Where might short circuit be?

I have a 2018 Heartland Pioneer PI RK 280 travel trailer. It appears that I have a WFCO Power Center (breaker/fuse box). I have an advanced degree in Electrical Engineering and I built my own house, including residential electrical. So I do know what I'm doing.

Yesterday, a Power Center LED light came on next to fuse #9. Yes, the fuse when removed was seen as blown. The radio and a couple lights no longer power on. I've confirmed with a volt meter and by testing other circuits that the LED indicates a short circuit to ground. I don't see how this is possible, or at least I hope it's not possible... but it is.

Specifically, it appears that the radio plus two LED lighting switches are supplied by this fuse. The switches are off and the LED remains on as does remain the associated short circuit. It seems to me that this can only mean that there is a wiring short between the Power Center and the switches/radio, or a short within the radio, or a short within a switch. The short can't be downstream from the switches because the switches are off. ANY IDEAS OF ADDITIONAL PLACES this circuit might go, of which I'm unaware? Perhaps a place with a higher likelihood of developing a short? I'm sure hoping it's not a problem in the ceiling, floor, or walls.

I can't recall for certain, but I believe this Power Center LED light was off before I started, and then on later while I was working. The dealer had long ago ripped apart the power supplying the led indirect lighting above the slide. I know what I'm doing. I was carefully replacing that lighting while the switch was off. Sure, I could have accidentally shorted the two together, but I'm pretty sure I didn't. And even if I did, turning off the switch should have later isolated my work area. Furthermore, I'm certain the wires are not touching at this time.

Yes, there certainly seems to be a correlation, so I must investigate this. Occam's Razor says this was the cause. What if I thought the switch was off but it was actually on. What if for a moment I did short the two wires? The only way I figure a persistent short could now exist, even with the switches off, is if that momentary short melted something upstream, causing an additional short upstream. That way, with the switches off (I never learned which was which) the short is persisting. AM I MISSING ANYTHING ELSE HERE?

All I figure I can do is remove the switches, isolate all the wires, and check both directions was well as check the switches themselves. This divide-and-conquer method should lead me to the location of the persistent short. I might also remove the radio and make sure it didn't generate its own short internally, since there's no external power switch for it.

But is there something else, something easier to check that I might check first? I don't think so, but perhaps I'm not seeing the forest for the trees.

Note that I have searched for doc. There seems to be no overall trailer wiring diagrams anywhere.

Thanks in advance!


Well-known member
I've been doing this kind of stuff for 50 years....no engineering degree, life degree. I always use the "cause and effect" rule. So, it was working before you started messing with the lights in the slide, and it had a problem when you finished? Obviously you need to take apart what you did and re-check. I'm thinking those LED lights are polarity sensitive, so did you observe polarity when messing around? I would probably unhook everything I had installed and check to see if the short was still there. Next, install one light at a time and re check. Good luck....


Well-known member
If the affected components are in a slide and you've moved the slide there is a possibility that where the wires enter the bottom of the slide may be compromised. Also check the junction box on the frame under the slide.
Thanks. I'm still looking for more applicable input, please. I'll explain...

@lynndiwagoner , thanks for the suggestion, but I never even got to the point of adding new stuff. I was confirming the presence of 12V before adding anything. The ONLY thing I did was strip the previously straight cut wires, so the ONLY possible short was temporary while stripping the second wire and touching the already stripped wire. I was careful not to do that, but could have not seen it. Anyway, there is nothing to remove. It was never there. All that is there is the power wire coming out of the ceiling, now both wires capped off. There's no way for those to be shorted. Furthermore, even if they were shorted, or if a short developed by pulling a little on those wires and causing a hidden junction in the ceiling (illegal in residential) to short, the fact that all switches are OFF would isolate that. So the problem can't be with the current state of the area I was working.

@Roller4tan , thanks for pointing out a junction box on the frame under the slide. I'll keep that in mind for other, future things. In this case, the lights are on the main body above the slide, not on the slide. So I wasn't clear enough. The wires come out of the ceiling, down the wall 2", and then were LEDs affixed to the main body wall. They don't move when the slide moves.

