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blessedbikerbabe

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I've got a 1978 FLH, I need to know on a coil # 31620-83A which post is positive?

This is a new style, high performance, not in the book
 
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If you have a meter, check and see if you have ANY continuity (using 200K ohm scale) between either primary post and the spark plug wire tower & also between either primary post and the body of the coil.

What I am looking to see is if the primary winding is Totally and completely isolated from the secondary winding.

Are you using some aftermarket special ignition or are you using the stock points setup /or what.
 
As the coil mounts on the bike the bottom is typically 12volts (positive side).




From my HD Service Manual

CAUTION
On 1980 and later models, use only ignition coils marked ELECTRONIC ADVANCE. On 1979 and earlier models, use the old style coils. Interchanging these coils could cause a failure in the electronic components.
 
Jonas brings up a Very important point. Are you using a stock points system because if you are you can't use a coil that was designed for CDI. A standard points system needs a primary that has several ohms of resistance where as a CDI coil can be just 1 or 2 ohms. Using a CDI coil under a regular points ignition would draw a ton of current and melt the wiring harness.

Not trying to scare you, just giving you a heads up because from the attachment above it looks like you may have points with no ballast resistor so you will require several ohms of primary resistance in your coil.
 
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