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!

[TMF89]

Alright guys, so I checked my trailer lights in anticipation of the coming season, and of course we have problems!

I turned on the truck lights and the E lights, couldn't check the brakes as I was by myself.

On the right side the rear light worked for both regular lights and flashers, on the left side nothing lit up. Some of the smaller lights on the wheel wells worked, some didn't. However I replaced those a few years ago and I wouldn't be surprised if I mucked it up. My first problem is not being able to check the bulb on the left taillight, because I cannot get the black bracket it sits on out of the housing! I pulled the tab out on the bottom, but couldn't get it to come down. I stupidly shoved it up to see if I could wiggle it up and down and loosen it, but now it's just shoved up even farther than originally, and I can't seem to pull it out. I'm guessing I might have to take the housing off? Here's a pic of the situation.

I noticed that both tail lights seem to have grounds on the front of them, are those just the grounds for the tail lights? Why are there three ground wires going to them? They're on both tail light housings.

Finally, I noticed there was another ground at the front of the trailer, right where the wiring splits off to either side. Is this the grounding point for the rest of the lights? To me, all the grounding points look fairly rusty, would it be worth cleaning/changing them? I don't have access to a rivet gun, but I would imagine a bolt and nut would get the job done as well?

Thanks for the help guys! I was considering rewiring the whole trailer anyway, so this might just be the catalyst for that.

[harvey lee]

Not 100% sure why all the ground wires but the 2 extra's on each light could be for the side markers on each side of the trailer. Thats if you have them. One of the grounds could also be for a light between the back sides if you have one locasted there. Like inbetween the back tires towards the back of the trailer, right in the middle under the motor.

By the looks of the ground clamps, I would at a minimum replace the ends.

On the left side, you could have a bad ground wire or a bad bulb but if nothing works, I would check the ground.

if the wiring is 5-7 years old, I would replace it all but that is just how I would do it depending on the age of the wires.

Seems for me at least, after 6 years or so, my wires start to give me issues and typically my wires are always broken inside the frame of the trailer tubing. This last time I rewired my trailer, I went with a much heavier wires and the coating was better and then those coated wires were inside another coating to stop the breaking of wires inside the frame.

To be honest, since you had to replace some of the wiring a few years ago and now have more issues, I would just replace it all, then you will be good to go for some years before any issues. You replaced some, maybe have a bad ground and it sure looks like it could be abd then if that fixes that, won't be long and the other wires will be another issue.

[mnguy152]

yeah and led then did mine works great and nice and bright too

[TMF89]

Thanks for the reply Harvey! I do have a center light under the motor (which works, FYI), and wheel well lights. Although on the left wheel well I have one light working, and one that doesn't. I would have thought they'd be in series, so if one got juice the other should too? I realize it could be a number of issues besides no power, just making sure I at least kind of know what I'm talking about! As far as "checking the ground" on the left tail light, does that consist of just cleaning up/replacing the connector on the front of the tail light housing, and possibly the main ground at the front of the trailer?

Do you know of any sort of "one-who-thinks-I-am-silly's guide to trailer wiring"? I've never wired up anything before (when I replaced the wheel well lights I just got new bulbs/housings and plugged them in to the existing wires), so I have zero experience with what I'm getting into. I believe in the adage "You don't know what you don't know", so I'd like as much information as possible before beginning. For example, am I going to have to get a bunch of extra wire to wire grounds for all those "extra" grounding wires I showed in my pictures, and wire a ground from each individual light? How do I go about wiring both tail lights up when I only have one brown wire (my dad bought the wire, and just got some generic trailer wire, without the separate tail light wires)? And last but not least, concerning that stuck bracket on the left side, how would you go about removing that? Is there a way to get the whole red housing off? Thanks again.

[TMF89]

Question, what's the best way to go about cleaning grounds? I was thinking I'd clean the three on my trailer, and see if that helped anything. Just undo the nut/bolt and sandpaper everything clean, then put it back together?

Also when I rewire, can I just run all the grounds up to one grounding point at the front of the trailer? Or even just run it right back into the truck via the wiring harness? I'm sure these are pretty easy questions but I'm a real newbie when it comes to electrical stuff. Thanks guys!

