what causes a coil to get hot & boil?

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

  1. rtwdoug

    rtwdoug prominent underachiever

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    Howdy
    so Im in Turkey on my Harley
    (but coils are about the same on all bikes)

    Coil crapped out a few days ago, so I got a new one from the HD shop in Istanbul.

    its for electronic ignition, & Im running points, but I've done the swap before with no ill effects.
    (you can run an elecronic coil with points, but not a points coil with elec ign)

    anyhow, new coil lasted about 4 hours, then died. the guts were spewing out of it, it got so hot.
    I found a couple car coils to wire up & get down the road, but Im afraid they will die also.

    Ive never had a problem of boiling coils like this before,
    any ideas what it could be?

    Thanks, Doug
    #1
  2. Beater

    Beater The Bavarian Butcher

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    Some electronic igintions can run hot. Their output can be of a higher wattage (volts x amps). This shouldn't be a poblem with modern electronic ignitions though ... It was the earlier one's that had beefier outputs ... i

    See if your elecronic ignition should have a resistor in line to your coil ... Was your bike points prior to going electronic? Probably?

    Wierd problem ...

    Just thinkin' out loud ...
    #2
  3. rtwdoug

    rtwdoug prominent underachiever

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    Im running points ignition, it always has used points
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  4. Mista Vern

    Mista Vern Knows all - tells some.

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    Really grsping at straws here, so forgive me - thinking out loud. Maybe you suffered with the low odds of having two bad coils, you need to add a balast resistor, or there is some gremlin in your wiring that leaves current to the points even when the switch if off? :ear
    #4
  5. Beater

    Beater The Bavarian Butcher

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    Modern electronic ignitions are for low currents ... they employ transitors and such ... therefore the 'output' is also very low wattage. Coils can be 'made for this super tiny current'.

    Points (generally) have a higher output ... and coils can be made to take the higher output.

    I *think* you have some coils designed to take a tiny ouput, and putting a big output (from your points) into them.

    Like Mista Vern, I am guessing here ... and correct me if I'm wrong ... but If this is a new coil (for a newer bike) ... and from the Harley supplier ... then it's probably made for electronic ignition ... and not for points.
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  6. rtwdoug

    rtwdoug prominent underachiever

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    I was riding when it boiled out & died, had been ridin about 90 minutes straight.

    I'm clueless, the odds of a new HD oem coil being bad are slim.
    specially to work fine for 4 hours of riding
    the 1st coil lasted about 10k miles
    #6
  7. rtwdoug

    rtwdoug prominent underachiever

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    Ive run elec coils with points before & been fine.

    in fact, Ive seen this coil listed as a direct replacement for the earlier coils in catalogs.
    #7
  8. Mista Vern

    Mista Vern Knows all - tells some.

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    Too bad there aren't any junk yards with Chevy in-line sixes running 12 volts there - be a snap to get a coil that would be bullet proof! :D
    #8
  9. Pigford

    Pigford British

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    Coils are rate by OHMS - eg: Big 70's Kawasaki fours like 3.0 ohms. This is the critical figure to match.

    Check all connections - could be dodgy earth ?
    #9
  10. rtwdoug

    rtwdoug prominent underachiever

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    I have a couple car coils on it now, wired in paralell. they are working ok, but seem to get too damn hot. but then I dont know how hot they can get under usual use
    #10
  11. larryboy

    larryboy Chopper Rider

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    Sounds like a bad positive wire to me.:dunno
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  12. Crisis management

    Crisis management Latte riders FTW!

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    I don't know my arse from my elbow about Harleys but so far you have cooked a couple of coils and the car coils are running hot? Coils have never been more than warm to the touch in my experience (shade tree mechanic here) so something electrical is making them hot, too much resistance in the circuit / poor connection. Find someone that knows auto electrics & owns a multi meter (or common sense), where you are they are good at making things live forever, failing that inspect all your connections and clean and tighten everything associated with the ignition. A random thought here, check your alternator output in case voltage is too high?

    Good luck.
    #12
  13. they

    they Draining

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    Any 12 volt points type coil should be fine. The only thing you have to watch for, is some have an internal resister and some require an external resister. No resister means hot coil and burned points.
    #13
  14. Stromrider 1583

    Stromrider 1583 Republican Adventurer

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    Is the bike electric start? Does it have a ballast resistor? If the ignition system is designed such that the coil(s) see full battery voltage whist cranking (9-10 vdc) and then voltage after the resistor (also 9-10 vdc) and the coil(s) get the full 12-14 vdc of a running engine they can overheat and fail.

