Trailer Light Dilemma

I've got a dandy going here that has me shaking my head and laughing/crying.

Last week I bought an older 12 ft MirroCraft tin boat with 15 hp outboard, sitting on an older trailer. Basically sound and can be used as is....almost...but there are a few things I've been working on "to make it better." As expected, lights on trailer were u/s, so tossed 'em and bought a cheap kit online that has both tail lights, both marker lights and about 25 ft of 4 strand wire with 4 pole flat plugs.

Late yesterday, I re-wired the trailer and mounted the lights. Lights wouldn't come on, so figured with all the rust and layers of paint I wasn't getting a good ground - the usual cause.

I drilled and tapped holes by each light and ran separate grounds to each one, then checked ground at car plug and showed power at all 3 terminals with a 12V test light. Plugged in trailer and no lights.

I clipped the grounding alligator clip to my new main ground on trailer, then poked the needle tip thru the insulation on each brown (tail light) wire and the yellow and green turn signal wires.....this is on the trailer side about a foot downstream of the plugs at hitch.

All showed power at the test light.....but still no lights on trailer - on the far end of the same wires. Went down to each light individually, clipped to ground and probed thru the insulation. Nothing on any of them. HTH can this be ?? I've wired a lot of trailers.....and lots of other stuff, too, and when you have an unbroken wire with power at one end, then there will be power at the other end, too, right ?? Hah.

I walked away from it for a while, then went back with a clear head and fresh viewpoint. No change. I don't know WTH to think. Can't be the bulbs (these are cheapie non-LED) cause no power is showing where wires enter lights but power is showing at other end of wires. Aaaarrggh ! ! !

Are you using the frame as a ground?

If you have power at the vehicle end, no power at the lights, and the wire has not been spliced anywhere down the line, then you have a wire that is likely broken inside of the insulation. It happens, could also be a factory defect.

Take the lenses off, bulbs out & use the meter on the contacts to verify power. I have cheap lights on my utility trailer & the wire is just pressed against the contact, no crimp or anything robust about it.

I have to mess with it every time I use the trailer, about 3 times a year.

Yea I should buy LEDs . . .

4 way flat plug

For no lights at all, it is unlikely that 3 separate power lines (green, yellow, brown) would be bad.

The ground (white) is the only wire that is common to all lights.

For trouble shooting you can use a spare wire and run from the plug back to the lights. Do not connect it to the trailer frame.

You can also use a small 9 volt battery and connect directly to the light wiring at the lights to verify they are working, Make sure and disconnect plug from vehicle and the lights will be dim when using 9 volt

I've had problems with using the trailer frame as the ground so many times that I don't do it anymore. I run an extra wire alongside the wiring kit for ground and bypass the trailer frame.

I'll take some pictures in the morning to try and explain more clearly. Thanks for the thoughts.

Really sounds like a bad return (ground).

You probably have 12 VDC at the lights, but without a return path to - terminal of battery you will not have lights or be able to measure any voltage with test light or multi meter.

When you tested at the plug and return (ground) point, you verified it is good to that point.

It is still unlikely that you have 3 separate power lines, 4 separate bulbs, and 2 separate lights that are bad.

Dumb question ... but was the boat trailer hooked up and dropped onto the trailer ball on the hitch receiver of your tow vehicle?

DaleH said:

Dumb question ... but was the boat trailer hooked up and dropped onto the trailer ball on the hitch receiver of your tow vehicle?

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From the description, this trailer has a return (ground) on the trailer side. Doesn’t seem to be one that gets the return through the ball and hitch.

I have had this problem with a few trailers. Usually the tilt bed ones. I find it best to just go ahead and run a ground wire to both trailer lights.

nytebyte said:

I have had this problem with a few trailers. Usually the tilt bed ones. I find it best to just go ahead and run a ground wire to both trailer lights.

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Absolutely correct

Some of the tilting ones have nylon, Teflon bushings, or rusty bolts and will electrically isolate the front from the back. Making the trailer a poor return path for lights.

Thanks for the responses, guys. Nytebyte hit the problem. It Is a tilt trailer and I completely forgot. Slap my silly head. Sure, the bushings isolate the front from the rear of the trailer. Thanks again.