I have a short somewhere? How do I find it?

[JimMartin]

Just installed a new roof on my 1999 Coachmen Mirada. Problem is that I must have pinched a wire somewhere? 12v Ceiling lights in my RV don't work now. Checked the fuse, it was blown, replaced it and it blew as I plugged it in. Removed all the light fixtures from the ceiling, unplugged them all from there 12v wiring. So now nothing should be powered up my that circuit right? Tried to put a new fuse in but it blew right away?

How can I find this break in the wiring? Do they sell a tracer of some sort that can test the wires and find a break? :?

[time2roll]

You need to separate the circuit until the issue is found.
So start pulling stuff apart and disconnecting wires.
Use a test light to verify conductivity vs a bucket of fuses.

[MrWizard]

he said he removed the fixtures
IF the fixtures are removed, the string should be disconnected
depending on where the "wire Nuts" or splices are tied together

break the parallel connections at the first fixture
work his way back or forward from there
find the section where the fuse blows, or doesn't
you find the section that has the screw thru the wiring

it will be a roof screw, around the vents, fans, A/c etc..

very unlikely to be and edge screw for the moulding/trim unless it is right above the fuse panel location, in their path from wall to ceiling, put that is a possibly

[westend]

You don't need to move bulbs around, power them, and blow more fuses. A meter will diagnose where the short is located. With light circuit NOT POWERED (pull fuse), Put one lead on the (+) wire from the distribution center that feeds the light circuit and the other lead to known good (-)ground. Check for continuity. You should have continuity if there is a short in the circuit. Remove (+) from closest light fixture and observe for continuity. If you have continuity, the short is between distribution and that fixture. If not, the short is downstream from that fixture.

[CA_Traveler]

Remove the screws? If his new roof was installed correctly none of them are currently visible. And even if the faulty screw is identified the wiring may well be compromised. Nor would I tear into a new roof.

Series or parallel connected bulbs? Maybe but why make that assumption. They could be wired in series or parallel or even a combination of both.

[JimMartin]

RoyB wrote:
This is not a bulb problem but a wiring problem. The two wires feeding a light fixture is shorted somewhere. Find where two wires feed a light fixture and then break (CUT) those two wires loose. i.e. two wires from the fuse panel will go to a light fixture close to the fuse panel then it will connect to two wires going to the next light fixture, and so on and so on. This is what I mean by daisy chaining... One of those cables is shorted together or to frame ground. The dark color wire will be +12VDC and the solid white wire will be frame ground potential. The fixtures should not be using frame ground connections but actually have a path all the the way back to the fuse panel for the NEGATIVE potential wire. i.e. both +12V and -12V feed all the way back to the fuse panel.

You want to disconnect each of these wiring connections and hopefully can determine which leg is shorted.

Not a easy task here as all of this was put in place before your ceiling was installed... The only holes you can look in are where the light fixtures are mounted. My space between the ceiling and the roof is full of foam... No way can can I pull any wires back or even see any of the wires except where they come out to wire into the light fixture...

In the end you may have to abandoned the whole thing and disconnect the two shorted wires from the fuse panel and run new wires the best you can to each of the fixtures. The wiring will be daisy chained I imagine. So if you can find the shorted segment that would be the place to cut and abandoned the wires.

In my TT trailer the air conditioner ducts run the whole length of the trailer and is a path cut from the foam for cold air to move thru. A perfect spot to run new cables...

I guess alot of screws went into the roof from the top...

You might just want to take it back to who ever put the new roof on and tell them to fix it for you... (of course that was probably you that put the new roof on haha...)

Roy Ken

Yes! I put the new roof on

[lanerd]

Bumpyroad wrote:

MrWizard wrote:

That indicates the short circuit is between the first fixture in the string, and the fuse panel t

don't understand why that is the case?

bumpy

Me neither....the lights are all connected in parallel, so with or without the bulbs, they are still all connected to the hot wire.

What you will have to do is:

Starting with the first light from the fuse panel, completely disconnect the hot and gnd wire from the light....you'll probably have to cut it.

Then insert the fuse. If it blows the fuse, you'll know the short is between the fuse panel and that first light.

If it does not blow, remove fuse and reconnect the wires (use wire nuts for now) and go to the next light and do the same process.

As soon as the fuse blows, you'll know the short is between the last good light and the light you just disconnected.

