Relative compression

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3 years 1 month ago #46477 by Mike T
Relative compression was created by Mike T
I did a relative compression test on my 1997 Dodge Ram 2500 5.9L Cummins it has some blow by. On the test it showed good compression but after every 6 compression spikes there was a down dip that I didn’t understand any help would be greatly appreciated.
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3 years 1 month ago #46479 by Matt T
Replied by Matt T on topic Relative compression

Mike T wrote: I did a relative compression test on my 1997 Dodge Ram 2500 5.9L Cummins it has some blow by. On the test it showed good compression but after every 6 compression spikes there was a down dip that I didn’t understand any help would be greatly appreciated.


Is that waveform current or voltage? It looks inverted which'd make your dip the top of the best cylinder's compression stroke....

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3 years 1 month ago #46482 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Relative compression
Exactly the questions I was going ask ?

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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3 years 1 month ago #46486 by Mike T
Replied by Mike T on topic Relative compression
It’s on voltage. I’m pretty sure it’s not inverted. The old truck runs good and strong.

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3 years 1 month ago #46487 by Mike T
Replied by Mike T on topic Relative compression
I can check it again tomorrow if I have time but I’m almost positive. And is there anyway to mark a cylinder on a diesel like this? Like you would with spark on gas?

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3 years 1 month ago #46488 by Matt T
Replied by Matt T on topic Relative compression

Mike T wrote: It’s on voltage. I’m pretty sure it’s not inverted. The old truck runs good and strong.


I guess I could have worded my earlier reply betterer. It looks like a cranking voltage waveform that hasn't been inverted. That makes it upside down, compared to a cranking current waveform, with the compression peaks at the bottom of the trace instead of at the top.

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3 years 1 month ago #46489 by Mike T
Replied by Mike T on topic Relative compression
1997 Dodge Ram 2500 5.9l down dip isn’t inverted

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3 years 1 month ago #46490 by Mike T
Replied by Mike T on topic Relative compression
So I should have inverted it?

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3 years 1 month ago #46491 by Mike T
Replied by Mike T on topic Relative compression
It was set at 2v coupling AC and I used my low amp clamp on 60v is that right or should I have inverted it? I’m not positive on if my clamp was on right or not

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3 years 1 month ago #46492 by Mike T
Replied by Mike T on topic Relative compression
Here is a pic of it starting
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3 years 1 month ago #46493 by Mike T
Replied by Mike T on topic Relative compression
Sorry I said I used my amp clamp. I didn’t use my amp clamp it wouldn’t fit over the wire. So I just used my yellow leads.

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3 years 1 month ago #46494 by AJeep18
Replied by AJeep18 on topic Relative compression
Yes your waveform is upside down. When the truck first starts cranking is when it will draw maximum current, but in your picture it is going negative. Hit the invert button on your scope.
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3 years 1 month ago #46495 by AJeep18
Replied by AJeep18 on topic Relative compression
Also if you notice on the original waveform picture, it looks like (because it is inverted) every single has the exact same amount of compression, and while they should all be very close, I don't usually seem them in a perfect line like that.
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3 years 1 month ago #46496 by AJeep18
Replied by AJeep18 on topic Relative compression

Andy.MacFadyen wrote: Exactly the questions I was going ask ?


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

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3 years 1 month ago #46497 by Mike T
Replied by Mike T on topic Relative compression
Thanks! Is there anyway to mark one of the cylinders like a coil on gas engine? And is there anyway to hook to the injector pump and test injectors

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3 years 1 month ago #46499 by AJeep18
Replied by AJeep18 on topic Relative compression
There is but I am not 100% certain how.
I *think*:
On those 5.9's there are those connectors by for the valve cover gasket/ injector wiring. I believe if you probed the command wire for a know injector this would allow you to sync TDC of that cylinder with your compression test. But some engines run pre and/or post injections so it you would have to figure out which one is the primary injection. That is the best I have.
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3 years 1 month ago #46500 by AJeep18
Replied by AJeep18 on topic Relative compression
Andy sent me this as far as testing injectors. (credit where credit is due :) )

www.picoauto.com/library/automotive-guid...sch-current-at-idle/
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3 years 1 month ago #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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3 years 1 month ago #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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3 years 1 month ago #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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