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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Blowing Fuse (12 Volt) in MH
Date: Sun, 18 Mar 2007 19:02:42 -0400
Message-ID: <>

I had one of those in my rig that almost drove me batty.  Turned out
to be a sheet metal screw penetrating a wall over the wiring that just
touched the wire.  Over the years vibration wore through the
insulation and the screw would occasionally touch, blowing the fuse.

I eliminated everything else and was starting to think the wire had
worn and was touching the frame somewhere.  I was yanking on the wire
trying to determine its exact routing when the short became
continuous.  I was removing the screws that held the portion of
cabinet on when the troubleshooting lamp I had hooked across the fuse
holder suddenly went out. Bingo!  After I got the hardware off I found
the worn spot in the wire and the little marks from arcing on the
point of the screw.

Start by physically tracing out the wire's path.  Remove the fuse and
connect a suitable lamp across the fuse socket.  When the short is
present the bulb will burn brightly.  My first place to look would be
under the light fixtures.  I've seen a lot of RVs where they use a
hole saw to cut a hole for the wires and leave the sharp edges
unprotected. Likely that vibration has worn the insulation on a wire
where it comes through the wall.  Don't be surprised if your test
light goes out as you remove the fixture.  If it does then you've
found the problem.

If removing fixtures doesn't find the problem then grab the wire at
every point you can and yank.  Fairly hard but not hard enough to
break it.  The movement should cause the short to break.

If you haven't found the short at this point, my advice would be to
snip the wires loose on both ends and run another.  The fault is
likely behind something that will be time-consuming to remove.  Just
not worth the effort.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Thanks Re:  Blowing Fuse
Date: Sun, 18 Mar 2007 23:27:41 -0400
Message-ID: <>

On Sun, 18 Mar 2007 20:30:43 -0400, Lee <>

>Hi Ben, Hunter and Neon John
>Thanks for your suggestions, will print it out and will give them
>a try tomorrow.  As Neon Joh said it can be frustrating and I have
>used quite a few of those 20 amp fuses just trying to find the
>problem.  Everything else was working fine so even if I don't find
>it before we leave we can run everything else.

Save your fuses by using the test light.  Just hook any old 12 volt
bulb you have handy across the fuse clips.  A short will make the bulb
glow full brilliance.  A load such as another lamp will cause it to
burn more dimly because the two are in series with each other.  Place
the lamp where you can see it while you work so that you can notice
even a brief flash as the short comes and goes.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Blowing Fuse (12 Volt) in MH
Date: Mon, 19 Mar 2007 15:30:23 -0400
Message-ID: <>

On Mon, 19 Mar 2007 09:07:58 -0400, Lee <>

>Bob AZ wrote:
>  > Unless you remove the power don't use an ohm meter. Instead use a
>> voltmeter. When the short is removed the meter will go to zero.
>> Connect the meter to the fuse holder. Positive to the feed and the
>> negative to the load.
>Thanks Bob
>If I remember correctly my meter does both, so will use it as directed
>will refresh my memory by looking at the instructions! (grin)  Lucky
>me there are only 3 lights in that circuit but as far as the routing of
>the wire it would be damn near impossible to follow it.  So am hoping it
>will be near the lights!  If not, time for a new RV? (right)

Don't try to use the meter.  It will only confuse you.  Even a tramp
leakage to ground will cause the meter to give a voltage indication
because it has such a high input impedance.  You can demonstrate this
easily enough.  Hook one side of your meter to the battery positive.
Hold the negative lead.  Touch something grounded.  Unless your skin
is parched dry, your meter will indicate almost full battery voltage.
The few tens of thousands of ohms of resistance your skin presents is
practically a short compared to the standard 10 megohm input impedance
of a DVM.

