RVelectricity – Cheap trick for finding a short circuit


By Mike Sokol

Dear Mike,
I have read through your RV Travel topics, bought and read your book on Electricity, read your Facebook page and done as much research as I know how. Here is the problem I am trying to fix, as well as use it to learn more personally.

My RV bathroom overhead lights continually blow their 3-amp fuse. I removed the lights so that both wires are not connected to anything (to eliminate the light fixture as the culprit), but it still blows the fuse as soon as I turn on the switch. The little glowing light in the fuse only tells me it’s blown, but not what’s causing it to blow.

I included two pictures [not included in this post]: Fuse box (FireFly) and the switch panel. My thought was that I run a wire from one side of the fuse to one of the two wires that would have been connected to the light, checking continuity. This may take multiple attempts because I don’t know which side of the fuse connects to which of the two wires (possibly white wire?). That is the only single thing I think I learned to try. Other than that, I am at a loss. Where could I learn more to troubleshoot this issue?

My great appreciation for all you do. As you probably know, I am a donator to your “I Like Mike” campaign, so I hope it helps. —Alan W.

Dear Alan,
You’re in luck… I’ve invented a simple troubleshooting technique for finding short circuits in DC electrical systems. Actually, I figured this out some 50 years ago when I was a young pup working part-time in a truck shop. Whenever there was a short circuit in any trailer lights, my mechanic/boss would do a “smoke test” instead of blowing a bunch of fuses. But there’s a much better Current Limited Fault Test which is much safer and faster. In fact, I’m getting ready to include this advanced technique in an RV training class I’ll be teaching for PRVCA  (the Hershey RV show group) in a few weeks. But you get to see it here first.

But first, take a break

If you want to take a break from all this troubleshooting stuff, here’s another Cheap Trick I can recommend. Yes, it’s the band Cheap Trick playing Dream Police.

Don’t do a smoke test…

Now, please don’t do this as it’s very dangerous and destroys wiring, but here are the basics. My boss would wrap a piece of foil around a blown fuse, then turn on the power. Now the wiring was carrying a LOT more fault current than it was rated for, so the wire between the fuse block and the short circuit would overheat and start melting and smoking. If you were quick enough you could find where the short circuit was before the insulation melted and caught on fire. See why you don’t want to do a smoke test?

Yes, I invented this test when I was 17 years old

So I developed a really simple alternative to the “smoke test” – which I’ve used for the last 5 decades to troubleshoot short circuits in vehicles – which is quick, cheap and easy. I really did come up with this in the truck shop where I worked part time, and every mechanic/electrician I’ve showed it to since then has loved it. So I’ll teach it to you (and everyone else here) now.

But what’s a short (or should I say shorter) circuit?

But first, let’s review exactly what a short circuit is, and how it works. As you can see from the diagram below, a short circuit is really a “shorter” current path in the wiring circuit. So instead of the current taking the full path through the fuse, switch, bulb and back to the battery, it takes a “short(er)” path, bypassing the light bulb. Now, this shorted wired could be something like the insulation rubbed off of a wire and making contact with a metal frame piece in your RV. Normally (without a short circuit) the bulb itself limits the current flow to maybe 1 or 2 amperes. But the “shorter” circuit allows the full amperage of the battery to flow through the wiring, which is what blows your fuse with 100 or more amps of short circuit current.

Don’t blow your fuse

Instead of going through a bucket of fuses, or wrapping foil around the fuse for a smoke test, I decided to limit the fault current so I could troubleshoot the path at my leisure. What I did was to take a 12-volt test light and clip it between the two contacts where the fuse should go. This did two things. First it limited the fault current to around 1 or 2 amperes, which was well within the current-carrying ability of the wiring so it wouldn’t do a “smoke test.” And secondly, the bulb would change brightness for troubleshooting. If the bulb was fully bright, then it was a dead short somewhere. But as you wiggled and flexed the wiring you could get the bulb to go off. And that’s exactly where the short circuit in the wiring was located. Once you figure that out it only takes a visual inspection to find and correct the problem.

DIY test fuse/bulb

Here’s what I had my reader Alan build to assist with the troubleshooting. I recommended that he take a blown fuse and solder a 12-volt running light bulb on a pair of wires, which in turn was soldered to the top of the already blown fuse. Now all he had to do was plug this blown fuse into the fuse-holder, turn on the light switch and watch the test light come on at full brightness because of the dead short circuit somewhere up in his ceiling. Take a look at what Alan whipped up.

But wait, there’s more…

And there’s one more addition to this test you can do if you have an inexpensive clamp ammeter that reads DC current in the jaws (not all of them do). By using a DC clamp meter along the wire, you can actually trace the short circuit current. So if your clamp meter is between the fuse/bulb and the short circuit it will read around 1 ampere of current. But once your clamp meter is placed after the short circuit point, the current will now be 0 amperes.

