Trailer light out – troubleshooting

So I have a driver side running light out on my boat trailer. Replaced the light it’s still out. Checked the ground connection at the light and it looks solid. I’m confident the connection (brown or yellow wire) is crimped and solid. What do I try next? It seems to me the problem is between the light and the plug at the tongue. I know it’s not the truck’s issue because I’ve hooked it to two vehicles. I might get to go fishing tomorrow so I’m hoping to get it fixed tonight. Thanks in advance!

Check to make sure you don’t have a pinched wire any where along the way. I just battled the same issue I had blinker and break light but no running light. Ended up having a pinched or bad piece of wire.

– It’s the brown wire, some where between the connector and the light assembly.
– It’s NOT a ground issue because all of the other lights are working.
– If a wire was pinched, that would blow a fuse in the truck.

– The brown wire from the connector goes to the running light on BOTH sides of the trailer, so it has to branch off somewhere inside the trailer. Since the running light is working on one side, the problem has to be a broken connection between the branch and the light assembly.

I’d look really hard at any splices. They’re notorious for causing problems.

S.R.

Thanks guys – I’ll double check the splice on the brown wire on both ends and hopefully that will do it.

If not I’m halfway tempted to take a speaker wire and just zip tie it along the frame if that doesn’t do it!

Check conductivity from plug to the light!

tight_lines wrote:

Thanks guys – I’ll double check the splice on the brown wire on both ends and hopefully that will do it.

If the splice is one of those pinch on quick POS splices replace it with a proper waterproof crimp on. You may as well do the other side as well. It will fail eventually anyway.

Grouse

Ok. So i checked the splicing at the plug – it was fine there. The only place I haven’t double checked is where the tail light actually connects to the brown wire. I don’t believe that is the issue though since it was a problem on the previous light (Which I thought I had burnt out – ended up swapping out for a whole new LED light cluster anyway). Here is a diagram showing where I am at. The amber light toward the front of the trailer is working so the short has to be somewhere between that light and the tail. No luck getting it to flicker when pulling on the wires at either end.

Do I just run a new line for that section or is there a good way to determine where it has a break? I have not opened up the amber light to see how the wire looks from there.

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First off it’s not a short, that would be blowing fuses. It’s a break or ‘open”.

The fastest way to fix this might be to do what you said, pull a new piece of wire from the amber light down to the rear light assembly. You know you have power as far as the amber light so the break has to be down stream from there.

S.R.

And to help eliminate confusion….the light in the cluster is called the tail light. A running light is the amber side light or side marker.

Have you used a tester to see if you have power back as far as the light. Is it a tail light or marker light

I just picked up a tester from my brother (Just the light up indicator style not a meter) but I have not tried it yet. My previous working tail light had broken – the plastic front cover popped off and was hanging when i came home. I epoxied the cover back on and added a piece of tape to make sure it would hold and that worked for 2 trips. Then the tail light died. Figured it wasn’t sealed great and that is why I replaced the light. But the brand new light has the same dead tail light (again only 1 side, and break and signal work). So I saved the old light for a backup as i doubt it actually has an out light and have just been trying to trouble shoot the break.

Take the tester and go from the hot post in the light to the trailer frame. Trailer lights are usually grounded thru the frame using the light mounting bolts

Light clusters like the tail/turn/stop have one common ground so if the other lights work its either in the lamp base or the wire to it. Try a splice at the side marker wire to the wire at the dead light.

If you get light, solder the splices and wrap with tape and have at it.

on my trailer the running lights are all grounded separate and a rusty screw will kill it! I brush all my connections and apply grease.

Really need to know if you have power at the brown wire back by the lefthand light when the truck’s parking lights are on.

A $5 Harbor Freight multimeter would work wonders here.

If there is NO power back at light on the brown wire, as Steve says, you have a break or open. I’d just pull new wire at this point and splice at the marker light.

Grouse

Not to hijack the thread, but my trailer light issues have to do with my ground wire always breaking on me right where it is attached to the frame. I have spliced a longer wire, taped the entire wire with electrical tape, etc. to try to help prevent it from breaking.

Any tips on what to try next? I feel like every other trip I am having to splice another connection on and additional wire…

drewbop wrote:

Any tips on what to try next?

On my last trailer, ran a ground wire to each light and skipped the “through-the-frame” grounding.

“Any tips on what to try next? I feel like every other trip I am having to splice another connection on and additional wire…”

I taped all four wires from the connector to the trailer together into one thick bundle. Then I used cable ties to tie that bundle to the trailer, using the exposed end of a bolt. The idea here was to make a bullet proof mechanical connection in one spot, and then after that make a separate electrical connection.

S.R.

If I can’t diagnose and fix the issue in about 5 minutes I usually just run a new wire and be done with it. If you have power at the plug run a new one and call it a day.

Matt, Matt, Matt, Come on may be a team player and help keep the thread running!

I usually check to make sure the factory has not run a ground wire all the way back and to each light. Most do not so I run a single ground wire full length of the trailer and to each light. This is then grounded thru the plug and truck wire system. I have a trail master trailer and they ran a separate ground wire to each light and I have never had any problems with the lights on it

Sorry to revive an old thread, but in searching this original post is the same issue I’m having with my trailer. Only difference is that neither the running light nor the marker lights on the driver side of the trailer are working, but the blinkers and signals work. I’ve also tested it on two vehicles. I’ve found where the brown wire branches to go to each side of the trailer – is there some way that I can test where I lose current? Can I poke the brown wire and test it with a multimeter and then just tape up where I poked?

Like you said check it with a meter for continuity. I would just shave some of the insulation off the wire until you see copper. I wouldn’t poke the wire because who knows if your lead is touching copper. You probably will have to rig up some longer wire to test it for a break.

stripping the isolation means you will have more problems in a short time.

stripping the isolation means you will have more problems in a short time.
the more times you poke the wire the more problems you are going to have down the road.

obviously you will be sealing this with some high quality electrical tape

i would recommend scotch 33 plus good to -18 degrees

fishingstar wrote:

stripping the isolation means you will have more problems in a short time.
the more times you poke the wire the more problems you are going to have down the road.

I’m open to other options.

So if I do expose some of the wire and put the red prong of the multimeter on it and the black prong on the trailer frame, should it read 12V even though the lights aren’t working? Obviously assuming I have the lights of the truck on…

Ended up nicking the wire up by the tongue (right before it entered the trailer frame to run down to the rear of the trailer) and it had about 12 V, at the light had zero. Pulled the wire harness out of the frame and sure enough the brown wire had a spot rubbed bare on it. Ran new wire and connected it up and let there be light!

glad you got it figured out….electrical problems can be frustrating.

So it blew a fuse? bare wire on a frame grounded it out and blew a fuse? Or the wire just wore through and broke?

slough wrote:

Ended up nicking the wire up by the tongue (right before it entered the trailer frame to run down to the rear of the trailer) and it had about 12 V, at the light had zero. Pulled the wire harness out of the frame and sure enough the brown wire had a spot rubbed bare on it. Ran new wire and connected it up and let there be light!