Use the lamp like I described.  It draws enough current that leakage
won't affect it.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Blowing Fuse (12 Volt) in MH
Date: Mon, 19 Mar 2007 16:15:09 -0400
Message-ID: <>

On Mon, 19 Mar 2007 08:46:53 -0400, HD Matt
<nospammbode@multiprintinc.comnospam> wrote:

>Is there some sort of reasonably priced device that would allow me to
>trace a 12 volt line through the ceiling? I have three lights in the
>bath area of my MH that work some of the time. I am sure it's a loose
>connection somewhere. I have pulled all the lights down starting at the
>one that still works on the circuit and soldered all the connections
>followed by a crimp on cable nut to insulate. Worked for a while but
>now the three light don't work at all. I suspect that there is a
>junction somewhere up inside the ceiling that is loose. Ceiling in not
>easily removable without major work and would be fairly hard to take
>down and reinstall without it looking like crap. I know there are
>"tone" injectors that allow the tracing on 110v line in a home and
>finding the correct breaker in the panel but, have never heard of such
>for 12v circuits. Thanks

Yes, there are devices for tracing telecom wires that work like the
120 volt ones except that they're battery powered.  I believe that
Home Depot sells one.  It may or may not work, depending on whether
your inside skin is metallic (metal or foil coated) or not.

You can rig something up that is much more likely to work in the
presence of metal because it works on the magnetic field instead of
the potential field.

Go to Rat Shack and get a crystal earphone.  Less than $5.  This is
the cheapo earphone that came with transistor radios in the 60s.  A
low impedance dynamic headphone, the more common type, won't work.  If
your Rat Shack doesn't stock the crystal earphone then for a little
more money (<$20), you can get a monaural amplified speaker that has a
high input impedance.  PC and iPod speakers won't work, as the input
impedance is low.  The one I have is stock number 32-2040.  I've had
it a long time so that may be an obsolete number.  It's labeled
"amplified speaker system" and has a "MIC" input jack on the back.

Next you'll need a solenoid with lots of turns.  The more the better.
An old relay is a good place to get one.  Buy a relay at Rat Shack if
you don't have any laying around and rip out the coil.  If the core
doesn't come out with the coil, find some nails or something similar
to stick through it.

Connect the solenoid terminals to the earphone or speaker.  You've
just made a magnetic field detector.

Make up a Jesus cord consisting of a 120vac line cord, a socket for a
100 watt bulb and some gator clips.  The bulb should be in series with
the black lead.

Disconnect the circuit in question.  Connect the bulb lead to the
circuit at the fuse panel.  Connect the other end to the neutral
(white) lead or simply ground it.  The bulb should light as current
flows through the circuit.

The current flow through the circuit sets up a weak magnetic field.
You're going to detect it with your homemade detector.  Put the
earphone in or turn on the speaker.  As you pass the coil near the
energized wire you'll hear a hum.  It'll get louder the closer you get
and the closer you align the axis of the solenoid with that of the
wire.  If you move across the wire horizontally you'll hear the hum
get louder, be loudest as you're directly over the wire and fade away.

It becomes obvious that you can locate a wire by passing the coil back
and forth and orienting the solenoid to produce the loudest hum.
Because the low frequency magnetic field easily passes through
non-ferrous metals like aluminum, you can find the wire even behind

This same trick will work to find wires inside the walls of your
house.  Simply plug a load into the outlet of the circuit you want to
trace, the larger the load the better, and start moving the coil
across the wall.  In this case, the sound will increase as you
approach, then decrease.  When you're over the wire the sound will
usually null out as the hot and neutral fields cancel.

I have a very old antique Western Electric telephone wire tracer that
works similarly.  It is built into one of those gorgeous
furniture-grade wooden boxes that instruments used to be built in.  It
contains a vibrator induction coil similar to a Model T ignition coil
except that there is no secondary and it vibrates much slower.  When
the points on the vibrator open, there is an inductive kick generated
of several hundred volts over a broad frequency band.

The coil is tapped at several places so that various voltages can be
generated. The wire under test (WUT) is attached to these terminals.
The high frequency impulse goes out over the wire and is radiated and
capacitively coupled away.  This results in current flow in the wire
even though it is open circuited.

The detector is a long coil that contains tens of thousands of turns
of fine wire hooked to an old-fashioned telephone operator's headset.
This coil is passed over the wire whereupon loud clicks are heard.  It
is amazingly sensitive, audible from several feet away from the wire.
The field penetrates aluminum like it wasn't there and even penetrates
thin steel such as what control cabinets are made of.  This is
something that modern single-frequency tracers can't do.