[amateurfishing]

I just had my trailer lights go out for no reason(right), only thing that works is on lights when I turn truck lights on, no turn signals or brake lights. Bought a new coupler as old one was falling apart at times but that did not fix anything. They r 3 year old waterproof led lights, where to start?

Connections?

Wiring?

Fuses?

[delmuts]

First thing I would check would be your ground. Is the connection thru your ball hitch and coupler, or do you have a separate wire to frame or battery ?

I got tired of light problems with the ground so now I run a wire from the battery to my plug. That has been a big help.

[Jeremy airjer W]

I just had my trailer lights go out for no reason(right), only thing that works is on lights when I turn truck lights on, no turn signals or brake lights. Bought a new coupler as old one was falling apart at times but that did not fix anything. They r 3 year old waterproof led lights, where to start?

Connections?

Wiring?

Fuses?

year, make, model, of the vehicle?

Have you checked the "trailer" fuses. Many newer trucks or vehicles with tow packages have separate fuses for trailer wiring. Many times there will be specific running, right, left, and stop fuses. Check the vehicle first either the fuses or at the plug with a test light.

[amateurfishing]

02ford explorer xlt 4 dr

Interesting, since I have no truck light issues assumed it had to be behind truck somewhere, will have to check manual for possible trailer fuses, weird that both of those would go out at same time though unless samd fuse (aha moment maybe?)!

[BAIT956]

As mentioned above, check the ground and fuses. I have had these types of problems and one of these was the culprit every time.

[amateurfishing]

Well, not trying to b cheap but trying to save time

Truck lights r fine which tells me its not fuses. Redid ground by stripping a bit off end & re screwing on, with no change. Only lamp lights work on trailer, no side lights, turn signal, or brakes.

Is there a common wire problem that would help me trace this faster, I assume it has to b a splice connection type issue, correct?

No specific trailer fuses, only have 2 fuses. 1. Flashers, turn. 2 brake, lamp. And since trucks all good I assumeits not this.

[Todd S.]

Have you checked your truck on a different trailer or tested truck connector with a tester by chance and or your trailer on a different vehicle? It sounds like your confident it's not the truck, but would want to test to confirm. I've see a a broken filament in one brake bulb incomplete the circuit and not allow trailer lights to work, along with trailering fuses in the truck randomly pop, which also is a side effect of possible filament popped. I to live in ER if you don't have another trailer to hook up to pm me your more than welcome to try mine, I also have a tester for your truck which is the best $5 money spent

[amateurfishing]

I did not know about a tester like that which will instantaneously give me direct feedback, will pick one up today, thx

[EBass]

It could be that black module piece where the truck back light wires merge into this black plastic box then to the trailer wires. I had that fry on me before. I went to the Hitching Post since I couldn't figure it out. Just another something that can go bad with trailer lights. This was on an old Toyota T100. Never had that issue on the other trucks I've owned.

[amateurfishing]

Well im gonna go to store for one of those testers and also hook up wifes vehicle to it tonight, c what I get and go from there for now

Big thanks for all ideas and suggestions. Will let you know

[Todd S.]

If it's the truck and it's not fuses I would follow the harness to where it T's into the vehicles wiring system, my guess would be near drivers side tail light assembly, but that's just a guess, it should be a plug an play harness that you can unplug, clean if filled with dirt or slight corrosion or just replace harness. Good luck, let us know what you find.

[amateurfishing]

Totally confused, all of truck lights working properly, bought tester and still have exact same results and with all truck lights working it means issue must be in towing harness, right? Looked under there & seems tight & solid, nothing is loose or worn looking at towing receptacle or going into it.

[Jeremy airjer W]

Fuse 13 in the battery junction block. It is the fuse for the trailer turn/stop relays. If that fuse is blown you will not get turn/stop on either side of the trailer. Only takes a couple minutes to check fuses.

[Jeremy airjer W]

In fact I show 5 trailer specific fuses for factory tow options on your vehicle.

[amateurfishing]

YeA, 5 fuses, where to begin???

Thanks for all the help, think I got it now, da fuse!!!