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Old 06-29-2020, 07:44 AM   #1
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Electrical Troubleshooting Idea

Jason Martin, owner of MTR Fleet Services in Cumming GA taught me a trick I had never heard of. As most of us know, using a VOM to read the voltage on a wire can show 12VDC under no load, but if somewhere along the path the connection is faulty, once a load is applied, the voltage drops or disappears. Jason had made up a test device that was an ATO fuse holder with long leads and alligator clips attached to each end. Using a 5 amp fuse in the holder, if Jason suspects a poor connection when the voltage reads OK, he attaches one alligator clip to ground and the other to the wire with the suspected poor connection. If the connection is poor, as suspected, the fuse will not blow. If it is good, the fuse blows instantly. Pretty clever idea! You could substitute the size fuse you thought was appropriate for the circuit you are testing. Of course, this method is useful only for circuits that normally carry at least a small load. It won't help with digital circuits or those that carry currents less than 1 amp.

I suspect Myron (YC1) , the 12VDC guru, already knows this trick, but it was new to me. I suppose the ultimate device using this principle might be a fused selection of resistors between 1 ohm and 10 ohms.

I would be surprised if some PAP (pontificating apocalypse predictor) did not post that this method might cause problems ranging from coach fires to global warming and world hunger. But for those of us who have spent many hours diagnosing electrical problems and crawling around and under coaches, it is one more useful tool in the toolbox.
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Old 06-29-2020, 07:58 AM   #2
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Great idea ---- now if I can remember when I need it LOL

Ray
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Old 06-29-2020, 08:02 AM   #3
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That's a great way to load test for various amperage circuits.

My old trick of using a 100 watt headlight bulb to load test the circuit is a little more clumsy, but I don't have to keep replacing the fuse.

I like that it will prove that it will carry a certain amperage, but you also need to be sure the voltage isn't dropping off as well.

Happy Glamping.
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Old 06-29-2020, 08:12 AM   #4
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why not just watch the voltage as you turn off and on the device? If it reads 12v with device off and lower voltage with device on, there is a resistive connection in the path
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Old 06-29-2020, 10:02 AM   #5
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Quote:
Originally Posted by gatorb8 View Post
why not just watch the voltage as you turn off and on the device? If it reads 12v with device off and lower voltage with device on, there is a resistive connection in the path
X2!
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Old 06-29-2020, 11:57 AM   #6
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why not just watch the voltage as you turn off and on the device? If it reads 12v with device off and lower voltage with device on, there is a resistive connection in the path
That works most of the time, when the connectors are easy to get to, and is a method I frequently use. These were six pins in the two multi-pin connectors in a harness that plugs into the ECM. Jason and I both prefer not to back-probe sealed connectors since you risk ruining the seal. Only way to check these without compromising the seal was to remove the connector and probe the pins. In that case, there is no load on the circuits, so you need some other method to assure the current-carrying capacity.

Another useful trick, suggested by Myron (YC1) is to locate a few reliable grounds throughout your coach and have a way to securely connect a ground "extension" lead to them so you can connect to the ground lead of your VOM. So many instances of electrical problems are the result of poor grounds.
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Old 06-29-2020, 12:00 PM   #7
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I use a old style 12V test light with a incandescent bulb over a digital meter plus it’s faster if checking a lot of fuses.
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Old 06-29-2020, 03:08 PM   #8
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That works most of the time, when the connectors are easy to get to, and is a method I frequently use. These were six pins in the two multi-pin connectors in a harness that plugs into the ECM. Jason and I both prefer not to back-probe sealed connectors since you risk ruining the seal. Only way to check these without compromising the seal was to remove the connector and probe the pins. In that case, there is no load on the circuits, so you need some other method to assure the current-carrying capacity.

Another useful trick, suggested by Myron (YC1) is to locate a few reliable grounds throughout your coach and have a way to securely connect a ground "extension" lead to them so you can connect to the ground lead of your VOM. So many instances of electrical problems are the result of poor grounds.
got it, thanks for clarifying!
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Old 06-29-2020, 10:19 PM   #9
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And, if you don’t like blowing fuse, substitute an ATO style circuit breaker. Back in the day before the ATO fuses, I used the circuit breakers with the threaded post. Now, I’m dating myself.

Actually, I was using this system more to find a dead short on a circuit. I used the circuit breaker with the alligator clips in the fuse block under the dash. Then I keep opening the circuit until I found the short to ground.

Van, your tech took it to the next level. I like that idea.
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Old 06-30-2020, 07:58 AM   #10
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why not just watch the voltage as you turn off and on the device? If it reads 12v with device off and lower voltage with device on, there is a resistive connection in the path
A digital voltmeter reads to slowly. I carry an old old analog meter and I have caught many a naughty issue by watching that beautiful long meter dip ever so slightly.
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Old 06-30-2020, 08:01 AM   #11
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As Van suggests, a fused lead is a very valuable tool. If you suspect a bad ground for example you can use that lead to safely and temporarily apply a temporary ground.

As for the digital meter missing important clues I like to use an old old 12 volt test light. One with an old dome light bulb in it. Then you have a decent load and if the light comes on bright you know you that is working. With a voltage test at the same time to be precise.
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Old 06-30-2020, 08:03 AM   #12
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If you don't believe a digital meter can fool you, try this test.

Go to your battery. Put the negative lead on the battery post - . Touch the positive battery post with a finger. Hold the positive lead of the meter in your other hand. The meter will read something. Wet your fingers and you should read the full battery voltage.

There is not enough current to light a bulb but plenty enough to fool the battery.
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Old 06-30-2020, 08:07 AM   #13
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A digital voltmeter reads to slowly. I carry an old old analog meter and I have caught many a naughty issue by watching that beautiful long meter dip ever so slightly.
Ah, I should have kept that old Simpson 260
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Old 06-30-2020, 08:11 AM   #14
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Now for one more trick and an invention I will take credit for because I have not seen them marketed yet. There are too many similiarities to other products on the market so not patent pending.

One of the most difficult problems chasing electrical issues inside or out of an rv is finding a suitable ground for the meter probe.

Many times I recommended using an extension cord and plugging into the ground lead. This is fine but may be dangerous to a novice.

So the simple solution. Grab any cheap three prong plug and install just one wire inside on the ground lead. Install an inline fuse for safety and an alligator clip on the other end of the wire. Any length will work for diagnosing and you can use this to apply a ground to something that is acting like it needs a proper ground. Common sense applys here of course.

I actually found a nicely made molded plug that is used near test benches but they are about $8. I bought two after making my own and have given them as gifts.

Before making this test equipment/tool, I had installed two places on my rv to connect long wires to. One near the tow hitch because there are always problems with tow lights etc. And the other is under the passenger seat which is near all those tempermental switches and wiring.
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