This principle seems the most difficult for some people to understand. But, powersports electrical parts made up of wire windings are the least conclusively troubleshooted using resistance tests. It may not be your fault if you don't accept this. But it is your fault if you regard yourself as expert at motorcycle electrical diagnostics. And you guys with radio or other kinds of electronics backgrounds--you who are so vocal on powersports forums--stop and consider that the ease with which you test radio and audio components has lulled you into complacency. Motorcycle charging and ignition systems endure more severe conditions than you realize.

The problem with resistance-testing wire coils is this: A piece of wire a couple hundred feet long coated with lacquer and wrapped into a coil has a habit of losing parts of its insulation as a result of constant exposure to high electrical stress, engine heat and acidic engine oil. Thus various parts of the winding can touch, resulting in lowered resistance of the part. But mostly when loaded. When not loaded, these wire coils often "lie" to the troubleshooter. They can meet factory manual resistance specification when their bad layers aren't touching. But when stressed by actually working, by carrying current, and thus warmer and affected magnetically and undergoing physical, dimensional expansion, it is then that they reveal the truth of their condition. Every career mechanic with even a little time on the job knows the truth of this.

I call this problem with resistance-testing wire windings the 95/50 rule. Here's what it means. When a wire coil resistance-tests "bad", it is almost surely bad, so the probability that the test result is valid is 95+ percent. Fine. We can work with that. That's the 95 part of the 95/50. But, here's the kicker. If on the other hand the resistance test shows the part is "good", now you have a problem. You have a dilemma. Because, with this "good" result, there's no better than a fifty-fifty chance that the part is actually good. Seriously. Because of the issue with wire coil failure modes described above. A part ohmmed good is a real challenge. The falsely good reading of a bad part will lead the technician on all kinds of diagnostic rabbit trails away from the real issue. Not to mention wasted shop time and revenue.

So, bad resistance = probably bad. But good resistance = maybe good, maybe bad. A crap shoot, in other words. Electrical parts having wire windings in them absolutely must be dynamically tested. Avoid using resistance tests in troubleshooting parts containing wire coils. Use instead tests of the parts while they are working. Troubleshooting will be faster and the results more conclusive. If you would like to check out an assenting voice on this, see Dan Sullivan's excellent automotive electrical testing book, Fundamental Electrical Troubleshooting, available on Amazon.

2017 Postscript: Had one of these just last week. Ignition coils on the money both primary and secondary resistance. But using one of several differnt possible dynamic ignition coil tests, no bueno.

2024 Postscript: Some of my customers, after learning of this principle, have also confirmed that resistance tests of their ignition coils failed to find their faults.