Trailer Wiring Issue

I’m a bit stuck

I have a trailer with 4 pin the usual yellow for left /turn signal/brake and green right turn/brake. There is also a brown which is being used for left side running lights and tail light and black for right side running lights and tail light. The brown and black are joined at the brown tail/running light terminal of the 4 pin.

All lights have two wires running to them. One papers and one returning to the white ground wire running through out the trailer.

I ran light this a few times but then noticed I kept blowing car fuse. I determined it was the tail/running light terminal blowing the fuse. Once I disconnected those two wires, my left and right turn/brake lights worked great.

I started to diagnose the issue and found when I connect either the brown or black directly to a battery, all the light light up without issue. I can’t find any indication of weak ground or connections.

However, when I connected a battery pack and also used an electrical diagnostic tool the battery pack would signal an error for the brown or black wire. The signal indicates a reverse polarity but the pack and manual are vague. The diagnostic tool indicates continuity.

I confirm that the worked turn/brake lights do not have continuity and work fine for the diagnostic tool, battery pack and battery. But both the brown and black, either separate or twisted together, both give a continuity alert on the tools. They don’t trip the breaker on the diagnostic tool which would indicate a short.

So….I’m confused. The working turn/brake lights do not have continuous unity. I didn’t take out bulbs but they are LED. 1. Do LED lights give no continuity when working properly?

The brown and black give me continuity alarm and alarm for on the battery pack. I’m puzzled why BOTH of these wires would give continuity to ground whether separate or together and also continuity to each other. It would indicate there is a short or something between ground and BOTH of these wires? The issue is that these wires are only run together for a short period and near the front they split left and right side of the trailer.

2. How could both wires show continuity to both ground and each other, alert and block the battery pack but yet if I use a battery the lights work fine?
3. Do non-LED lights show continuity if I don’t remove bulbs and that is normal?
Thanks

I’m not familiar with your diagnostic tool or what it’s trying to tell you. But if you connect the running lights to the car and it blows a fuse, that tells me there is a short somewhere between the brown and black wire and ground.

If it was me, I’d separate the brown and black and try them one at a time to see what happens. Then you can narrow it down to one side of the trailer.

I’m going to have to get the ohm meter out and see what LEDs look like. I know what old incandescent bulbs acted like, but haven’t looked at LEDs yet.

SR

Basic multimeters measure just amps, volts, and ohms. To test LED lights you will need a multimeter with a diode setting.
LEDs do not have a linear relationship between current and voltage so they cannot be modeled as simply as a resistor using Ohm’s Law

Steve Root wrote:

I’m not familiar with your diagnostic tool or what it’s trying to tell you. But if you connect the running lights to the car and it blows a fuse, that tells me there is a short somewhere between the brown and black wire and ground.

This ^^^^.

You have a short somewhere on the black or brown wire OR on the lights that these wires power.

Since you can separate the black and brown wires, I’d connect one side at a time to the vehicle and see which side of the trailer blows the vehicle fuse.

That will at least tell you which side the short is on, left or right.

But from there, you could spend hours in diagnostic hell trying to find the short in the wire or the lights. OR… just replace the wire AND both the marker light and the tail light on that side and be done.

FWIW, I strongly suspect you have a bad LED marker light or tail light that is the cause of your problem, but running this down to a specific “there’s your problem, lady” moment is (again IMO) not worth the time.

Thanks for your help.

That’s what’s confusing me. I did separate the left and right side and confirmed that no lights share these circuits. Yet each one can blow the fuse as well as when they are wired together. Since none of the lights share these wires, I’m a bit lost. I figured it was something related to a ground or a screw through the cable before the wires split into left and right but I can’t find anything. Having a bad LED or a reverse polarity LED or a LED + Incandescent COULD make sense if they installed a kit of lights the same way on both sides.

Keep in mind, if I apply the battery without a fuse everything works but the fuse and the diagnostics don’t like those two circuits.

I’m going to measure amp draw on those two vs the yellow and green circuits to see if there is anything different. The yellow, green, brown and black all use the LED tail/turn/brake lights so for now I’m assuming the issue is elsewhere.

I’ve already removed the little working dome light and the rear running light from the brown circuit but it didn’t fix it. I’m going to remove the side running lights next which means that circuit will only power the tail light. I’m hoping I can isolate the issue.

I’ll take a fresh look at the trailer from that perspective. Its a horse trailer so a lot of the wire is run in the walls of the trailer. But I really didn’t examine whether I could pull new wires through the space. If the are free and I can snake new wire it might be worth it. The trailer has two side markers on each side plus a top front and top rear running light in addition to two dome lights inside. So that aa lot of wiring and 6 lights to replace. But….as you said….I might spend as much time trying to fix an issue with no resolution!

Thanks for giving me the kick to think about replacing vs fixing. Probably better in the long run!

if it’s 4 pin plug there is no black wire.
green- right blinker and right brake
yellow- left blinker and left brake
brown- running light and tail lights
white- ground

A guy can pull his hair out for days trying to figure out trailer wiring issues. I keep those led trailer wiring kits on hand and replace everything when there is a problem. Wiring, lights, plug. All new. Problem solved in 30 min.

Coming out of the brown pin, this trailer sends brown to the left side running and tail lights and black on the right side. But they are joined just prior to connecting to the 4 pin.

The wiring is clear but something is causing tail/running lights to blow a fuse

Being a horse trailer does it have brakes and are they electric, or just a two horse without brakes?

It is a European brender up two horse trailer with inertia brakes for towing with cross over. So no powered brakes. Just 4 pin

I did a little research on LED lights. LEDs operate at a much lower voltage than 12 volts. I was able to find the schematic diagram on a few units, and as expected they use a Zener diode and pass transistor setup to supply a constant voltage to the LED.

What this means is that you won’t be able check out and LED light assembly with an ohmmeter. With an old fashioned incandescent bulb the filament was either good (zero ohms) or bad (open circuit or high resistance). With the LED’s all you’re going to see it the circuitry that feeds the LED itself.

So like everything else, the new technology has some advantages but adds some complications.

SR

Ground each light separately to a ground. It’s ALWAYS the ground.