I got the idea for the technique I described above from using the
detector wand to trace 60 hz wires.  I noticed that it was VERY
efficient at detecting the 60 hz field.  Thus I could detect energized
wires without having to de-energize them and connect the induction

I'll mention in passing one other technique for finding hidden faults,
one that power companies use on buried cables - high voltage arcs. The
idea is to cause an arc at the fault strong enough to be detected at a
distance.  The gadget is a trailer-mounted rig called a "Thumper".
The trailer contains a generator and a bank of high voltage capacitors
and the necessary controls.  It dumps a high joule high voltage pulse
into the cable at about one second intervals.  One then walks the
right of way.  The arc is intense enough that it thumps the ground.
One can feel it as he stands over the buried fault.

The shots are strong enough to blow out most shorts and cause the arcs
that can be felt.  Sometimes when the cable is shallow buried the arc
actually blows away the dirt.  Sometimes spectacularly.

If I were going to try this technique on your problem I'd get one of
those disposable cameras that contains a flash.  The strobe capacitor
stores about 360 volts and considerable energy.  After unhooking
everything from the circuit, I'd dump that cap into the circuit using
simple jumpers and using the flash tube as the high voltage switch.
Just hook the circuit in series with the tube and take a picture :-)
If the short is against aluminum there will be a little aluminum
vapor/air explosion (well, really just a pop) that is easy to locate.

I know that this is probably more than you wanted to know but I
thought I'd do a brain dump while I'm at it.

Please let me know what you find.  I'm curious.  Also how you end up
finding it.

>By the way haven't seen any "travelogs" from you lately. Boring places
>or just tired of writing them. I am assuming you are still on the road.

Funny you'd ask.  I'm at home now for a little vacation.  I turned in
my truck last Thursday.  The company has apparently gotten itself in
financial trouble and has lost a lot of customers. It got a little
tiring getting my butt chewed by customers because of the bad service
when I walked in to pick up my load.  I've spent much of my time since
Christmas sitting and what running I did was repetitive short haul
stuff.  Nothing much to write about other than being in the
mini-blizzard of about a month ago.

I don't know if I'll go back out driving a semi or not.  Probably not,
as several other opportunities have presented themselves.  One
discussion I'm having involves setting me up in a motorhome as a
traveling tech support engineer.  Basically on the road full time
supporting the company's industrial controls customers.  If I were to
write my own job description this would be it so I'm hoping it comes
to pass.

I have some friends in Texas who've been pestering me to come there
and re-open my BBQ joint using their money.  That's also an enticing
possibility, though I could do without the heat of Texas.

Right now I'm taking a month off and maybe the whole summer to relax.
I don't much like working in hot weather so I may just hole up in the
mountains for the summer.  Between RV excursions, of course.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Blowing Fuse (12 Volt) in MH
Date: Mon, 19 Mar 2007 18:21:21 -0400
Message-ID: <>

On Mon, 19 Mar 2007 16:25:31 -0400, "Steve Wolf" <>

>A friend of mine obtained a dynamite deal on drywall labor.  After they were
>done he realized they covered all his outlets.  I lent him my tracer and he
>was able to find each of the lines and boxes without a problem.  Ask around
>to see if anyone has one of these:

Heh.  I'm reasonable certain that this is the one I saw at Home Depot,
only under the Greenlee brand.  About the same price.

>You'd be surprised how many have them as they are so useful.  Disconnect the
>power, hook the transmitter up to the circuit and you're likely to be good
>to go.  Yes, the signal is weak.  Even so, I have also found wire routings
>quite deep in a wall.

Yep, very handy thing to have around.  I still use the old inductive
one I described when I know the circuit doesn't have anything
connected to it because the signal is so much stronger.  The inductive
kick will probably damage anything solid state connected to the
circuit so I have to be careful.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Blowing Fuse (12 Volt) in MH
Date: Mon, 19 Mar 2007 18:22:37 -0400
Message-ID: <>

On Mon, 19 Mar 2007 16:15:09 -0400, Neon John <> wrote:

>The current flow through the circuit sets up a weak magnetic field.
>You're going to detect it with your homemade detector.  Put the
>earphone in or turn on the speaker.  As you pass the coil near the
>energized wire you'll hear a hum.  It'll get louder the closer you get
>and the closer you align the axis of the solenoid with that of the

Oops, meant to say "perpendicular to the wire axis."  Think one thing,
type another.


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