Trailer Wiring 101

[Jeremy airjer W]

The trailer lights are giving you trouble and you’re not sure where to begin. The first thing you need to determine is what is or is not working. There are two things that will make your lights work incorrectly. One obviously is the lack off power which will result in everything working except for one light. The other is grounding issues which will usually cause strange things to happen.

Power Issues

This first part will deal with power issues. Our objective is to narrow down the possibilities until we have isolated the problem. The information below already assumes that you have checked or replaced the bulbs just incase they where burnt out.

The first thing I want to do is find out if it is the truck or the trailer. The simplest way to check the truck is with a test light.

Since most of us use a four flat style connector I will be basing everything on that. Whether it’s an aftermarket four way connector or a 7 pin to four adapter the male terminal on the four way connector on the vehicle is the ground. Simply hook the test light to the ground then turn on the trucks lights and four way flashers and probe the other three terminals. If the other three terminals light/flash the test light than you can be reasonable confident that the vehicle side of the wiring is fine.

What if one of the terminals does not light up the test light? If it’s an aftermarket harness and everything else on the truck is working than the harness itself may be at fault since the aftermarket harness uses the same power that is provided to the lights on the vehicle. If it’s a factory wiring setup than the fuses are first on my list of things to check. Typical factory trailer wiring will have there own fuses for each function of the trailer wiring.

What if all the terminals don’t illuminate the test light? My first thought is bad ground. We have all had some pretty shady looking connectors and if there green they aren’t going to work.

If there clean then I would try finding another way of grounding the test light and rechecking the three terminals (when finding another ground source it doesn’t hurt to make sure it is a good ground by checking a wire that you know has power to it to make sure the test light lights up). If the test light lights up then we know we can concentrate on the ground if the test light still doesn’t light up then we need to dig a little deeper into the aftermarket harness or there may be a problem with the 7 to 4 adapter. If you suspect the adapter you can remove it and check the running light, left turn and right turn pins on the seven way connector. If they work at the 7 pin, than the adapter is likely your problem.

So the truck checks out fine. Now what? The first thing I would do is look at all the exposed wiring on the trailer. Some things you should look for are broken wires, pinched wires, corroded connections, and bare wires. Connection made with either wire nuts or scotch locks should also be looked at carefully as many times they can be the cause of the failure since they do not “seal” out the elements which allows the wires to corrode.

Also pay close attention to the lead buttons on the bottom of the bulbs. I have run into many of these where the buttons have worn out and causing them to loose contact with the socket. In this case the buttons have worn enough that they are touching, the symptom was that the brake lights would be on as soon as the customer turned on the tail lights.

More often than not trailer lighting problems can be resolved by repairing one of the problems mentioned above.

If the wiring looks to be in good shape, than the next step is to check for power at the socket. Again the simplest way is to use a test light. Socket for the tail/brake light will have two contacts on the bottom. One will supply power when the brake/turn signal is applied and the other when the lights are turned on.

(Photo courtesy of Shackbash)

If there is no power than its as simple as tracing that circuit back until you find the source of the power loss.

Ground Issues

So your trailer lights are acting weird one light blinks opposite the other when it’s not supposed to. When you step on the brakes all the lights go out. The lights are really dim. The lights flicker when traveling down the road. These are all possible ground issues and are not that hard to figure out.

The first thing I do when I suspect a bad ground is hook up a test light to a known good ground on the vehicle (preferable the negative battery terminal using a long jumper wire). Next, with the trailer plugged in to the vehicle and with the symptom present, touch the test light to the trailer (preferable a clean metal surface). If the test light at any point lights up then there is a main ground issue with the trailer. Usually between the trailer plug-in and where the white wire is attached to the trailer. Make sure that the area where the white wire is attached is clean and corrosion free, as well as the wire and/or connector. A star washer does a good job between the wiring terminal and the trailer frame to make a good ground contact, or in between the washer and the nut on the back of the taillight.

(Photo courtesy of Shackbash)

There still can be isolated ground issues that will not light up the test light in the scenario above. This could be anything from a corroded socket, poor contact between the lens assembly ground post and the trailer, or the socket ground wire and the trailer. All of these can be checked buy using a jumper wire to a good ground and then grounding the suspected component to see if the problem corrects itself.

(This lens assembly uses the mounting bolts to ground itself to the trailer - Photo courtesy of Shackbash)

Most trailer lighting issues can be solved with simple repairs. Remember to start with the simple things first.

-Identify any and all issues that may be present.

-Determine if the source of the problem is the vehicle or the trailer.

-If it’s the vehicle check fuses and connections

-If it’s the trailer check the wiring, bulbs, sockets, and connections

-Make sure there is a good power supply and a good ground. These are the two most common problems.

Special thanks to Shackbash, Marine_man, and 4wanderingeyes for there help!

[Jeremy airjer W]

Some harnesses will have two brown wires so that they can run tailight wires down doth sides of the trailer. It is stilled wired the same. Both the browns still get attached together to the single brown on the new connector.

[tonkapat]

I use connectors from the home.... depot that are heat shrink with solder in the middle. push the wires in heat it up and you have one great connection. I use them for all my splices. Check them out you will like them.

[sheryl96razor]

Everything that's really concerns with wiring's making me feel uneasy if it'll never gonna be used properly.

[Xplorer]

Just discovered late last night that I have a wiring issue, and have a trip for MEA weekend with my son leaving early thursday AM. Flat 4 hookup.

Condition:

When truck headlights are off, all lights work perfectly (signals/brakes).

When I turn on my headlights, i have no running lights. However all of the signal and brake lights still work fine.