So, once more, this leaves me with some secondary damage that might have happened if I unwittingly shorted the wires while working. The secondary damage must be inside the "off" switches or else upstream from them. (Or could be inside the "off" switches *and* in the ceiling downstream from the "off" now-broken-ON switches.) This leaves only the radio, other switches, and the control panel.

The only way I know to divide and conquer is to open up all the switches, disconnect the wires, and probe for shorts that shouldn't be there. This includes perhaps removing the control panel and disconnecting the whole cabin, then checking both directions at that point: into the control panel and out to the cabin.

I'm just still hoping that somebody has an idea for a different hidden thing that might be on that circuit branch, that may have disliked the color of my shirt while I didn't actually short anything...


Well-known member
Seems to me that I remember an old timer way of chasing short circuits while not buying out all of the fuse stock in town. The ideas was to substitute an incandescent (filament) lamp of the correct voltage in place of the fuse that keeps blowing, then keep trying disconnecting points on the circuit until the short is located (i.e. , the lamp no longer lights with power applied to the circuit). The lamp should light and limit the circuit current while power is on and the short is still present.

Of course, you could try the daring way of trying to find the short point by bypassing the fuse, applying power for a few seconds, and trying to detect the smoke/heat source area at the short.
LOL, @wdk450 , I can simply use a volt meter (on continuity tester position) to see if there's a short. No need for lightbulbs or smoke/heat. That's the proper tool for this job. Remember, I'm an Electrical Engineer. I've got thousands of dollars of test equipment, and voltmeters can probably be had from amazon for 10 or 20 bucks, anyway.

I guess nobody has thought of a different PLACE where there's a likely secondary problem...
I just temporarily separated the offending branch from the distribution center (fuse box). This was trivial enough to do. The problem is indeed in the branch and not in the distribution center board.

Note that it's NOT a simple short to ground. There's at a minimum some capacitance here. That makes me wonder if the Radio went south. Specifically, a continuity tester between the branch and distribution center incoming ground beeps for a moment and then stops. A voltage check between the ranch and incoming ground show a fraction of a volt, not "zero" as open circuit would do. After decades and decades of diagnosing electronic boards, I recognize this as a capacitance on the line. This is NOT a simple wiring short, but something that wants to draw more than the 20A fuse likes, yet won't measure as a short on the continuity tester. The ohms measurement decreased over time, which is another indicator that it's a cap. All this speak to circuitry, not wiring. So I think the next place to go is to disconnect the radio.

I don't think anything on the levels monitor should be on this circuit, which is another place with something more than wiring, switches, and LEDs. The level monitor is illuminated while the offending branch has no fuse driving it.

Again, I *believe* this branch has only two switches controlling two LED strings, and the radio. Nothing more. So this capacitive behavior points at the Radio. Of course, the Radio could show this behavior on the open branch and still not be the problem. But by disconnecting the radio I should better see any other behavior. After that would be disconnecting the two switches. After that is fishing through walls and floor - oh no!

Anything else somebody might think of that's on this branch that I didn't think of? All the obvious candidates are powered and running on other branches/fuses.


Well-known member
Dr. Lightning:
I hope that disconnecting the radio shows you where the excess current draw is.


Well-known member
LOL, @wdk450 , I can simply use a volt meter (on continuity tester position) to see if there's a short. No need for lightbulbs or smoke/heat. That's the proper tool for this job. Remember, I'm an Electrical Engineer. I've got thousands of dollars of test equipment, and voltmeters can probably be had from amazon for 10 or 20 bucks, anyway.

I guess nobody has thought of a different PLACE where there's a likely secondary problem...
the PROPER tool for the job would be a "short circuit tester"...... they are so affordable that everyone who owns an RV or a Boat should have one in the tool box.... a short circuit tester can shorten an all day job into about an hours worth of work, because it doesnt take long to fix once you find the location of the problem.....


