until the next cylinder hits compression when it will drop again and so on until you stop cranking. There is actually no need to invert your pattern because that WOULD make the result confusing. Also, unless there is an uneven cranking rhythm (like in your capture of the other Dodge), I would not be too concerned with trying to find a way to synch your capture with something. At some point in the past, I thought Snap-on had a way to synch a diesel by means of clamping some device connected to your scope to a diesel injector tube.
I hope this helps. Take care,
DougS
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07 Feb 2021 09:21 #46570 by Mike T
Replied by Mike T on topic Relative compression
I haven’t found anything to clamp to the injector tube yet. I will look in to that. Thanks for the help

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