Bad ground or fuse? why would the lights still work when the headlights are on/off?

Which wire carries the running lights to the trailer?

Thanks in advance for any help. I'll have time tonight to troubleshoot if more info is needed.

[marine_man]

The tail light power comes through the brown wire.

It does sound a bit like a ground problem to me... but odd that it works fine with the lights off.

Have you verified that the brown wire / pin has power from the pickup (with the trailer connection off) to isolate trailer vs vehicle?

marine_man

[Xplorer]

mm,

Nope, it was almost 11pm when i discovered the issue, and I had to get up at 4:30 this am so figured I'd leave it til I get home today. I'll start at the pickup flat 4 and work back from there with the tester. Just puzzled me that everything worked fine when the truck lights were off, and only the running lights don't work when the truck headlights are turned on.

Now that I think of it, I have both a 7 pin and a flat 4 on the truck, I'll check it out using the 7 pin adapter also. If it works on that then I'd know its the flat 4 that is the issue.

[Big Buck Buster]

Xplorer,

What year,make,model vehicle are you having the problems with? If it is a newer GM or Ford, you may want to look for the trailer tail lamp fuse in the Power Distribution Center under the hood to see if it is blown.

[Smoker]

I had the same prblem. It was the flat plug on the truck was bad. I just got the 7 to 4 adaptor and problem fixed. Plus you can take the adaptor out and your plug is still covered and won't caroad as fast.

[Xplorer]

Thanks everyone.

Got home tonight and started at the beginning of the run. Fuse OK, power to the 4 prong, but none to the taillights. I have a 5 wire trailer config (2 browns, 1 for each side, white, green and yellow ) and 3 grounds (one from the hitch, and one from each side of the trailer). Checked all connections, good. Cleaned and replaced rusted nuts on the back end grounds, and all is well now. Not sure which it was but both were rusted pretty good. Replaced with stainless locknuts. I'll make sure to clean all terminal's before and after season from now on!

Cast and blast weekend starts in 30 hours!!!! Vermilion eye's beware

PS: was a '10 Tundra, so I really didnt think it was the truck harness.

[Jim Almquist]

PS: was a '10 Tundra, so I really didnt think it was the truck harness.

Tundra NEVER !

[BAIT956]

I had the same problem as Daze Off. Checked the fuse box like you sugessted Airjer, and aha, a blown 15 amp fuse in the trlr park box. I will pick up a new one, but this should be the solution. Thanks Airjer, great post.

[Mlaker3]

I got this idea from a buddy of mine. He uses lite outdoor extension cords on his small trailers. The kind where you have an outer layer of rubber over the top of your load carrying wires. Start at the front and a four prong pigtail, keep track of your color transfers and you have a wire system that will keep out the salt and wear. The wiring harness you buy just aren't meant to last thru hard use. FYI

[Tofishem]

I used extension cords to re-wire my trailer. Solder, shrink tubing & elec. tape never again problems from tongue back. LED lights the way to go!

[JIvers]

I just re-wired my whole boat trailer; I ran the new trailer wiring through soft PVC tubing to try and protect it from wear and tear.

I don't trust myself enough to solder, or otherwise attach, the trailer lights to extension cords. I picture melted wires, blown fuses, and all sorts of other problems if I tried that.

[Jeremy airjer W]

Once you figure out how to solder you have a "huh, that pretty easy" moment. It just takes a little practice.

[triggertrav]

I am having issues and hoping someone here can help me out....

My left turn/brake trailer light is not working. it seems that the ground is good and i swaped out bulbs out and that didn't work. The bulbs are the dual running and brake/signal bulbs and the running lights work on that bulb.

Any thoughts? Would it be that something in the actual brake light housing/assembly went bad and that needs to be replaced? Or some other wiring issue?

Just was hoping to see if anyone had any input prior to me tearing the wiring apart or buying a new light assembly.

thanks

[SpecialK]

Are both filaments good in that bulb? It really sounds like the bulb is bad or you have a broken wire.

[triggertrav]

bulb is fine, i have a spare and that didn't work.

[SpecialK]

There should be 2 or three wires going to the light assembly. One is ground, one will be the normal tail light (which as I understand you say is working) and one wire for the brake and turn signal.

Take off the cover and I would use a length of wire that will reach from the from pigtail connector to the tail light. Strip the ends of the wire and put one end in the socket that corresponds to the correct wire color that you need to test and using a meter, test for continuity for those two wires. More than likely one of those 2 wires will be fine and go to 0 ohms and the other will show OL (on a digital meter) meaning it's broken.

Once you figure out which wire is broken you can either run a new wire and splice it in or replace the entire wiring harness.

The only other things I can think of is that the pin in the connector is pushed in and not making connection, the wire is broken right at the connector or the wire is broken on the towing vehicle side.

This is assuming that there isn't a fuse blown in the towing vehicle.

[triggertrav]

got it fixed. it was the six prong to four prong connection. must have been a short in it the plug. thanks for helping.

trav

[SpecialK]

Glad you got it fixed.

[rocstar]

Is there an easy way to check for a dead short in the trailer wiring?

[SpecialK]

Using a digital meter put one lead on the wire you think is shorted and the other lead to a ground spot on the trailer. If it shows a number there is conductivity (a short). if it shows OL it's an open circuit i.e. no short.

[rocstar]

When you do this do you have to pull all the bulbs? I assume you would get continuity through the bulb filiments. My running lights (brown wire) appears to be the problem.

[SpecialK]

I would pull the bulb just to be sure because it probably will read thru the filament.