® The Case Against Resistance Tests
of Charging and Ignition Systems

I have said this elsewhere on my website, many times, but allowing for some highly specialized exceptions, resistance tests are next to useless in electrical troubleshooting powersports vehicle charging and ignition systems. The fact is, there are very compelling reasons to avoid using resistance tests on alternator stators, field coils and ignition coils. That this should still have to be emphasized; that so many on Internet forums are unaware of this or worse, waste time debating it, this late in powersports history, simply and absolutely astonishes me. Any professional powersports tech knows the truth of this. But this does indeed need emphasizing, so let's look at the facts, one at a time. Those of you who can think for yourselves, please do and if these points raise questions, don't hesitate to ask them. Others, including those who have taken me to task for saying these things, well, I make no apologies and add only this: these things weren't learned in a classroom. They came from a lifetime in the trenches, day after day, job after job, living in the real world.

  1. Reason number one. Manufacturers hedge their bets where resistance tests are concerned. Factory service manuals, though they describe resistance tests, let the proverbial cat out of the bag when they do one or both of two very curious things. First, they will often say "...using XYZ multimeter, part number 123456.". This is not an attempt to sell multimeters. This is an obvious admission that the specification in question is not real-world, being subject as it is to a certain test meter, under certain conditions, including temperature and humidity, just for starters. Second, factory manuals frequently follow resistance specifications with "Replace with known good component.". Again, this is nothing less than tacit acknowledgement that the specification is tentative and not to be taken at face value. Incidentally, none of the multimeters recommended in factory service manuals that are over ten years old can be purchased today, making depending on these tests all the more nonsensical.

  2. Item number two. The horror of resistance matrix tables. The resistance matrix tables in factory manuals for voltage regulators is one of the all-time gigantic farces and scandals in the powersports industry. The fact that perfectly working regulators fail these tests and that bad ones often pass them is known to even the most plebian practitioner in repair shops all over the world. These tables have been one of the accepted wonders of the powersports world for nearly as long as there have been motorcycles. Avoid them.

  3. Number three. The unfortunately biased nature of factory manuals. Factory manuals started changing in the early 1970s, until today they are written from conception to completion to merely serve as legal documents prepared to be ready at hand for the manufacturers' defense teams. This is reality. Oh sure, lot of good mechanical stuff is in there, most of it invaluablr. But that they communicate anything useful to the electrical troubleshooter is increasingly recognized within the industry as pure bonus.

  4. And now the biggie. Number four. The previous three items are debatable, maybe, if you just gotta argue, though no one in any service industry argues them. What is unarguable is the fact that a piece of wire a couple hundred feet long coated with lacquer and wrapped into a coil has a habit of shorting and doing crazy things after repeated exposure to hot, acidic engine oil and when it does, it almost never does these things when tested with an ohmmeter. That's it. Bottom line. Everything else is secondary. Most of the components on a motorcycle that are resistance tested are wire coils, that is, windings. Even those outside the industry acknowledge that such parts are not reliably measured with an ohmmeter. One simple reason: they're not being stressed. Wire coils have a habit of lying to the troubleshooter. When cool and not carrying an electrical load, they often meet factory manual resistance specification. But when stressed by actually working, by carrying current, and thus warmer and undergoing physical, dimensional expansion, they often will not function. This leads to a problem well known in the powersports industry, what I will call the 50/95 rule. What it means is, a wire coil that tests good unloaded might be good and it might not be good. You can't be sure. There's no better than a fifty percent chance. Lousy odds. But, if the resistance test result is bad, the chances are very much better. Almost, but not quite one hundred percent. So, good = maybe, bad = probably. 50/95. Now, many folks may not see the problem here, but believe me it is a problem. A technician isn't going to recommend to his customer a parts replacement based on a "maybe"! The customer will be mad when the part doesn't fix the problem, and he (the tech) will be on the hook for its cost. Alternator stators for example, cost $300 to $700, and voltage regulators are also in the hundreds. Worse, the falsely good reading of a bad part will lead the technician on all kinds of diagnostic rabbit trails. The only correct, conclusive (a key word) way to test wire coil based parts is by actually measuring their performance, and this is not done with a resistance test. Testing a part by measuring its actual performance is called in the industry "dynamic testing."
I have had many conversations with those who were troubleshooting their bikes and also with those trying to help would-be troubleshooters. The reluctance on both their parts to think like a mechanic is troubling, and I lay at least part of the blame on the Internet. The net is at the same time a blessing and a curse. A blessing because it quickly codifies major thought streams on a subject. But a curse also because often the Internet user forum as an organism breeds a decay that colors and constrains that accumulated information. Every forum has its house "experts," and who knows how they got to be that, and pity the one who disagrees with them. The result is a one-dimensional thinking pattern that regards problems in very superficial ways, and ways that are more often than not ill-informed. Silver bullets don't fix powersports electrical systems. The systematic (that is, heirarchical) application of logic (if this, then this) does, and when that system is born of real-world experience, meaning an understanding of how the parts actually work, not simply their design specs, then troubleshooting scales down from the shotgun to the scalpel, and becomes meaningful, real-world, and effective.
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