Relative compression

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03 Feb 2021 17:10 #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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03 Feb 2021 17:35 #46479 by Matt T
Replied by Matt T on topic Relative compression

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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03 Feb 2021 18:19 #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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03 Feb 2021 19:40 #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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03 Feb 2021 19:43 #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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03 Feb 2021 20:07 #46488 by Matt T
Replied by Matt T on topic Relative compression

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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03 Feb 2021 20:13 #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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03 Feb 2021 20:18 #46490 by Mike T
Replied by Mike T on topic Relative compression
So I should have inverted it?

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03 Feb 2021 20:35 #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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03 Feb 2021 20:36 #46492 by Mike T
Replied by Mike T on topic Relative compression
Here is a pic of it starting
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03 Feb 2021 20:53 #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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03 Feb 2021 20:54 #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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03 Feb 2021 20:58 #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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03 Feb 2021 20:59 #46496 by AJeep18
Replied by AJeep18 on topic Relative compression

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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03 Feb 2021 21:05 #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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03 Feb 2021 21:11 #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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03 Feb 2021 21:13 #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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