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Thread: Why Reproduction "Harley" coils Fail

  1. #1

    Why Reproduction "Harley" coils Fail

    I had a stimulating phone conversation with a well-known Harley historian tonight. We covered the bases, so to speak, in an hour's conversation, but what I want to report is what came up in one part of that conversation. He axed me what was up with repro coils. He told me that one theory someone proposed to him related to engine heat, and that Flatheads, with more top end heat, seemed to fry repro coils before OHVs, based on anecdotal evidence.

    It turned out that it has something to do with heat, but it turns out to be heat at the sparkplugs' tips, between the center electrode and the ground electrode, and two electrical principles and coil polarity, as it relates to a pair of electrical theories: the electron theory and the theory of thermionic emission, which are factors in automotive wasted spark ignition. (AKA: "Why do Tedd's V-Twin repro coils so often fail prematurely?") All this discussion of direct current, alternating current, electrons, heat and current flow stimulated my wetware, and between what he said and what I said, after we hung up I came to more completely understand what I thought I knew "for years."

    The Answer: Basically, the coils fail because their primary-coil is wired backwards from the direction intended in the designs of OEM Harley 6v coils (1930-47, and the next, 1948-64), and yes, the locations of the "+" and "-" terminals do matter.) If you buy a dash-30 (uses two u-bolts) or dash-48 (bolts top and bottom to blocks welded on the frame seatpost) repro ignition coils, DO NOT attach your power wire from the main switch to the "front" terminal (as the coil is mounted on a bike) on the coil, and your points wire to the back terminal. This is the recipe for disaster (early failure).

    The primary winding within the repro coil (about 220 turns of wire) is "thrown" in the opposite direction/rotation within the box from the OEM coils that used to be supplied by the MoCo. The secondary, "hi-tension" coil (about 12,800 turns of extremely fine wire that connect to your sparkplug wires) are excited in the opposite direction ("+" and "-") from the way they were designed, if you follow the old manuals and wiring diagrams when installing your repop coil. This creates an opposite polarity at the sparkplugs from the way sparkplugs normally like to be treated, and leads to their early demise, if not that of the coil, itself.

    And after we got off the phone, I did a little internet research and got the details and kerrect terminology:

    This is the basic diagram of a Harley "wasted spark" ignition system

    What you have to do is connect the switch 'ign hot' wire to the back low-tension terminal on the repro coil, opposite of the diagrams in Harley Service manuals, and opposite of the way it was done on the bikes originally, and attach the wire that connects to the timer/the points, over on the opposite side of the bike, to the front terminal of the coil, on the side facing the fork and front wheel.

    If you examine a recent Tedd's repro coil, you may see a tiny "+" embossed in the tin case on the back, near the terminal, but it is a fairly recent addition, and is not publicized anywhere. It's only essential to the life and reliability of the coil, is all.


    All seem to agree: Tedds' coils fail because polarity does matter, and backwiring leads to eventual sparkplug failure. Yup.

    Perform the "lead pencil test" described above, in the third reference, the "International Harvester Manual," & shown on "Next Page: Condensers" at its top right. Do it under low light, and check your current polarity at the plugs. On a UL, you can use a tiny c-clamp to hold a wooden pencil alongside the adjacent cooling fin and the lead point will be in the perfect position for the test, right next to the top of the rear plug, with the rear plug wire removed and placed right on the opposite side of the pencil. The spark should appear between the plug wire and the pencil, and the 'flash' of an orangish color, between the pencil and the terminal on top of the plug. Look quick, for the spark on one side and the "flash" on the other. It's quick. You can snap "closed" points if you're using them. I had to kick over my electronic ignition and make it turn to fire off the coil.
    Last edited by Sarge; 03-31-2012 at 10:30 AM.
    Sarge, Gerry Lyons, Fla.
    AGENT ORRNGE Survivor. So far.
    The Friendly Fire that Keeps on Burnin'.
    F.O.G. member:
    "It's a 1937, and the downside is, it's out of warranty"

  2. #2

    Re: Why Reproduction "Harley" coils Fail

    Thats interesting.I wondered why those coils never lasted.I went through 2 of them before I just install a newer type coil because I got fed up.I thought they were just made cheap.

  3. #3
    Join Date
    Oct 2010
    back at South Bend

    Re: Why Reproduction "Harley" coils Fail

    Thanks Sarge. NOW will the repop manufacturers correct the direction of their wiring wrap, or will we just have to settle for their bass-ackwards ways??
    Last edited by Ar Amytas; 03-31-2012 at 06:53 AM. Reason: pre-coffee post
    A poor boy plays by poor boy ways

  4. #4
    Join Date
    Aug 2008
    East TN, just north of Chattanooga

    Re: Why Reproduction "Harley" coils Fail

    I read about this before before I installed my repo coil so I wired it backwards (which is really frontwards). So far at almost 2000 miles no's hoping

  5. #5
    Join Date
    Sep 2009
    Missouri City, Texas

    Re: Why Reproduction "Harley" coils Fail

    Nice job Sarge.

  6. #6

    Re: Why Reproduction "Harley" coils Fail

    The primary field is just a coil of wire wrapped abound an iron core.
    HOW does the direction the electrons travel effect coil life?
    HOW does the coil fail?

  7. #7
    Join Date
    Feb 2012
    Barnesville Pa

    Re: Why Reproduction "Harley" coils Fail

    Good stuff! Thank you for diverting a future headache for me.............

  8. #8
    Join Date
    Jul 2008
    High in the B.C. Rockies

    Re: Why Reproduction "Harley" coils Fail

    Thanks Sarge. You've given something to ponder. I may just give Taiwan Tedd another chance!