Well-known member
Thanks, @centerline . That's what a "continuity tester" on a regular multimeter does. The multimeter also gives you other diagnostic tools. Amazon has them as cheap as $15. More digit accuracy and known brand Fluke for $237. I think the typical RVer should be fine with $15. (@wdk450 followed up with a link to a "short tester" while I was typing. $6. I think $15 much more worth it. It beeps rather than lights up. That's nice when you're upside down and twisted reaching under something with the probes. I guess a light would shine from behind your head, but if you can hear the beep, that works too. Plus you can confirm 12VDC, 120VAC, check current, etc. It can't be too hard to learn. I don't know when I got my first, but I must have been a teenager. )

OK, I was going to wait until after more investigation tomorrow, but because @wdk450 said something, I'll give a midway progress report.

Not germane: The radio is the wiring hub. After separating everything, I find the wire from the rear wall control center goes to the middle wall radio. From there it goes to the partially forward wall switches. Then that goes into the ceiling and back to the wall above the slide, where the final LED string goes. Below the radio is a usb jack. Believe it or not, it was the usb jack causing the red light on the control center to illuminate. When I disconnected that, the light went out. It was surely also the source of the capacitive behavior. That usb jack will certainly have a switching power supply to change 12V to 5V, and those have a decent amount of capacitance on the front end.

Here's the funny thing, that usb jack was simply fooling the control panel into thinking there was a short on the line. Thus the control panel lit up the LED. That, of course, was while there was no fuse. Having found this situation, I went ahead and risked putting in a fuse. The control panel LED went out and Everything worked. That's with or without the usb jack connected. THEREFORE, the control panel warning LED that there was a short in the branch still was AN ILLUSION. With the fuse removed, it was subject to this illusion. When I put the fuse in, it powered things, and made the warning LED go out. WIthout the fuse, when I separated the usb jack, the warning LED went out. But with the fuse in, the warning LED stayed out regardless of whether or not the usb jack was connected.

So the warning light was misleading me, preventing me from trying to burn another fuse. HOWEVER, know that TWO fuses were blown in the same spot. The first fuse most likely blew while I was working. I thought the switch as off, thus isolating the area where ALL I DID was strip the incoming unused 12V wires. But maybe at one moment the switch was on and my tool touched both. Dunno. However, I later replaced that fuse and it blew again. I forget the timing, but there's almost no way I allowed a short this second time. So there's still a big question as to why the second fuse blew. My memory may be failing, but I do know very well what I'm doing.

Now here's the kicker. I pulled out the switch for that indirect lighting led string above the slide (in the main body wall above the slide). The downstream side of the switch has TWO wires. I continuity tested and indeed the white-with-red-stripe wire goes directly from that switch to the indirect lighting led string. But the second wire is white-with-orange-stripe. Where the F does that wire goes? That means the switch contains TWO different loads on it.

Now, the dealer had physically TORN OUT the wiring connection before I bought the trailer. They didn't even cut it. They just tore the end of the led string off. I jumpered that together long ago and found the string was spotty, partially on and partially off, looking bad. That's why the dealer tore it apart. Out of sight; out of mind. i only learned of the indirect lighting while watching a Heartland promo video. Well, now that I find TWO wires coming out of the switch, I wonder where is the second load. The switch operates indirect lighting. The other wire almost has to be lighting, and most likely also indirect. I can't check again until tomorrow (maybe I'll peak today) but when I do I need to go searching for the other missing light that perhaps the dealer ripped out. Did the dealer leave a potential short, dependent on road shaking? Well, without invoking goblins, maybe THAT will turn out to be the only possible logical reason for the SECOND fuse to blow. When the second fuse blew, I was certain that I had no shorts in the indirect lighting, which was simply the wires coming out of the ceiling, and certainly pulled apart. Maybe the hidden torn light wire is randomly shorting, and happened to short to blow that second fuse whenever I tried the switch again.

If not the above, then there's a ghost somewhere!


Well-known member
I have found, regardless of the definition for the red light being, "over current, or short circuit", that when the circuit is simply "open", like removing the fuse, or disconnecting the circuit wire from the screw terminal next to any fuse, you will get the red light. Maybe leave the USB out of the circuit to see how the new fuse fares. Knowing that things like the USB are poor quality imports, and was showing high capacitance, this may be your culprit.

Thanks @2019_V22 . I'll add more clarification to the warning light in the distribution panel. If the downstream circuit can be fully turned off, such as with a light switch, then the warning light WILL go off, even with the fuse removed. But if the downstream circuit is connected full time, such as by even the highest quality USB power supply, the warning light will likely come on when the fuse is removed.