Clamp meter in action

Since you’re not going through a bunch of fuses, and there’s only 1 or 2 amperes of limited short-circuit current flowing (instead of 100 amps that will blow a fuse or melt the wiring), you can take your time to snoop around with a DC clamp meter. One caveat is that an incandescent bulb can get pretty hot after it’s been on for a while, so don’t lay it down on anything meltable. Or better still, buy a cheap running trailer light from your local auto store and solder it to a blown fuse. Then you’re ready for your next short circuit adventure.

Wrap it up

And yes, I got an email back from Alan W. the very next day saying that he quickly located the short circuit up in the ceiling using this method and was able to splice in a new piece of wire. So, problem solved without having his RV sit for weeks in a repair shop, and he didn’t have to pay for hours of troubleshooting by a technician who doesn’t know this cheap trick. Way to go, Alan!

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.


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Matt C
3 months ago

I have owned a Fluke/Bell current probe for a long time and I am glad to see that similar are now available at consumer prices. But, I was doing this stuff long before that and many years ago had to locate the damage to a trailer’s harness. It was a solid short in the brake light circuit. So, I provided the circuit with roughly the same current as a pair of 1157 bulbs. Then, I did my best to follow the harness with a compass and watch for where the pointer moved. This worked. I found where a part of the cradle (it was a boat trailer) had shifted and crushed the harness. I have used this method since, but not always with great success, but it is an inexpensive thing to try.

Don Biancone
3 months ago

Say Mike how about instead of a light bulb, using something that makes a sound like a buzzer so you can search for the short at a distance from the fuse box?

3 months ago
Reply to  Don Biancone

That is exactly what I do… a 10 cent piezo soldered into a blown fuse. 99% of the time I expect an “off” circuit to simply open when the short is fixed. I’ve also used bright opposed LEDs (polarity moot), but i’m generally more likely to be able to hear rather than see.

3 months ago

Now how about telling how to find a short in the RV ceiling! No Mfg wiring diagram, long distance from fuse to light socket, stapled down wire bundles…..?

2 months ago
Reply to  Mike Sokol

Can you expand on what you are calling a “Fox & Hound” tester?
About a month ago, I used my Idea Tracetone to find the electric brake lead (7 pin connector) back up under the dash of my son’s RV.

3 months ago
Reply to  jim

That was exactly the problem. No wiring diagram and the manufacturer was out of business years ago. With guidance from Mike Sokol, I followed the wire from the back of the 12volt fuse panel, opening wall access points and locating the wire by clipping the clamp meter around each wire. Then, I would turn off the light switch (I could tell it was off because of the test light he had me build). Then, turn it back on at the switch. It was easy to find out which wire was for my bathroom ceiling light that way. I kept doing this until I ended up with the wire going up over the ceiling (top of the shower stall). It only had one exit at that point, and that was terminated at the light that did not work. No power got to the light itself, so that told us it was between the last “good” point where the wire went into the ceiling, and the “bad” spot was where it exited to go to the bathroom ceiling light. I simply routed a new wire from the “good” spot, to the light.

Lew Engel
3 months ago

Another method uses a old fashion directional light flasher and a small plastic ammeter like the ones used to check the alternator output current. Connect 2 wires with alligator clips to the terminals of the flasher and clip the alligator clips to the fuse terminals. The flasher will start flashing, causing current to flow in short bursts down the wires, now use the ammeter to follow the wire. it will be jumping from the current of the flasher until you reach the short. This works good in wiring harness where you cannot get a meter close to or around the wires.

3 months ago

Again, just an outstanding article and explanation. I can’t believe how simple this makes the entire troubleshooting process, especially for something that seems like such a mystery to many of us! Thanks Mike! Worth a Million I tell ya!

Michael Roach
3 months ago

Excellent write up. I love how you keep it simple. Thanks for sharing your knowledge with us DIYers.

Mike Sokol
3 months ago
Reply to  Michael Roach

I tell my students that I’m really just a simple guy who has to do complex things. Once I can break a process down to the basics, it becomes useful for all kinds of things.

3 months ago

I have used the bulb trick as a auto mechanic back in the 60s. However using the clamp meter is pure genius. I don’t recall we had DC clamp meters back then. Great write up. Thanks Mike.

Mike Sokol
3 months ago
Reply to  Casey

You are correct. I had a Triplett clamp meter in the mid-70’s, but it would measure AC current only. The DC (Hall Effect) versions of the clamp ammeter have only been affordable for the last 10 years or so, but I think they’re the handiest meter you can keep in your RV troubleshooting kit. And now they’re available for less than $50 or so.

3 months ago
Reply to  Mike Sokol

I have a Fluke 325 and I have to admit it is sure a handy tool to have. Even measures temperatures.

3 months ago
Reply to  Mike Sokol

Not sure if it is fun but certainly easier on the nerves. LOL

3 months ago

Spot on Mike

3 months ago

Excellent advice! Thank you Mike.