Trailer wire short

Where’s a good place to start looking for a short? The port side tail light works but no turn signal or brake light. When the standard halogen lights were in, the brake light would gradually come on. There’s new LED lights in there now and just the running lights work. The starboard side works great as it should.

Any thoughts?

I would check the plug first especially if you use an adapter. Ground wire on that light may be loose or corroded also.

That will give me a starting point...

I would say bad connection or broken power wire to that light. Could be a bad ground but I doubt it since the running light works. Clean the plug good and wiggle it around to see if you get a temporary connection. Sometimes a single wire, or more, will break right over the axle or where the wire comes out of the plug.

When a lamp doesn't illuminate, for some reason people immediately think "short circuit". If you blow fuse after fuse in the tow vehicle, you have a short circuit. If you do NOT blow fuses in the tow vehicle, it is extremely unlikely that you have a short circuit.

The most common problem with lamps not energizing is an OPEN CIRCUIT. This means that a conductor isn't making contact. Either the positive conductor or Ground conductor could be disconnected at some point along the circuit; either one or both gives the same no-workie result.

Trailer lamps at a given lamp fixture share one common Ground. Since your tail light works, that tells us the Ground is making contact at the lamp housing, so the Ground connection isn't the issue. This narrows it down to a single wire - the positive conductor for the brake & turn function. [Note that the same LED or filament provides both brake and turn functions.]

Turn/Brake on the left lamp housing is energized by a yellow wire if the trailer is wired correctly.
Turn/Brake on the right lamp housing is energized by a green wire if the trailer is wired correctly.

Your issue is with the yellow wire - or some connection for that wire. Corrosion in pluggable connectors or other wire splices, or a faulty connector installation (butt splice) are the typical culprits. Occasionally, the wire itself may be found to be physically broken due to an encounter with road debris or getting snagged on something when the trailer moved.

You seem to not have an issue on the right side, so swap the two fixtures (temporarily just for a plug-in test). If the issue is the trailer, the left side will still have a problem when you plug the other fixture in. If the problem moves to the right side, then the issue is with the lamp assembly you had installed on the left side.

When you unplug the connectors, inspect them for corrosion or heavy tarnish. They may need to be cleaned up with a soft metal bristle brush or fine grit emery cloth (not typical sandpaper - way too coarse). Also trace the yellow wire to see if it is damaged along the path to the trailer tongue. Realize that a disconnect in the yellow wire could be all the way at the trailer tongue as well. The splice that attaches the tow harness connector could have an open circuit. A digital voltmeter or test lamp is useful for diagnosis of problems such as this. Problem isolation should take mere minutes with this info if you have a test light or DVM. You'll need to connect the tow vehicle and turn on the left turn signal to chase down the open circuit using the tools. I can judge most by sight, and fix the obvious without bothering with more effort if my guess fixes it....

It is the trailer. I did swap both lights this morning and same issue. My next step is to follow the yellow wire from front to back. I’m guessing what I’m looking for is a broken / cut wire or exposed copper wires?

Personally I would start checking the plug on the truck first to make sure the wire harness connections on the truck are good. Then right behind the trailer plug that plugs into the truck receptacle then the butt splices where the plug pigtail is connected to the trailer wiring. The plugins are the first always for me to check since it gets the most action of all components on the trailer wiring. Hooking up in the dark and buddies or kids helping can lead to messed up plugins. Sometimes just normal wear and tear over the years. If you can see the the wiring from under the trailer without crawling under it you might look for obvious cuts or scraped wires also

jbobo has some good points.

Go with the things that are more likely to be the cause as the first to inspect, and leave tracing the full length of wire along the frame until last. The wire run is the least common place for the issue, but it does happen. The problem is most likely at a connector, whether one you unplug and replug, or a permanent one (crimped splice).

A 12V test light (inexpensive, available at auto parts store) is an excellent test tool. You can use it to confirm whether the tail/stop contact at the vehicle's tow connector is energized. If you don't know how to use one, contact me. There is no sense spending hours trying to diagnose a trailer when the tow vehicle's connector has an issue. [I didn't suggest this previously since you told us the old lights worked without an issue. This would mean the tow vehicle's wiring was working at least at that time.] Experience tells me the problem is where the wiring system was "messed with", which is at the left rear corner of the trailer. I'm going to guess the yellow wire butt splice at the pigtail was weak, and is now open circuit due to handling to plug in the new lamp.

