You are right.. Coils don't usually go bad.  You see cars in junk yards with 1/4 million miles on it with a perfectly good coil.  There is no mechanical movable parts inside the coil. It is basically a transformer constructed in a cylinderical shape.  It consists of a few winding of very thick wires in inner core (primary winding) and a few hundred thousand windings of very thin wires around the inner core making up the secondary winding.  So, the physics is high currrent and low voltage in tranformed to low current and high voltage out (power in == power out!)  For this to work, the wires in the windings must be insulated from each other.

The most common way for coils to go bad is as it ages, the insulation materials around the windings developes minute cracks and hair line fractures. The very high voltage (40KV plus) in the secondary winding contribute to this. It will show no ill effect.  A few months, or years go by, these minute factures absorbs moistures during the summer time and in winter when it gets to way below freezing, the frozen water molecules inside the cracks expands and increase the cracks some more.. finally enough cracks in the insulation layer between windings  let high voltage current jump between winding and the coil gets weaker and weaker until it stop working.

btw, I hope you didn't get a nasty shock testing the coil!

Yau-Man Chan
87 GL

>>> "Seth C. Bruggeman" <scbrug@MAIL.WM.EDU> 01/08 10:39 AM >>>
Well, thanks to everyone's input (esp. Yau-Man who provided the exact coil
checking method I was looking for), I traced my troubles to a bad coil,
replaced the offending part, and am now back on the road.  One last
question, though...am I treating a symptom of a greater problem?  In other
words, why would the coil go bad?  I realize the bad coil covered a lot of
miles, but coils seem like the kind of part that goes bad generally for
reasons other than mechanical failure.  Any ideas...???

Seth B.
'84 Westy