  9. #9

    Re: Why Reproduction "Harley" coils Fail

    Quote Originally Posted by 1950panhead View Post
    The primary field is just a coil of wire wrapped abound an iron core.
    HOW does the direction the electrons travel affect coil life?
    HOW does the coil fail?
    U can tell who axt his question before he took the time to read the three referenced explanations that tell you just why, all in slightly different language, but that all reach the same conclusion.

    (Just pullin' yer leg, Jerry! I'm pickin' on ya because I knew in the back of my mind as I wrote the post that there would be someone, and it turned out to be you, who would, after all of that, re-ask the basic, underlying question: "Why?" We also all have different preferred ways of learning. I'm not blamin' you, Jerry!)

    So here's the cut-to-the-chase, the most direct, right from the International Harvester Truck Repair manual:

    And remember to go up and click "Next: Condensers" near the top right, to see the next page. The graph is stunning, showing how much harder a coil hooked-up backwards has to work, and the "lead pencil test" is shown next to it. The condenser explanation, and why condensers don't fail, anymore, like they used to (better sealed against moisture than they used to be), is also good reading.

    The cut-to-the-chase is that it's far more important to our sparkplugs than most people realize that the electrons leap from the hottest part of the plug in a running combustion chamber: the sparkplug's center electrode, the plug's "core," to a "colder" part, the grounding electrode (the little prong that we "gap" before installing), ALL because of the (not commonly understood) electrical principle of "thermionic emission." I.e.: electricity likes to go from hotter surfaces to cooler surfaces, and hates to go the other way. Imagine it as trying to push extra ions into a heat lamp! All those heat ions are coming at you, and you're trying to push (requiring increased voltage) something just like it back in, being forced to push even harder, across a plasma gap, against the flow!

    Anyway, here I've quoted the important part of the International Harvester manual that directly answers your question, Jerry, and anyone else who may still be confused:

    "What difference does it make whether positive or negative voltage is supplied to the spark plug terminals?

    It directly affects the amount of voltage required to fire the spark plugs. When polarity at the spark plug terminals is positive, it's harder for the voltage to jump across the air gap than when polarity at the plug terminal is negative. Just why this is so is related to a pair of electrical theories--the electron theory and the theory of thermionic emission.

    According to the electron theory, all current flows from negative to positive. The theory of thermionic emission states essentially it's easier for electrons to leave a hot surface than a cold surface. Combining the two theories, one finds that electrons will always leave a negative charged surface for a positive charged surface, and they will leave the negatively charged surface with more ease when the surface is heated.

    Spark plug design is such that the center electrode almost always operates at a higher temperature than the ground electrode. Since it's easier for electrons to leave a hot surface, it is preferred to have the electrons "jump" from the hotter center electrode to the cooler ground electrode. When the center electrode is negatively charged (negative voltage at the spark plug terminals), this is what happens. Stated another way, putting the negative charge on the hotter center electrode causes the gap to be ionized at lower voltage. (Ionization is necessary to permit passage of the spark through the high resistance of the gases in the cylinder.)

    When positive voltage is supplied to the plug terminals, which happens when coil polarity is accidentally reversed, the hotter center electrode becomes positive charged. Consequently, electrons must leave the negative charged ground electrode and move to the positive charged center electrode. But, since the ground electrode is cooler than the center electrode (and remember, it's easier for the electrons to leave a hotter surface), it takes more voltage to make the current jump the gap--in fact, up to 45 percent more."

    So, being forced to work :"up to 45 percent more" eventually (also), overheats Taiwan Tedd's "mis-thrown" (backwards-wrapped) coils enough to BREAK just one of those internal "12,800 turns of extremely fine wire" that make up its secondary, high-tension (17,000-volt!) winding. HEAT, Jerry. Just extra, unnecessary heat, at certain points deep inside that little, amazing, electrical transformer, cause it, working extra-hard, force it to push electrons backwards, uphill, against the way they naturally want to flow. ONE hair-thin wire melts and "pffft!" a seventy-dollar coil is junk. "No returns on electrical parts."
    Last edited by Sarge; 03-31-2012 at 11:38 PM.
    Sarge, Gerry Lyons, Fla.
    AGENT ORRNGE Survivor. So far.
    The Friendly Fire that Keeps on Burnin'.
    F.O.G. member:
    "It's a 1937, and the downside is, it's out of warranty"

  10. #10

    Re: Why Reproduction "Harley" coils Fail

    Very good sarge, very good.
    You plum wore me out with your two page post and by the time I got to "firetrucks and equipment" I was kaput and could read no more and started thinking about how this all worked.
    Just about the time you think you know it all something silly like this happens and I start to think not this again.
    From "firetrucks and equipment",
    "According to the electron theory, all current flows from negative to positive.", this is a fact by the way, not a theory.
    "The theory of thermionic emission states essentially it's easier for electrons to leave a hot surface than a cold surface"
    Notice that it says "theory", meaning it's an idea that has no been proven, otherwise it would be a fact and not a theory.
    So, sarge, assuming the "thermionic emissions" theory is true, would you like to take a crack at WHY it is true?
    And would you like to take a crack at WHY the center electrode is hoter then the ground electrode and how much hoter is it?
    So if we accept these theories as true we are guessing the Vtwin secondary (high tension) windings are being overheated until the insulation fails of one of those tiny wires melts and thats the end of the coil.
    It would be interesting if the original coils (who made these by the way?) would suffer the same failure rate if connected backwards.

    When I attend the AMCA meets this summer I am going to ask everyone I see tell me about "thermionic emissions" and see what they say.
    Last edited by 1950panhead; 03-31-2012 at 01:06 PM.

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