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Thread: Ignition Coils

  1. #1
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    Default Ignition Coils

    Does anyone know of a way to test old H-D square coils with an ohmeter? I have several -48 style coils I'd like check before installing them.
    Thanks, VPH-D

  2. #2

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    Though it is not with an ohmeter, there is a bench test for coils in Palmer's book on page 400...Bones

  3. #3
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    Default

    Anyone? Anyone?
    VPH-D

  4. #4
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    Default

    If ya can find a Duo Gap tester that is what the MoCo used.
    Be sure to visit;
    http://www.vintageamericanmotorcycles.com/main.php
    Be sure to register at the site so you can see large images.
    Also be sure to visit http://www.caimag.com/forum/

  5. #5
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    Quote Originally Posted by Chris Haynes View Post
    If ya can find a Duo Gap tester that is what the MoCo used.
    That's a really cool picture Chris. You can see a Weidenhoff generator test bench off to the right.

    Quote Originally Posted by VPH-D View Post
    Does anyone know of a way to test old H-D square coils with an ohmeter? I have several -48 style coils I'd like check before installing them.
    Thanks, VPH-D
    Quote Originally Posted by VPH-D View Post
    Anyone? Anyone?
    VPH-D
    Attached is an internal coil wiring diagram (courtesy of Uncle Frank). You can see the two external terminals are the primary winding and the two plug wires are secondary. I don't have a coil here to take readings off of, but the primary should have low resistance (< 10 ohms) and the secondary should have high resistance (> 10,000 ohms) and there should be no reading between primary and secondary. I think a coil can still be bad if it passes those readings, but it won't be good if it doesn't ... Perry
    Attached Images Attached Images
    Last edited by Perry Ruiter; 11-24-2010 at 12:47 AM.

  6. #6
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    Default

    [QUOTE=Perry Ruiter;103729]That's a really cool picture Chris. You can see a Weidenhoff generator test bench off to the right.

    The Duo Gap tester is also a Weidenhoff product. H-D used a lot of Weidenhoff stuff. Weidenhoff today is known as Sun.
    Be sure to visit;
    http://www.vintageamericanmotorcycles.com/main.php
    Be sure to register at the site so you can see large images.
    Also be sure to visit http://www.caimag.com/forum/

  7. #7
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    Default

    The values I usually read for the primary side of the coil are around 1.2 Ohms and for the secondary side are around 17,000 Ohms.
    An infinite reading on either side would indicate an open circuit and values a lot lower would indicate some of the turns are shorted. Any reading within maybe 20% would indicate your coil is good.

  8. #8
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    Quote Originally Posted by Chris Haynes View Post
    The Duo Gap tester is also a Weidenhoff product. H-D used a lot of Weidenhoff stuff. Weidenhoff today is known as Sun.
    And if I understand the corporate hierarchy properly, Sun is owned by Snap-On ... Perry

  9. #9
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    An ohms meter is just giving you nothing more than a quazy continuity test. Is there a way to load test it?

  10. #10
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    Default

    Energize and trigger it with the leads held off ground, should see a strong spark. They can still go thermally intermittent, but that's the best way I can think of to test them. If you have a running bike just disconnect the coil that's on it and use some jump leads to the coil you need to test but all you really need is a good battery, some wire, contact points and condenser. I've seen many 12 volt coils pass the ohm tests and still not run right.
    Brian Howard AMCA#5866

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