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Posted by D Sherman [72.47.9.228] on Tuesday, July 20, 2010 at 00:49:23 :
In Reply to: 6V coil test posted by Glen Idaho [64.126.154.220] on Monday, July 19, 2010 at 21:33:46 :
I suspect an ohmmeter isn't a fool-proof test of a spark coil just like it isn't a fool-proof test of a motor winding. The trouble is that coils of wire tend to fail by adjacent turns shorting to each other. Out of the total amount of wire in the coil, a shorted turn or two isn't going to make any noticeable difference in the resistance, but it less efficient and hotter running. That in turn will eventually cause another turn to short, and so on, until it gradually degrades to the point where it's obviously bad.
An ignition coil circuit is electrically the same as a flyback circuit in a TV. The old-time TV repairman would check for shorted turns in a flyback transformer by "ringing" it electrically -- putting a sharp electrical pulse on it and watching with an oscilloscope to see how quickly the resulting oscillations damped out. Faster-than-normal damping meant excessive losses and probably a shorted turn. Something similar would work for spark coils, but I've never heard of anyone testing them that way. It's easier to just swap in a known good unit.
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