Trailer Light Problem

I'm having issues with my gooseneck tractor hauler lights. All lights work except left turn and left brake lights. I have power to the plug in and power to the junction box on the trailer, it seems to me the problem is in the wire going from the junction box to the tail lights. All of the wiring is inside conduit so it's almost impossible to see where/if there is a bad place in the wire. The crazy part is that the first time I pulled it out of the shed, I had no brakes and no left turn signal or left brake lights. I unplugged it and when I had time to work on it a few days later, now I have brakes but no left turn or brake lights. I checked at the back of the trailer and have power for the running lights but not for turn or brake lights. I installed a new taillight but that didn't help, I didn't think it would, but had to be sure. How can I check the wire from the junction box to the back of the trailer? Or should I just run a new wire for brake lights?? Did I mention that I hate working on lights?? I have a small multi tester, could I add enough wire to run from junction box to the back of the trailer to test continuity of the brake light wire, the trailer is 27'+ a 5' beaver tail, so close to 35' of wire to check the continuity with my little tester. Thanks, Chris

The issue is likely at the light fixture. Wire in conduit rarely has issues. gain access to the light fixture (remove it if rear side wires are not accessible. Buy a can of Liquid tape from a autoparts store. Using a knife, nick a bit of insulation off of the turn/brake lead in wire at the fixture. Test the voltage there to assess where the break is. There is likely a connector between the conduit wire and the fixture wire. Test both sides of that connector. When fixed, use the liquid tape to seal up the wire nicks. Jim

Is your truck a ford ?there is a separate fuse for trailer turn signals on my wifes 09 f-150 check at the light plug receptacle at the rear of the truck for current with your tester

The truck is a 98 Chevy 3500. I have some liquid tape, I'll try that. I wondered about the ground at the light, but the running lights work, so that should rule out the ground issue. Thanks, Chris

Make sure you have a good ground from the trailer to the pickup.
That lack of ground can cause a lot of odd, weird, strange problems,
including individual light problems.
While a multimeter is very useful, a test light is also useful.

I grease the ball and the top of it is shiny, ought to be a good ground from truck to trailer.

(quoted from post at 19:17:08 06/05/22) I wondered about the ground at the light, but the running lights work, so that should rule out the ground issue.

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Not necessarily. I have a trailer that I ended up running a new and separate ground wire to each lower light (not simple clearance lights; enclosed trailer) in addition to the original ground. Hadn't had a problem since.

That is not enough for ground you need a good wire at least to the frame. I run ground wires for all circuits.

Sometimes it is just a poor connection at the plug. Try spreading the contact in the receiver slightly, with a small screwdriver, or knife blade>

X2!!!

There is a reason for the ground wire location in trailer plugs, use it. Make sure the ground wire connection is good on the tow vehicle and install a good ground wire (clean connection point to bare metal and use a bit of anti-seize to slow rusting) to the trailer. Use 12 gauge or larger with electric brakes.

Your turn lights and brake lights use the same wire from the signal light switch back. If you had one and not the other, it would indicate the truck signal switch was bad and that would show up in the truck lights as well.

If you have steady (blinking with that side's turn signal on) power for the left signal/brake lights in the trailer junction box, it is not likely the plug connection. In the junction box, check directly on the wire going back to the lights, not just the stud to be sure it is not a bad connection.

At the back, did you check the power to the lights in the light socket or the wire ahead of the connection to the light? Even though still connected, connections can corrode and stop current passing through. Is there a break in the conduit, near the axles, where the brake wires connect? A junction box for the break wires near the axles? Any opening in the conduit is a possible location where the wires could be damaged.

It's unusual for a continuous piece of wire to have a break inside conduit, but anything is possible.

Trailers are known for having shabby wiring from the factory. They like to use the crappy fold over connectors. The sooner you can rid it of those the better! Solder, liquid tape, shrink tube is the way to do it!

Also look for bad grounds. Test with only one function going at a time, as in don't turn on the running lights and turn signals at the same time, it will give confusing feed back if there is a bad ground.

I had that problem with my 22,400 PJ. If I pulled a side light out and pulled the wire behind it forward the brake and signal would work for awhile. This told me that there was a problem with the wire in between. The wires run in the 2.5 x 5 tube frame, and apparently at the point where the beaver tail welds onto the main part of the frame, some roughness in the weld wore the insulation off. I pulled new wire in attached to the old wire as I pulled it out. Problem solved.

(quoted from post at 23:43:45 06/05/22) I grease the ball and the top of it is shiny, ought to be a good ground from truck to trailer.

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That's funny stuff there. Also not very smart.




This post was edited by Carlmac 369 on 06/06/2022 at 09:44 am.

Those little fold over wire connectors should be outlawed.

You need a 12 v test light with an alligator clip on the wire to ground and a nice sharp point on the probe. Start at the truck receptacle and verify function on that terminal. Plug in the trailer and work your way back until it malfunctions.
If your lights are grounded through the trailer frame you will have problems because of so many dissimilar metal connections. When you eventually rewire your trailer ground everything through the wire harness.

If you have power at the junction box in front, but do NOT have power at the lights at the rear, it HAS to be the wire running in between.

It could be as simple as the connection in the junction box. I use the boxes that have seven color-coded posts, and use heat shrink ring connectors on the wires.

Easy way to test is run a wire out on the ground and bypass the wire in the conduit. If the lights start working, there you go.