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Ignition Coil Resistance

Discussion in 'Early CJ5 and CJ6 Tech' started by Kiowaflier, Jul 31, 2017.

  1. Jul 31, 2017
    Kiowaflier

    Kiowaflier New Member

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    Can someone clarify for me what the resistance on an ignition coil should be, and what that reading should display on a digital multimeter? From multiple posts, I understand the primary resistance on a 12v internal resistor coil should be about 3.0 ohms, and secondary resistance should be about 8,000 to 11,000 ohms. With my multimeter, I get exactly 3.0 on the primary, but only about 9.5 on the secondary (with the selector switch on either 20k or 200k ohms). Is the assumption to multiply the readout by 1000? Probably a silly question but I'm trying to eliminate all possibilities here...
     
  2. Jul 31, 2017
    mickeykelley

    mickeykelley Well-Known Member

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  3. Aug 1, 2017
    ronnie victor

    ronnie victor Member

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    I don't understand how you can have two multiples. 9.5 x 20000 = 190000. and 9.5 x 200000 = 1.9 million ohms. Can you clarify? Is there something about your multimeter that suggests 20k and 200k positions equate to a factor of 1000? My old Fluke doesn't work that way, as I recall.
     
  4. Aug 1, 2017
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    The selector switch will give you a range of 0-20000 or 0-200000 resp. I expect the MSD (most significant digit) of the display only shows off (0), 1 or 2. So if you have a display like XX.XXX that will show 20.000 at 20000 ohms (20K). Your reading of _9.500 is 9500 ohms or 9.5K or 9.5 * 1000 - all equivalent expressions. When set to 20000 and the resistance connected to is more than 20000 ohms, you will see INF or all dashes or something, signifying the meter is out of range.

    Buy a packet of assorted resistors at Radieux Shaque or another electronics store and experiment with the meter. You can learn how to read and use the meter and also learn how to read the color code on resistors. BBROYGBVGW - Definition by AcronymFinder - the acronym I remember is not suitable for a family oriented site!
     
  5. Aug 1, 2017
    ronnie victor

    ronnie victor Member

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    Okay, thanks. But a good Fluke meter is essentially a direct read. No ranges. Only multiples in orders of magnitude; 1k, 1m, etc, x what's expressed as ohms in readout window ( e.g., 9.5 x a K underneath that number in the window). Easy to use.
     
  6. Aug 1, 2017
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    The Fluke meter is what's called an "auto-ranging" meter - an expensive feature. Fluke is a great meter, but you don't need a Fluke to measure resistances for your car.

    For the professional, a major selling point for the Fluke is the ruggedness of the electronics. You can abuse the Fluke a lot more than a cheap meter ... it's designed to withstand high voltage spikes and such, much better than a cheap meter. The Fluke is also "true RMS" (always giving you the root-mean-square voltage of an AC signal) which your inexpensive meters will not be. Again, not of much value when working on your car.

    I have one of these Extech MN16A Compact Autoranging MultiMeter with Advanced Functions - - Amazon.com which I use on my bench indoors. The similar meter without auto-ranging is less than half the price. I also have a meter for my toolbox, but I don't recall the brand.
     
  7. Aug 1, 2017
    ronnie victor

    ronnie victor Member

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    Check. Good information for those planning to buy a volt-ohmeter for their Jeep troubleshooting and general workshop activities. Thanks.
     
  8. Aug 1, 2017
    tomtom

    tomtom Sponsor

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  9. Aug 1, 2017
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Indeed.
     
  10. Aug 2, 2017
    Kiowaflier

    Kiowaflier New Member

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    Interesting...so, continuing with my troubleshooting, looks like my ignition coil is indeed good (and the one I bought to replace it, thinking it was bad...ahh wishful thinking). So now, I'm narrowed down to the electronic distributor I recently bought from KW (Omix Ada...I know I know...) Is there a way to test an electronic distributor with a multimeter? I had my wife crank the engine while holding the distributor end of the coil-to-distributor wire up to a good ground, and only got very weak, sporadic sparks. Same story when holding one of the actual spark plugs to a ground. I have good resistance on each of the five wires...3k on the short wire, 8-9k on the plug wires.
     