    Did you change anything else on the bike before it started eating coils?

    Try swapping the 12 vdc ignition input from one coil to the other and see if the symptom transfers.

    Have you checked alternator diodes? A failed diode will still charge but can introduce A/C into the system. The coil may not like this.

    My bet it that something has changed on the bike. The coil didn't just fail on its own.

    R
    #14
  15. datchew

    datchew Don't buy from Brad

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    just like in real estate, sometimes failure or success of electronics is "location location location."

    Is it shielded from engine heat? Does it get cooling air? Got tinfoil?

    If those 3 answers = yes (at least the first 2) then you know it's really electrical.




    I've only had things melt for 2 reasons. bad/inconsistent ground or incorrect wiring.


    You know what you're doing so I think you'll just need to tinker and test and inspect and you'll figure it out.

    Good luck. Bring me a good souvenir or t-shirt.
    #15
  16. OldPete

    OldPete Be aware

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    I say BINGO!

    Reversed polarity or a failed condenser will not cause over-heating.
    No condenser will cause(edit: points to burn up from arcing) rough running, back-fire and no high rpm operation.
    Power will be weak but the coil will not over-heat.

    Automotive coils of old were oil filled and should be mounted in a comparativly cool place, vertical with the secondary up.

    I had to zip-tie a NAPA coil to the Goose to get home once(700 miles).
    #16
  17. motu

    motu Loose Pre Unit

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    Electronic igns have current control by module or computer,and usually have less resistance.Points coils just have to take however much current the points feeds them,and have higher resistance.You need to check primary resistance on those coils,but seeing as they are both now buggered,not much point.Some early electronic igns used a ballast resistor....you need to find one.

    Luckily you are in a place where you might just find one on the side of the road.Have a look through a wreckers yard - you are looking for a white ceramic resistor next to the coil.Also look for a waste spark coil pack,plenty of cars have them,talking Japanese here.A 4 cyl will have 2 coils on a bracket - that gives you two twin output coils,hopefully very much cheaper than a HD coil.
    #17
  18. Plaka

    Plaka Brevis illi vita est

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    The coil is overheating because you are pulling to much juice through it's primary.

    The circuit is strait power from the ignition to one side of the coil primary. The other side of the coil primary goes to the points and then to ground. So when the ignition is on and the points are closed, juice flows through the coil to ground.

    When the points open they interrupt this flow on the ground side causing the high voltage spike in the coil secondary windings that is then routed to the plugs to make sparks. The condensor is parallel to the points just to suppress arcing when the points open.

    If you sit with the ignition on, engine not running and points closed, there will be a constant flow of juice through the coil primary, which can overheat it. Ditto if the points are shorted to ground---but then it will run like major crap if at all.

    The electronic ignition can switch a lot more power than points so you use a coil that can take more power. It will have a lower primary resistance to allow the flow. maybe 1.5 Ohms for a single dual nose coil firing two plugs on two cylinders.

    The points setup can switch less juice so think 3 ohms for a single dual nose coil firing two plugs on two cylinders.

    A pair of single nose coils in series firing two plugs on two cylinders will have the combined resistance of the coils. So two .75 ohm coils in series gives a 1.5 ohm circuit. But put them in parallel and the resistance halves. Two 3 ohm coils in parallel gives a 1.5 ohm circuit.

    If the resistance is too low, there will be too much current and you might cook the coil (and the points). If there is an external resistor and it is bad and not resisting enough, this could do it.

    As something heats up, it's resistance increases. Just to keep in mind.

    Pull the hot feed to the coil(s). With the ignition off and the points open, measure the resistance from the hot input terminal of the coil to the "hot" (wired) side of the points. If the resistance is too low (I'd guess under 3 ohms) figure out why.
    #18
  19. AntonLargiader

    AntonLargiader Long timer

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    I didn't see anyone mention dwell so I will. Points saturate the coil for a certain (fixed) percentage of the crank's rotation. The spark happens when they open, then they close again and charge the coil. Because of the inductive nature of a coil, coil current increases as this 'dwell time' gets longer, so if you have the points closed for too long you will heat the coil exponentially/logarithmically/biorithmically/something..

    Points setting specs are basically dwell (often set by the gap) and the timing itself. They go hand in hand. It's possible to get the right timing with too small a gap, meaning you get too much dwell. On a slow-running motor, with lots of coil soak, that could make a difference.
    #19
  20. AliBaba

    AliBaba Been here awhile

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    Dwell-time can also be increased by mechanichal failure at the points (the part that touches the cam) and the cam itself.
    #20