Re: PO15 Coil Testing

[Tom]

"Normally, a resistance value is measurable from the secondary post to the coil’s ground point."

This is true, most of the time. But for some opposed twin outboards, it’s not the case. The Johnson coils of that era tend to be very reliable under normal use. If you haven’t done the test specified by Garry, do so before you write off that coil.

Here’s a little more detail: Most coils have a primary and secondary winding which are grounded together. The other end of the primary winding goes to the points. The other end of the secondary goes to the spark plug. The complete secondary circuit, then, is from the secondary lead, across the spark plug gap, to the engine block, to the mag plate, and back to the secondary coil through the ground.

The PO coil windings are independent. One end of the secondary winding goes to one spark plug, the other goes to the other spark plug. The complete secondary circuit, then, is out of one plug lead, across one plug gap to the engine block, across the other plug gap, and back to the secondary through the other plug lead. So if you are testing one spark plug wire at a time without grounding the other wire to the engine, you will get no spark. And the coil will be damaged if repeatedly tested without grounding one of the seconday leads.

Tom