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Thread: The separate coil ground wire issue.

  1. #1
    Senior Member 34nineteen's Avatar
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    The separate coil ground wire issue.

    This is interesting. Many people have commented that their bike runs better when the coil has an auxiliary ground wire to the battery. However, according to the service manual (2004 XBS), on page 4-99 it states the “coil will function without being attached to the frame”.

    So, is this wire actually solving an issue or is it a bandaid for ground maintenance???

    Discuss...

  2. #2
    Senior Member Cooter's Avatar
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    Its all an illusion! Haha
    An ignition coil does not need a ground on the chassis to operate. In fact, if anything in that primary circuit was connected to ground, the coil would not work at all, and your ignition fuse would blow.

    The "coil" is one wire (primary) coiled around another winding (secondary) thats coiled around an iron core. The (-) that makes that circuit is the spark plug threads on the other side of the electrode gap. Thats the only way you get spark across that gap. If you put the secondary windings to ground = no spark.

    The (+) charge on the windings wrapped around the iron core make a magnetic field. It is the act of the (-) signal from the ignition that causes the magnetic field to collapse and shoots the voltage out of the secondary windings at a drastically higher number.

    By grounding the chassis of the coil, maybe you can reduce some ignition noise (the magnetic field collapsing every time a plug fires) but it won't give a bigger spark or anything silly like that. But hey, whatever makes people feel good ya know?

  3. #3
    Senior Member 34nineteen's Avatar
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    Wait, what? The coil needs to a complete circuit on the primary side to energize it. Once you open the connection, the field collapses and transfers to the secondary side through induction. Then the high voltage goes through the HT leads to ground. But you have to have a ground on the primary side. The part you don’t need is a positive source for the secondary since it gets that from the primary.

    Just because the circuit is complete doesn’t mean the fuse will blow. The coil (primary) is creating a load, like a light bulb. However just like back in the points days, if your distributor stopped with the points closed you could burn up a set of points if you left the ignition in the ON (not ACC) position. Been there, done that while making out with my GF back in my HS days in my Porsche 914.

    I always thought the feeling was that the ground wasn’t sufficient on an XB which wasn’t allowing the coil to create enough of a charge on the primary side, and the supplemental wire resolved that. But, after reading in the service book that it doesn’t require being attached to the chassis to work, is the wire just a placebo?

  4. #4
    Senior Member Cooter's Avatar
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    I did the same with the points in a 51 chevy Ya never forget that one! I couldn't even afford new ones so I spent 1/2 a day un-welding them and filing them back into shape.

    The ignition trigger on the primary windings IS the "ground" to collapse the coil.


    Terminal 1 would be the ignition hot (+) and terminal 15 would be the (-) trigger, but theres no chassis ground. It doesn't need one to operate because thats not where the 'circuit' is. The circuit completes across the plug gap.


    The ground wire your talking about just goes to the outer steel sandwich plates that hold all the windings around the iron core. It's for ignition noise so it doesn't upset the ECM.

  5. #5
    Senior Member 34nineteen's Avatar
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    Ok, so we are on the same page.

    That’s how I got home that night also. I replaced them the next day, but i was able to break them free fairly easily. Luckily I didn’t just sit and crank the motor until I wore the battery down. I had heard the rumor about that happening to points but Murphy’s law needed to wait until the time I needed the car to start the most. That car never left me stranded, but after that time everyone thought it was unreliable. I do not miss points!

  6. #6
    Senior Member rb70383's Avatar
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    Just stick a suppression diode across it. That way no need for the dangerous high voltages...

    Points....When a matchbook was also in my tool box.

  7. #7
    Senior Member 34nineteen's Avatar
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    Cooter is so old he put the illustration in with the external ballast resistor. Lol.

  8. #8
    Senior Member Cooter's Avatar
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    RB: Where does one even find a book of matches anymore They're dang-er-ous!

    34:19 Totally right?! I'm working on the '57 right now and feeling a little torn about upgrading the ignition. Point ignition is EMP proof, so when the bombs go off I won't have a car to drive with Mad Max. LOL

  9. #9
    Senior Member Endopotential's Avatar
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    Matchbook??? was that to file down the points? Sorry, was a little bit before my time.

    Tried to google it, and ended up going down the rabbit hole with threads on Reddit for sharpening hypodermic needles for IV drug users. Yikes

  10. #10
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    Matchbook for proper gapping



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