You'll then have to run new wires between those two lights....somehow.

Go back to the wire nuts, remove and solder the wires together and cover with shrink tubing.

Good luck

Ron

[Matt_Colie]

OK Jim,

Yes you have a short circuit, they are much more difficult to locate than a break.

If putting the new roof on involved putting screws where none were before, that is where I would start. Get some way to put a light bulb where the blown fuse was. It will light. Get a friend to watch the light. You go up on the roof with a screwdriver and remove screws one at a time. If your watcher doesn't shout, go to the next one. When the light goes out (Hope-Hope), start putting all the loose screws back in. If the stays out, you only have to relocate one screw.

There are wire tracers that put a signal on a wire that you can trace with the receiver from the same set, but that won't get you much in this case. If could tell you the wire's path, but it would be very difficult to actually identify where the short circuit really is.

Good Luck

Matt

[RoyB]

This is a schematic of daisy chained wiring for several light fixtures.



Looking inside all of the fixtures you should be able to see all of the wiring something similar to what I show here going from fixture to fixture... Your wiring may be connected with wire nuts which would be easy to disconnect...



You will need to find where the wiring goes from one fixture to the next and disconnect that wiring to see if the short goes away. Then all of the light fixtures from the fuse panel up to this point is good...

This is what I was trying to get across...

Not going to be a fun project for sure... About all I can offer - will get out of your hair now haha...

Roy Ken

[newman_fulltime]

Move the wire on the fuse panel you could have a bad cradle for the fuse

[newman_fulltime]

Then the short is between the first lit and the fuse panel.go to Walmart in the auto section and get a inspection camera

[Bumpyroad]

MrWizard wrote:

That indicates the short circuit is between the first fixture in the string, and the fuse panel t

don't understand why that is the case?

bumpy

[CA_Traveler]

Even with foam I've punched holes through with a wire puller which may/may not be possible for you as Roy suggested.

If need be run a new UV resistant wire on the roof. Use Dicor or similar for holes and to secure wiring. See my son's solar installation for example

[MrWizard]

JimMartin wrote:

newman fulltimer wrote:
Sounds like you screwed a screw into the wire.run a jumper wire from light to light until fuse blows.then you will know where the short is at

With all the lights unplugged and removed I still can't get the fuse panel to take a new fuse without it blowing.

That indicates the short circuit is between the first fixture in the string, and the fuse panel

Climb on the roof and look for new screws you put in
New roof , lights on center line of ceiling?
Possibly a screw at the ceiling vent closes to the fuse panel or fixture

That's where I would start

[CA_Traveler]

You can also replace the fuse with a bulb and and have someone watch it while you disconnect the ceiling wires to track down the short.

The ceiling wiring could be any combination of serial/parallel wiring. Once you identify the bad segment connect a temporary wire and make sure that all of the lights then work.

[RoyB]

This is not a bulb problem but a wiring problem. The two wires feeding a light fixture is shorted somewhere. Find where two wires feed a light fixture and then break (CUT) those two wires loose. i.e. two wires from the fuse panel will go to a light fixture close to the fuse panel then it will connect to two wires going to the next light fixture, and so on and so on. This is what I mean by daisy chaining... One of those cables is shorted together or to frame ground. The dark color wire will be +12VDC and the solid white wire will be frame ground potential. The fixtures should not be using frame ground connections but actually have a path all the the way back to the fuse panel for the NEGATIVE potential wire. i.e. both +12V and -12V feed all the way back to the fuse panel.

You want to disconnect each of these wiring connections and hopefully can determine which leg is shorted.

Not a easy task here as all of this was put in place before your ceiling was installed... The only holes you can look in are where the light fixtures are mounted. My space between the ceiling and the roof is full of foam... No way can can I pull any wires back or even see any of the wires except where they come out to wire into the light fixture...

In the end you may have to abandoned the whole thing and disconnect the two shorted wires from the fuse panel and run new wires the best you can to each of the fixtures. The wiring will be daisy chained I imagine. So if you can find the shorted segment that would be the place to cut and abandoned the wires.

In my TT trailer the air conditioner ducts run the whole length of the trailer and is a path cut from the foam for cold air to move thru. A perfect spot to run new cables...

I guess alot of screws went into the roof from the top...

You might just want to take it back to who ever put the new roof on and tell them to fix it for you... (of course that was probably you that put the new roof on haha...)

Roy Ken