  11. Aug 2, 2017
    Glenn

    Glenn Kinda grumpy old man Staff Member

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    FWIW I bought one of those distributors and never could get the Jeep to start. Luckily I also bought a spare module, installed it and it cranked right up. I did that after going crazy trying to figure out what the problem was. Bottom line, even if you discover it is some other problem having a spare module is a very good idea.
     
  12. Aug 3, 2017
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Better to test the coil in place, rather than look at the resistances. All you need to do is ground the coil lead momentarily and lift it, and the coil will spark. Color of the spark and the gap it will jump is a good test.

    Also, you may as well buy an extra coil. Every Jeeper should carry an extra coil... so you swap in your spare to test.
     
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  13. Aug 3, 2017
    tomtom

    tomtom Sponsor

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    I bought one of those distributors too. First module lasted about a year, second one less than that. I had been carrying my old point distributor in the under seat box and decided to upgrade that with a petronix module and use that instead. That has been going for 5 or more years now. Also, just need to carry points and condenser for the trail spare now.
     
  14. Aug 4, 2017
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Always suspect the newest thing first. Especially electronics.
     
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  15. Aug 26, 2021
    Mark T.

    Mark T. Member 2022 Sponsor

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    Are the positive and negative posts on the coil where the primary resistance can be checked? I have a single choice ohm meter that shows 1.8 for the check on the terminals (primary?) and 11.5 for the positive to center part (Secondary?).
    It's a 12 volt that says "use with external resistor" from Standard Plus. It looks very old and I took it off the engine while it was being rebuilt. I am not at the point where I can crank anything up yet to check it.
    My manual says a Prestolite 200691 3.9-4.2 ohms for primary resistance and 9400 - 11,700 ohms for secondary but doesn't say anything about an external resistor model.
     
  16. Aug 27, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Here's how they work internally -

    upload_2021-8-27_9-2-40.png

    The primary and secondary are tied together and grounded through the (-) terminal. (-) to (+) is your primary resistance. (-) to the center post is the secondary resistance.

    JMO_ - resistance is something of a blunt instrument when you are testing a coil's condition. I expect you have read this thread all the way through from the top. A specialized meter that can read low resistances would be better. I have one of these - Capacitor Wizard In-Circuit ESR Meter | Howard Electronics - this works pretty well for resistances in the range of an Ohm. Your generic meter may not do so well. Be sure to check the resistance of the leads before you measure the coil.
     
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  17. Aug 27, 2021
    Mark T.

    Mark T. Member 2022 Sponsor

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    Thanks Tim, I think tomorrow is the day I get another meter. Wife doesn't need to know anything. Any recommendations on better than other coils? gonna pick up a couple of those as well.
     
  18. Aug 28, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    Are you asking about a meter or coils?

    For the coil, we used to sell Standard Ignition (now SMP) at the parts store. Their flagship coil was the UC-12X, their premium offering. UC-12 is good too. Avoid anything with the "T" suffix from SMP. Any coil used with points will need extra resistance in series to current-limit the discharge. There are coils that use external resistors ("ballast resistor"), and coils that include one internally. The external resistor is more flexible, allowing bypass of the resistor when starting for faster/hotter sparks.

    Lots to choose from. Big price break between $10 and $20, likely warranted.
    1968 JEEP CJ5 3.7L 225cid V6 Ignition Coil | RockAuto

    Lots of coils out there. You can go with one of the hot rod coils like MSD which comes in an "oil can" package like the original. The cheapest hot coil I know of is the Ford square coil from the mid 80s? This resembles the GM HEI coil, but made weatherproof. You should be able to get these at the junkyard for a few dollars - take the bracket and the connectors too. Or buy new - not expensive.
    1985 FORD F-150 5.0L 302cid V8 Ignition Coil | RockAuto
     
    Last edited: Aug 28, 2021
  19. Aug 28, 2021
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    If you want help on meters, pick some from Amazon and I'll comment. Really depends on how much you want to spend, and what features you want. There are a bunch around $25-50 that would work fine for automotive.
     
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  20. Aug 28, 2021
    Mark T.

    Mark T. Member 2022 Sponsor

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    Exactly what I needed Tim. Thank you.
     
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