The voltage drop test

Tips and Recommendations from Guru Mike Nixon

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mikenixon
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The voltage drop test

Post #1 by mikenixon » Wed Oct 04, 2017 9:02 pm

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 half the wiring harness. Try a voltage drop test instead. What's a voltage drop test?

Well, what happens when you touch a multimeter's test probes to a battery's terminals? You read battery voltage, of course. But why? Because you are actually reading the potential difference between the battery's two terminals. That's what makes voltage, i.e. a differential, and that's what battery is, a "differential box". Now put both meter test probes on the same terminal. (Huh? Don't laugh). Now what do you get? You get 0 volts, as expected. But why, technically speaking? Because the battery is dead? No, because the two probes resting on the same terminal lack the necessary two points of reference from which to gauge a difference, a differential. No differential, no voltage. All voltage readings, no matter what kind, are in reality differential, or "voltage drop", readings. A volt drop test then is merely a voltage test from one point to another in the system, wherein the meter's reading is simply a direct indication of that difference. To ensure that we always read only that difference, the multimeter is connected across, that is, straddling, the switch, connector, or component.

What should the reading be? A connector should theoretically drop 0 volts. However, we can allow up to a very tiny 0.2 volt drop, as all connectors have a slight amount of resistance. However, too much resistance absorbs electrical pressure as if it were a light bulb. Is a connector supposed to use electricity like a light bulb, or merely carry it like a wire? You know the answer. Not use, but carry, naturally.

Let's volt drop test power to the ignition coil. One meter test probe to battery positive, the other to the ignition coil's black/white wire. Turn the key on. Say you get a 4-volt drop. Not good. Remember what we're straddling. We're straddling the entire ignition coil power circuit from battery to coil. What's in this power path? Main fuse, key switch, engine stop switch, and at least four or five connectors. Somewhere along this electrical path is a problem. To find the glitch, keep the positive meter probe at the battery, and move the negative probe closer to the other test probe onto the next switch or connector in the system (have the factory schematic in front of you). Note the reading. Move the test probe again, on to the next connector in the line, and keep moving until the reading decreases suddenly a large amount. That’s the tip-off point. Somewhere between here and the last point tested will be the bad spot in the circuit. It works, and it really is this simple.

The voltage drop test is one of the most powerful electrical troubleshooting techniques in your troubleshooting arsenal. Focusing on a conductor’s voltage instead of its resistance provides two benefits. First, it speeds up the job. Resistance tests require disconnecting the component, and even then the results are not always reliable. Second, the voltage drop test delivers more accurate values. It can even uncover resistances too small for an ohmmeter to measure. Just don’t forget that the key switch must be turned on for each test, or you will get some misleadingly large readings. Happy troubleshooting!
Mike Nixon
www.motorcycleproject.com

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CYBORG
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Re: The voltage drop test

Post #2 by CYBORG » Thu Oct 05, 2017 9:49 am

Thanks for the lesson. I've been doing that for years, and it is both reliable and fast. People who understand electricity know these things, but a lot of people consider it a black art. I'm sure your information will help many on this site. Keep them coming

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mcgovern61
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Re: The voltage drop test

Post #3 by mcgovern61 » Thu Oct 05, 2017 10:01 am

Great tip Mike!

Question. When testing our systems, we can only access the connectors and components. There are many splits or branch circuits within the wire harness that are covered in tape. It is these connections that are difficult to access for checking except for going from the battery to the key switch. For example, the red wire from the 30 amp fuse goes directly to the key switch, but then the black wire that leads out from that same red wire down to the fuse panel also branches off to other parts in the system such as the regulator rectifier.

Short of opening up the harness, is there a way to use this method to determine if a connection is at fault before or after one of these branch offs?

Wire Harness 001.jpg


82 Wire Harness 011.jpg


82 Wire Harness 004.jpg
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mikenixon
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Re: The voltage drop test

Post #4 by mikenixon » Thu Oct 05, 2017 3:28 pm

Hi, mcgovern61! Now there is some hands-on experience!

Yes, buses they're called. And they sure can be troublesome. I recently resigned from a job on the inside of the industry and can tell you with confidence manufacturers continue to struggle with these even today. Notice they are not soldered, but like everywhere else in the system, crimped. Kawasaki whike I was there changed their wire harness vendors more than once, and on Jet Skiis in particular is still experiencing headaches due to concealed buses.

The volt drop method can be used to identify not only connectors and switches that are giving trouble, but wire buses too. If a drop is evident along a certain stretch of wire and there is no connector or switch within that span, then it has to be something in the wire itself, and a bus would certainly be included. The rule of thumb is, once a drop is detected, move the meter probes toward each other until you have pinpointed the spot.
Mike Nixon
www.motorcycleproject.com

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salukispeed
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Re: The voltage drop test

Post #5 by salukispeed » Thu Oct 05, 2017 6:23 pm

This is a very good method of diagnosis and can be far better than an Ohm meter, An additional observation when troubleshooting is If the load is heavy like a headlight or something else moderately large you can sometimes feel along the harness and feel for a warmer spot than could indicate such a connection as the crimped buss. Excellent advice for these ol girls and many other vehicles


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