There are two places most any trailer has a butt splice: 1) very close to the lamp, made to attach the lamp housing pigtail, and 2) close to the trailer tongue, to attach the trailer's wire run. One of those could pull free, be badly corroded, or broken right at the butt splice, causing the issue. Find those splices and look at their condition. Do that before tracing 20+ feet of yellow wire.

A nicked wire, in and of itself, doesn't guarantee an open circuit. Cuts in the insulation can allow corrosion because the wire strands are exposed to the elements, especially if non-marine grade wire was used (and it typically is, sadly). [Note: wire strands with a metallic orange color are bare copper strands, and not present in marine-grade wire. In marine grade wire, each copper strand is coated with tin, giving it a gray appearance.] Cuts in the insulation may not be an issue if they aren't touching other conductive items such as the trailer frame. I repair minor nicks with liquid electrical tape to seal the conductors from the elements.

You posted: "The port side tail light works but no turn signal or brake light." As mentioned above, this means the brown wire (brown wire is lights) and the ground are good to the module. Port side is left and left is the YeLLow (L in yellow for left to remember / gReen is right, R for right). This means somewhere the yellow wire is not energizing to the light. At the vehicle, turn the left signal on and check for voltage on the yellow wire (with the signal it will be on/off/on/off etc.) If voltage there, then apply direct voltage to the yellow wire at the plug (don't forget to ground the trailer) If the light turns on, then it's possible the wire is corroded, bad connection, etc allowing for a voltage drop from the vehicle. If light doesnt' come on, I would disconnect the yellow from the light, tie another yellow on in the front and pull a new wire, connect on both ends.
John

First I'd cut and re-splice all butt connectors on the yellow wire - Even the ones you think are good. Sometimes the wires corrode and break off inside the connector and you wouldn't even know by looking at it. I recommend using heat shrink butt connectors and liquid electrical tape to seal all connections. My saltwater trailer had the plug type connectors from the lights to the wires - the posts themselves were corroded and cracking. If you get to looking and have several bad connections, It might be time to strip it and start completely over with a fresh wiring harness.

I was constantly having similar issues on an older trailer so I ran the white ground wire all the way to the back and used a ring terminal to connect directly to the ground bolt of the light housing (this is how it was grounded). Seemed to eliminate all problems on this particular trailer (the trailer itself was losing conductivity at some point).

Check all splice locations.

Someone who has to ask for help in basic wiring issue diagnosis probably should NOT be given a suggestion to cut out and replace all splices without confirmation of the need to do so. If they can do good splices, they probably wouldn't be asking what this OP is asking. Such an inexperienced person is more likely to CAUSE more problems than they repair if they cut/replace all splices. It is best when diagnosing an issue to leave as many things alone as possible. The first rule of diagnosing an issue is not to destroy the evidence - if you can't figure it out, turn it over to someone else.

On your own trailer issue:
A conductive trailer doesn't "lose" conductivity, I assure you. What was lost was continuity at some connection point. When you ran the Ground wire, you did so with a NEW ring terminal as the Ground contact point. Sanding the corroded spot on the trailer and the fixture's contact point to the frame would probably have yielded the same result.

That obvious suggestion has been made numerous times already.

Probably quick splice connectors at vehicle.

Those should be illegal to sell.

As I said earlier, it's his port side signal/brake light. that is the yellow wire. It shouldn't be connected to any side lights, just a straight wire from front connection to light. Pull a new wire is less time than trying to redo splices that will cause problems again.

If you aren't proficient at troubleshooting electrical problems hire someone to run a new wiring harness. You will be money and time ahead. They are cheap and it's about a hour job. Once you begin having to track down bad connections and splices on trailer wiring, it goes downhill from there and unless you know what you are doing, you will end up with worse than what you have now.

I’m actually considering taking it to Uhaul and have them run new wires and install of my new LED lights. I found a bunch of Optronics LED’s on amazon cheap. I’ve been hit once from behind pulling my last boat, so I figure a bunch of LED lights will help stop that.

Ok well I try to live life by K.I.S.S..... keep it simple stupid... tonight I’m installing my new LED’s to my trailer and I see to wires unplugged / broken, well 2 + 2 = 4, splice both wires back together and it’s alive.

All is good again, thanks for all the help guys

Glad you got it going Mike. For future reference or anyone else having problems like you had is checking (if you have the sealed oval lights) is check the connection on the back of the light fixture itself. The connectors are thin brass that has a slit in it (for spring tension effect), sometimes these can get spread out and not make contact with the plug itself. You have to get a pair of needle nose in there and squeeze it together be careful though, like I said they are thin brass.