I'm still wondering where that second switched wire goes...
Well, the mystery second wire on the switch controlling the indirect lighting over the slide is INDEED the problem. It didn't before, but today it measures as a short to ground and in fact blew another fuse. I have it disconnected. I confirmed it's shorted to ground at this time. I confirmed it's NOT the second wire I found going to the awning. (Two separate wires to the end of the awning? Some mods were happening there... But that's not the problem.)

Please see this post:


Well-known member
The USB jack probably doesn't use a switching type power supply. Switching power supplies are generally for stepping up a voltage, not down. I suspect a simple solid state type 5V regulator chip, which probably has a capacitor in it. You stated that you did not see a direct short but rather something that looked like a capacitor, which is probably included on that little, cheap, regulator board. Also, did you reverse the ohm meter leads to see if maybe you were seeing a solid state type resistance? If you know how to check a diode then you know what I'm talking about. Glad you found the problem.
Just between you and me, @lynndiwagoner , I appreciate your comment about the USB jack. Shop talk: yes, it might be a cheap linear regulator. I've been focused on battery operated devices for a few years, and would never use a linear regulator to get from 12V to 5V, instead opting for a switching regulator with 85% or maybe 90% efficiency - beating the roughly 40% efficiency in this 5V/12V case. Of course they all have caps on the input, and that's what I was measuring. None of this is here nor there with respect to my OP. (Note I don't have memorized Buck vs Boost language, but I just checked. Yes, a Buck converter is the one that does step-down.)

I still want to know where that mystery white-with-orange-stripe wire goes. I haven't been able to figure it out. Yes, I found which wire as the short, from the upstream end, but I don't know where the wire goes and so I don't know where the actual short is. Oh, yes, I've got my other post.

More shop talk: I've thought of this in the far past, but never tried it. I wonder if my function generator on high freq, driving the wire, plus my spectrum analyzer with a hand bent tiny antenna, would work to trace within the walls where that darned wire goes, LOL? I don't have enough spare time to risk such an experiment just yet...
Cool! A new toy to play with. Thanks, @mlpeloquin . This uses exactly the concept I was talking about, but is in a tool where they've already figured out the details. I can support my charity via smile.amazon that I always move over to, and it seems you provided a smile link in the first place. It comes fast, and is returnable if it doesn't do what I think it should. (That language in the Q&A was pretty poor.) I'll keep it if it works, rather than use once and return.


Well-known member
Thanks, @centerline . That's what a "continuity tester" on a regular multimeter does. The multimeter also gives you other diagnostic tools. Amazon has them as cheap as $15.

a multimeter is NOT a short circuit tester.... a multimeter IS an extremely helpful tool (if one knows how to use it), but a short circuit tester only needs to be connected to either end of the faulty circuit, and then you use the sensor to follow the hidden wiring that is BEHIND the walls/floors/panels/ceilings, as the circuit gives off a signal that the sensor picks up.... when you get to the spot where the signal stops or changes dramatically, there you will find the problem..... a multimeter will never do what a short circuit tester can do, BUT, if one has the time and intimate knowledge about how to use a multimeter, it can be used to find the short.... but so can a simple test light.... using a multimeter usually requires one to remove panels and dig into the wiring at intervals to check for voltage, where as a short circuit tester is just so much faster and precise in locating the exact spot of the problem without going thru all the other un-necessary troubles... and is cheap enough that everyone who does their own electrical work should have one....
Sorry, @centerline , I thought you were referring to the link provided by @wdk450 (https://www.ebay.com/itm/1855525184...CIfogCAnvhKsC7tMWXDIycfFk=|tkp:Bk9SR_SxlITsYA) which is nothing more than a light bulb on wires, which indeed, is less useful than a multimeter. I now realize that you, @centerline , were referring to something like the link provided by @mlpeloquin (https://smile.amazon.com/General-Te...245&sprefix=short+circuit+tes,aps,435&sr=8-15) which is indeed much better than a multimeter for this task.

BTW, I got one of those now and I will try at my next opportunity.