Johnson HD25 Coils… Why 4 wires?

[bayham3261]

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!

A little information is a dangerous thing!

[Tom]

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]

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.

• This reply was modified 5 years, 3 months ago by frankr.
• This reply was modified 5 years, 3 months ago by frankr.

[bayham3261]

Thanks for the insight into these antique ignition coils!

A little information is a dangerous thing!

[Author]

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