what causes a coil to get hot & boil?

Discussion in 'Old's Cool' started by rtwdoug, Aug 30, 2010.

  1. Lokey

    Lokey redneck

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    I had a similer problem on my 59 panhead many years ago. It was frying points within just a few minuets of installing them. Turned out it was a bad wire. As has been mentioned, when the coils are drawing too much amperage through them they will get hot. In my case the wire going to the points was broken inside the insulation (you couldn't see it) and in order to push the voltage across the bad area, the current went way up.

    I would check the wire on the positive side of the coils and see if it is nearly broken some where. If you have a bad ground (as has also been mentioned) that could cause it also, but I would expect it to be frying light bulbs too.
    #21
  2. MightyChosen1

    MightyChosen1 I wanna be sedated

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    Does it have a resistor in line with the coils? old school car coils use a low value resistor ( About 1 Ohm ) to lower 12 volts down to about 9 volts to the coil. I had this problem years ago on an old truck because someone bypassed the resistor. The coil would get so hot it would start missing then
    work agaain after it cooled.
    If you have a meter check how many volts are at the coil.

    Hope this helps. D


    [​IMG]
    #22
  3. MudWalker

    MudWalker Long timer

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    This happened to my 85 Range Rover......small engine fire lead to some burnt wires that I replaced but the power wire to the coil was listed in the diagram as a "resistor wire". I tried in-line resistors of all types but had a hard time finally getting 9 volts going to the coil. And yes for a while there I had a hot coil although it never failed me.
    #23
  4. tradelakecycle

    tradelakecycle Turnip Eater

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    Any HD running points or an intermediate 79-80 electronic ignition should be have 5 ohms total resistance in the coil. Most elec are 3 ohm and will work but poorly. Part number 31609-65A should be the ticket if you can get it OEM. I have officially sworn off anything but factory Harley electrical products. Quality and very reasonable. Hope this helps. Craig
    #24
  5. El Hombre

    El Hombre Banned

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    Not unknown for a new part to fail. If the insulation in the coil primary fails and it bypasses some of the windings, the resistance will fall and it will pull more amps and run hot. Most points can handle about 3 amps, 13v at the coils and you need about 4-5 ohms to keep everything working.
    #25
  6. Yarddog

    Yarddog Been here awhile

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    If there's a Harley shop there, then there oughta be an auto parts store there, and there would be your source for an automotive points type coil. Also, when you went to the Harley shop, I'm certain they sold you something that was for an eight year old Harley at the very oldest, because they only carry stuff back that far...I think there's a good deal of suspicion here that you should be using a points type coil for your points ignition, makes sense, if you think about it, so take care of your problem thusly, and I'll bet you won't have any more problems...
    #26
  7. Lornce

    Lornce Lost In Place

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    Just saw this thread. You're getting some good info here, Doug.

    Hot wire continuity.

    Good ground.

    Correct dwell.

    Correct coil resistance.


    It's gotta be one of those or a combination of a couple of them.




    Don't go out in the sun without your new hat. :thumb


    [​IMG][/


    :rofl
    #27
  8. ericrat

    ericrat Long timer

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    Maybe put a dwell meter on it. Excessive dwell will cause the coils to overheat. This seems unlikely on a twin, but is possilbe and is easy to check. Most dwell meters only go down to a 4 cylinder setting, so double the expected dwell spec for your bike. This ASSUMES a singe set of points. Anything from 70 to 150 degrees should be fine.
    #28
  9. rtwdoug

    rtwdoug prominent underachiever

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    hey guys
    thanks for the tips & ideas.
    the car coils are hot, but working
    I got a set of russian Izh coils as a backup, as spare parts aint everywhere here.
    when I get somewhere more civilized, I'll look into it better

    Doug
    #29
  10. mpanther

    mpanther KotW - Just Ride!

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    Did you ever find out what was the cause of this issue Doug?
    #30
  11. rtwdoug

    rtwdoug prominent underachiever

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    of course.
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  12. mpanther

    mpanther KotW - Just Ride!

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    :rofl
    :huh
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