® The Volt Drop Test

You're working on an electrical problem. You suspect you've got a bad wire somewhere, but you aren't looking forward to resistance-testing have the wiring harness. Try a voltage drop test instead.

What's a voltage drop test? Okay, what happens when you touch a multimeter's test probes to both of a battery's terminals? You read battery voltage, of course. But why? Because you are actually reading the difference between the two terminals, one fully charged, the other having no charge. At one terminal is13 volts, at the other 0, and the voltmeter indicates the voltage dropped from one terminal to the other. Now put both test probes on the same terminal. Now what do you get? You get 0 volts, naturally. But why? Because the two probes resting on the same terminal lack two points of reference from which to gauge a difference. No voltage is lost, so no reading results. All voltage readings are in reality voltage drop readings.

For an example of a real test, let's probe with our multimeter both sides of a bike's keyswitch. That is, put one probe on the input side of the switch (red on a Honda, white on a Kawasaki) and the other probe on the output side (black on a Honda, brown on a Kawasaki). Turn the keyswitch on and note the reading. In most cases the reading wil be zero. Why? Because there shouldn't be any difference in voltage between the inout and output sides of a switch. You might however find a tiny voltage, say 1 volt. What does that mean? I means the switch contacts are dirty or loose and thereby present enough resistance to "push back" against voltage, lowering it. More practically, this resistance absorbs electrical pressure as if it were a light bulb. Is a switch supposed to use electricity like a light bulb, or merely carry it like a wire? Not use, but carry, naturally. So we don't want a switch to act like a load, or it will make circuits not function properly, not to mention it will get hot and start melting its plastic connectors. Zero volts drop is the goal then in a wire, connector, or switch, because these all are examples of electricity *carriers* and are not intended to be electricity users.

Does that mean a switch that reads a small voltage between input and output is no good? Technically, yes, but in reality we can allow a very tiny amount, that is, no more than 1/10 volt. This would read 0.1 volt on your digital multimeter. Why? Because sometimes it's the best you can do, though it isn't ideal.

Alright, another example. Real-world, too. You have an early Honda or Kawasaki charging system that has an electromagnetic type rotor. You're getting low charge to the battery, and you have aleady bypassed the regulator to dynamically test it, and you then dangled a feeler gauge near the alternator and turned the keyswitch on an off to teste for magnetism, and got none or very little. The next step is to volt drop test the rotor circuit, starting at the battery. That would look like, battery to rotor positive brush. You'll probably pick up 1 volt, and that is okay, but no more. Don't forget you're encompassing several connectors, a lot of wire, and a handful of switchs and even a fuse or two, so a whole volt is okay, maximum. But what if you get more than that, say 3 volts? Then you know the problem with not having much magnetism is related at least in part to there being excess resistance in the rotor circuit. The technique now is to systematically move the probe that is on the battery positive terminal closer to the other probe on the rotor's positive brush, which you are keeping there, until the drop goes below the maximum acceptable level. What was that level, class? Right, 0.1v. You move the one probe closer and closer to the other until that point it goes under the acceptable maximum, and the last place touched contains the fault. And you already know what it will be -- a switch, a connector, or in a few cases a bit of bad wire.

The voltage drop test is one of the most powerful electrical troubleshooting techniques available. Focusing on a conductor's voltage instead of its resistance provides three advantages. First, it speeds up the job. Resistance tests require that you disconnect and electrically isolate the component, and this takes time. Second, a resistance test is the least reliable and conclusive test you can make, due to temperaureand other variables, and due to the simple fact that things such as wire coils, which make up a lot of the systems on your motorcycle, fail most demonstratively when stressed and often not at all when at rest. Third, a voltage drop test can actually pick up resistances too small for most test meters to read. Remember, that is what a volt drop test is, a dynamic resistance test, that is, a resistance test done under load conditions. Just remember one simple thing. Whenever doing a volt drop test, voltage *must* be present for the test to work. The keyswitch must be turned on. If you forget to turn on the keyswitch, you will have highly inaccurate results, so don't forget!

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