Johnson HD25 Coils… Why 4 wires?

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    bayham3261
    US Member - 1 Year
    Replies: 15
    Topics: 3
    #164205

    Hi Everyone, I’m working on a HD25 right now. It’s a fun little project. I noticed that these old horseshoe coils have 4 wires instead of the typical 3 wires (primary, secondary and ground). Can anyone explain why these have 4 wires? I think these coils are ok but the condensers will need replacement along with a point cleaning.
    Does anyone have a resistance spec for these coils?
    Out of curiosity has anyone ever pulled these old coil windings off and installed a new one? I think the laminations are too small for a typical OMC universal coil winding substitution.

    Thanks!

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    Tom
    Tom
    US Member - 1 Year
    Replies: 329
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    #164208

    Secondary resistance should be 3.0 to 4.5 kilohms. The coil should fire at 1.5 amps on a coil tester. These coils, like most old outboard coils, are actually two coils in one… the primary and the secondary. Usually, one wire from each is tied together, leaving three wires coming out. This coil doesn’t have two tied together, so there is a separate ground for the primary and the secondary.

    Tom

    frankr
    frankr
    US Member - 1 Year
    Replies: 3930
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    #164217

    Going way back, some Johnsons had “maverick spark suppressors” to prevent stray unwanted “maverick” sparks. On those, the secondary winding was grounded through the spark suppressors instead of directly to the armature plate. The secondary winding was grounded through the suppressor, but not the primary because it would not work if the primary was not directly grounded. Thus the reason the primary and secondary were individual wires.

    BTW, the secondary was not truly grounded, but only close, via the suppressor. The spark current had to jump that almost-connected gap, resisting stray, unwanted sparks.

    No reason the 4-wire coil cannot be used with non-suppressed mags, so no reason to make another part number for the non-suppressed mag.

    EDIT: If you attempt to ohms-test the SUPPRESSED secondary winding by connecting your meter lead to the mag plate, it will indicate an open secondary. On those mags you have to connect the meter lead to the secondary ground WIRE, not to the plate.

    Ignorance is simply a lack of education. Ignorance can be cured. There is no cure for Stupid.

    • This reply was modified 1 month, 3 weeks ago by frankr frankr.
    • This reply was modified 1 month, 3 weeks ago by frankr frankr.
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    bayham3261
    US Member - 1 Year
    Replies: 15
    Topics: 3
    #164242

    Thanks for the insight into these antique ignition coils!

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