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Thread: How many of you ohm your spark plugs?

  1. #1

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    How many of you ohm your spark plugs?

    So I have been doing the 5000 ohm check on plugs when doing the annuals for a few years now, I am finding a lot more failures that surprised me than what I was expecting. Now some of you may say that Champion says that it is not a valid test, but when you have an open circuit inside a plug, that is a fail.

    I am not finding a lot of high ohms (especially with tempest), but I have found 3 plugs with open circuits with he fluke meter. Oh and the mil-spec for aircraft spark plugs does call out an ohm requirement for the plug. I have found 2 Champion and 1 tempest fail this way. If you have a weird miss, do a good check of you plugs, you may be surprised.
    Last edited by astjp2; 10-24-2019 at 04:03 PM.

  2. #2

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    I do it at every annual.

    Had a Pawnee from a visiting glider club fail a mag check this past summer. My fellow mechanic immediately wanted to pull the mag for reasons he could not fully explain. I successfully pissed him off to get him out of the way as I pulled the plugs from the suspect mag. Didn’t find any lead fouling but did find one open plug and another way outside the 5000 limit. Replaced the two plugs with used serviceable ones and the mag problem went away. Pilot was happy he could go back to towing and the club happy they didn’t need a new mag.


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  3. #3

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    When I was building my cub, for some reason, I checked the Champion plugs. Two of the eight showed open circuits. Since then, I check them all.
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  4. #4

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    I shake them before I measure them. Yes really. I have three champion plugs now from three different airplanes that the center electrode will move in and out. Only the first one was leaking gases into the high tension lead.
    You can't get there from here. You have to go over yonder and start from there.
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  5. #5
    Steve Pierce's Avatar
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    Aren't the older Champions carbon pile and the ohm check doesn't work on them?
    Steve Pierce

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  6. #6
    hotrod180's Avatar
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    from https://www.aircraftmagnetoservice.n...shooting-guide

    "Most “bad mag checks” are spark plug related. The spark plug is fouled and shorted to ground or is open and the magneto, which is functioning normally, is unable to fire it. A typical bad plug will cause an immediate drop of 250 or more RPM, at the mag check. The key indicator is the suddenness of the drop..."

    I ohm-test my plugs at every annual.
    In my experience, it seems like that plugs are either within spec or waay out of spec--
    very seldom are they "just barely" out of spec. FWIW.



    Cessna Skywagon-- accept no substitute!

  7. #7
    Steve Pierce's Avatar
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    That is why you swap plugs top to bottom when you have a bad mag drop. If it moves to the other mag it is the plug, if it stays I check the lead. Have had both plugs on the same cylinder fail a couple of times on transient students airplanes. They get real excited when that thing is shaking like crazy.
    Steve Pierce

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  8. #8
    Eddie Foy's Avatar
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    They get more excited when it gets very quiet!

    Quote Originally Posted by Steve Pierce View Post
    That is why you swap plugs top to bottom when you have a bad mag drop. If it moves to the other mag it is the plug, if it stays I check the lead. Have had both plugs on the same cylinder fail a couple of times on transient students airplanes. They get real excited when that thing is shaking like crazy.
    "Put out my hand and touched the face of God!"
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  9. #9
    Dave Calkins's Avatar
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    Have been doing the resistance check for years now. IT NEEDS TO BE DONE

    It is part of plug maintenance and ignition system troubleshooting in my world.

    Anyone wonder how a magneto and harmess are affected by high resistance plugs??? I wonder!

  10. #10
    algonquin's Avatar
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    I just got a Champion spark plug tester and cleaner. Also my 9 yr old Grandson loves annealing the washers with me. If it looks like it has a bit of danger he’s right on it.

  11. #11

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    Quote Originally Posted by algonquin View Post
    I just got a Champion spark plug tester and cleaner. Also my 9 yr old Grandson loves annealing the washers with me. If it looks like it has a bit of danger he’s right on it.
    for $.25 I will replace the washer, my time is worth more than what it takes to anneal them...vs the cost.
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  12. #12

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    Quote Originally Posted by Steve Pierce View Post
    That is why you swap plugs top to bottom when you have a bad mag drop. If it moves to the other mag it is the plug, if it stays I check the lead. Have had both plugs on the same cylinder fail a couple of times on transient students airplanes. They get real excited when that thing is shaking like crazy.
    Steve, I check them while they are out...before I put the back in rotated. It usually finds the problem before I put them back in.
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  13. #13
    Steve Pierce's Avatar
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    Quote Originally Posted by astjp2 View Post
    Steve, I check them while they are out...before I put the back in rotated. It usually finds the problem before I put them back in.
    My response was to hotrod180 and his comment about the bad mag drop on one side. At inspection I clean, gap test etc but when there is a bad mag drop I don't always have access to my tester and I can swap them, see if the problem follows the mag and work accordingly. I usually have a spare plug in my airplane.
    Steve Pierce

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    Will Rogers

  14. #14

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    Yep - I've been checking my plug resistance for several years now. My mechanic (who used to exclusively use Champion plugs) "resisted" (pardon the pun) doing the ohm testing until I showed him a Mike Busch article on the topic, the MILSPEC document someone else referred to, and finally the Tempest document on their website.

    Right after that, my Citabria (O-200) failed a mag check on runup while I was doing my tailwheel endorsement. He checked the plugs, and found two that were WAY beyond spec... He had a batch of "serviceable used" Champions, so we began testing them prior to using in my plane. Six of the 10 plugs he had in his stockpile failed the resistance checks. (He actually cussed me out that day, but apologized and thanked me later...) Once we swapped out the bad plugs with good ones, the engine started so much easier it was hard to believe.

    On the Commander (IO-540) I used to own, during the pre-purchase (which we turned into an annual once we got past the "yes, I'll buy it" stage), we found 6 plugs that failed the resistance test. We did a 100% replacement, using Tempest fine-wire plugs. Never had a whisper of a mag check issue the whole time I owned that plane.
    Jim Parker
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    ?? Bearhawk Patrol - Building
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  15. #15
    algonquin's Avatar
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    I have a jar of washer done and when I get a small pile needing to be done I spend 15 mins with my best buddy doing them... priceless.
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  16. #16
    Steve Pierce's Avatar
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    I have a coffee can overflowing. Every now and again I torch and quench some for my own airplanes.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
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  17. #17
    www.SkupTech.com mike mcs repair's Avatar
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    never have bought new washers 30++ years .... simple and quick to anneal them...

  18. #18
    Charlie Longley's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    never have bought new washers 30++ years .... simple and quick to anneal them...
    Have you read the Continental and Lycoming spark plug service bulletins?

  19. #19
    Charlie Longley's Avatar
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    I am not a big Mike Busch fan but this is a pretty well written article on spark plug resistance-
    https://blog.aopa.org/aopa/2015/03/1...to-acceptance/

  20. #20
    Steve Pierce's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    never have bought new washers 30++ years .... simple and quick to anneal them...
    How many gaskets do you do a year?
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

  21. #21
    sjohnson's Avatar
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    I found an intermittent plug with an ohm test. Vary the pressure on your probes a little while doing the test.
    Idaho drinks more wine per person than any other state in the country.

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