Cabin lights problem

[western traveler]

So here is an update.
My friend came by with his meter. He checked the voltage 12.8v at both the light locations.
Then we disconnected everything from the fuse block. We checked resistance on each wire (forgive me for not knowing the proper terminology. I think that is correct. A lifetime block) then resistance on both together. Nothing however the moment he completed the connection between the two wires with his finger the needle jumped. When he removed it flat again.

Then we hooked up one light location at a time and hit the on/off switch the fuse blew.

Then we hooked a negative jumper with only the two wires neg/positive on the block. Negative jumper first and the fuse blew.
We then switched things around and used a jumper on the positive side and the light and switch worked. So for what ever reason there is a problem with the positive wires running to those two locations.

My battery powered overhead lights arrive Monday…

[twisted lines]

Good to know where it is so you can move on
Bad thing is it doesn't sound like its going to be ez to pull new wires.

[pchast]

Normally you have a negative ground, Yes? I read that several times. Now I'm confused also.

Are you sure there is no grounding in the lights or their attachment to the tear? Are the lights themselves wired properly? The action of switching a light on should not provide a direct short(blow a fuse). You always have power at the switch. It controls the light(resistance) which should work normally even if there is a short after it. In order to have power at the light there can be no shorting problem before the switch.

When you checked the resistance of the wire runs were they all identical or nearly so? Did you use a digital multimeter that could read milliohms'. Do you have 2 individual returns from the lights? With the lights mounted and power off or disconnected what happens to the ohm reading to ground when you switch on the circuit? Can you compare that reading to that of the return line to your circuit block.

These problems are always annoying.

[tony.latham]

But you are getting 12.8 volts at the wires at the light, right? And there isn't any resistance with these two positive wire runs, right?

If I understand it, the only time a fuse blows is when one of the light fixtures is hooked up, correct?

Buy a different type of light.

Tony

[western traveler]

But you are getting 12.8 volts at the wires at the light, right? And there isn't any resistance with these two positive wire runs, right?

If I understand it, the only time a fuse blows is when one of the light fixtures is hooked up, correct?

Buy a different type of light.

Tony

Pchast and Tony,

I will try my best to address your questions..

Tony, correct to all those points except I tried my galley light and it blew the fuse as well.

Again, a bit of background. It makes no sense to me but I do have this problem. Clearly I did something wrong along the way and there is an issue with the positive side.
This is a woody build. The point being there are no fasteners in the vicinity of the wire run that I can figure.

I followed the book with the exception that to error on the side of caution I moved the wire channel out away from the wall about 3 1/2 inches because the wall was the only place that would get crown staples (3/4”). Everything else was fastened with a T50 stapler and those were removed when the glue dried.

I placed the headliner in before wiring and removed the few staples that missed the spars before wiring.

Using a Blue Seas block and these were the first runs from the block to a location. The shortest runs there are with the exception of the fan. An individual black and red and their own fuse for each light. Also the lights are in two separate bays, one in front and one behind the roof fan and well back from those doubled spars because I knew I would be using crown staples around the hatch.

It was not a digital meter but it was sensitive. It showed no resistance in any of the four wires. Two to each light.

All these lights work away from these two locations (well except for the one I hooked directly to the battery to see if they had the positive and negative marked incorrectly. That ones switch no longer works. They were marked correctly. They work great at other locations and are really nice lights.

Using the positive wire inside the roof and a new negative wire strung from the block to the lights locations blows the fuse. Using a positive wire jumper and the negative inside the roof and everything works. Go figure…

Maybe a digital meter would identify the problem but it is not possible to run another wire inside the roof and I am not willing to place a wire channel on the exterior of my headliner. It looks too good right now. Maybe down the road after I have beat the hell out of it with use or I get tired of buying batteries (unlikely).

Gremlins perhaps…

Ralph

[Tom&Shelly]

Then we hooked a negative jumper with only the two wires neg/positive on the block. Negative jumper first and the fuse blew.
We then switched things around and used a jumper on the positive side and the light and switch worked. So for what ever reason there is a problem with the positive wires running to those two locations.

So the positive and return wires run next to each other? How about if the insulation rubbed off of both somewhere, and turning on the switch happens to apply just a little mechanical pressure on the wires, causing the two to short?

Just a guess, but it would seem to explain the symptoms.

Tom

[tony.latham]

Ralph:

So the only time the fuses blow is when these same-brand light fixtures are attached, correct?

Tony

[pchast]

Ok. This is driving me up a wall too.

Do you have pictures of the light installs. How did you connect the wires to the lights? How did you attach the light to the tear? Are the wires well taped/protected where things are connected? Are they in a roomy box/not crushed together?

Otherwise, Just confirming here, your fuse panel fuses the positive wires. These are routed to the light that has a switch. You just had it tested at 12.8V at the entry to the light... There CAN NOT be a problem with the positive wire to the light. It would break the fuse when power is applied if there was. There would not be a power reading in a DC circuit otherwise.

The problem is in the fixture or the connections behind it.

When you throw that switch something happens that bypasses the light to short the power to ground. Without the resistance of the light itself there is too much current draw and that blows the fuse.

Pressing the switch presses something inside the fixture that avoids the bulb and shorts to ground.

You said there was one way that you jumped things that it worked......... What did you avoid doing that? Did you connect things differently? did you activate the switch differently?

That may help you figure it out.
Please let us know.