Lights with one wire?

[2ndmile]

I searched but did not find anything.

Getting ready for my first build. I have ordered most of my lights and all of them are showing up with only one wire. I remember from my 5th grade science fair project that you should connect a light to positive and then negative.... so where is the other wire? I know this is something obvious since all the lights only have one wire.

Thanks

[Mightydog]

The 'other' wire would be ground.

Bench test the concept to be sure I'm not misunderstanding your question. Run the wire to the + side of a battery, touch the metal body of of the light fixture to the - side and see if it lights up.

If you're mounting the lights on something that isn't grounded, isn't conductive or if you want to be sure it works all of the time, run the second wire to one of the screw holes and make sure it connects well. You now have your two wires!

We ran ground wires to all lights in our trailer to be sure the lights worked well. Most of the 'lights don't work' questions around here seem to center on the lack of proper grounding for the lights.

[tonyj]

Many lights come with one wire because either the backing plate or mounting stud acts as the negative, or ground side of the circuit. This requires the light socket to either attach to the grounded trailer frame, or will require you to make some provision to run a ground wire to the backing plate or stud of the light socket. Trailers and many car circuits use this same wiring principal to reduce the number of wire runs. The trailer (or car chassis) acts as a big ground circuit, which is then attached to the negative (ground) battery lead.

This is why faulty ground circuits between the tow vehicle and trailer cause so many problems. Make sure you have a good ground lead from the tow vehicle either through the trailer wire connector, or a dedicated, detachable ground from the tow vehicle to the trailer. Many older wiring schemes depended on grounding through the trailer hitch itself--very unreliable. The white wire on on the trailer wire connector is the ground circuit. The short white lead on the trailer side of the connector should be attached to a reliable grounding spot on the trailer. Make sure and use an ohmmeter to check from the connector to the ground attachment point to make sure you have a good ground.

[2ndmile]

I assumed that was the case. I think I will run a wire to all the lights as some of these will be mounted on the aluminum skin and some are mounted on the inside of the roof. Once some of these are in they are not accessible except for dismantling.

[angib]

I would make two suggestions:

If possible, run the ground wire from outside through the back plate of the light (maybe with the other wire through a grommet?) and make the ground connection inside the light - this means this connection is in the (fairly) dry area inside the light and may last for many years. Trying to use the mounting bolts/studs to add a ground on the outside will work to start with, but not for long....

The simplest and most reliable method for wiring is to run two wires to every electrical fitting. Making connections to trailer metalwork, to use that as the ground wire, will come back and bite you eventually, as these exposed connections start to fail.