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Trailer Wiring 101


Jeremy airjer W

Question

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.

testlight.jpg

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.

corrodedplug.jpg

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.

7pin.jpg

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.

wirenut.jpgscotchlock.jpg

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.

bulbterminals.jpg

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.

taillight.jpg

(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.

trailergrounds.jpgmainground.jpg

(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.

tailightmounts_grounds.jpg

(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!

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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?

mad

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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.

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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?

mad

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.

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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?)!

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

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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.

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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

full-35223-48288-image.jpg

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

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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.

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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

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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.

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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.

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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.

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

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YeA, 5 fuses, where to begin???

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

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