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!

[sgalbo]

Hello, I am having a trailer nightmare! I put a new set of lights on my trailer but they will not light up at all. So i purchased a tester that you plug into the 4 pin on the truck ( Nissan Pathfinder 2006) and all test lights work exactly as they should. Now when I pull the tester out a little so that I can get a test light on the actual connectors they mirror what the tester is showing, then I unplug that and plug back in the trailer, and do the same thing but the tester is showing power from all connectors. Then if I hook my test light up to the ground and touch my test light to any where on the trailer it lights up, which means it is detecting power. I have tried grounding the trailer to the truck by jumper cables etc.. but nothing changes, and I am at a loss as to how to fix this and advice would be appreciated!

[Jeremy airjer W]

Run a ground wire from the trailer plug all the way back to one of the trailer light grounding studs.

[lilwalter]

My problem-no left turn or left brake light on trailer. Switch vehicles and trailer works fine. Try truck on different trailer same issues. Voltage meter says no power at plug. Truck side wires are all wrapped! Where to start? Thanks.

[SpecialK]

My problem-no left turn or left brake light on trailer. Switch vehicles and trailer works fine. Try truck on different trailer same issues. Voltage meter says no power at plug. Truck side wires are all wrapped! Where to start? Thanks.

Depends on the vehicle but I believe there is a fuse for each side of the vehicle lights so I would start by looking for a blown fuse.

[Jim Almquist]

When checking for a blown fuse make sure to look under the dash and under the front hood.

[Jeremy airjer W]

My problem-no left turn or left brake light on trailer. Switch vehicles and trailer works fine. Try truck on different trailer same issues. Voltage meter says no power at plug. Truck side wires are all wrapped! Where to start? Thanks.

Year make and model would help a lot to direct you to the exact location of what is probably a blown fuse.

[lilwalter]

Its an 05 nissan titan. I'll check the fuses to start with. thanks

[icefisker]

Not sure if this has been covered yet... My boat trailer running lights work, but not the brakes or blinkers... I have recently towed a different trailer and all of the lights worked on that trailer. What could be the issue?

[LightningBG]

Not sure if this has been covered yet... My boat trailer running lights work, but not the brakes or blinkers... I have recently towed a different trailer and all of the lights worked on that trailer. What could be the issue?

Could be many things.

Bad ground, bad wires, bad plug, blown fuse.

[icefisker]

Change lightbulbs... problem solved!

[Craigums]

Not sure if this has been covered yet... My boat trailer running lights work, but not the brakes or blinkers...

[SpecialK]

Craigums, if they are standard trailer lights you should be able to find them at any auto parts store.

Before you buy new bulbs tho take them out and look at the filaments, if they look like they are intact you can jiggle them (do not shake them hard you can break the filaments) and see if they do not make connection between the two probes. If the filaments are good putting new bulbs in may not fix your problem. Put them back in and see if they work as there may have been some corrosion in the socket that broke free when you removed the bulb. Do not be alarmed if there is a grease like substance on the socket or metal end of the bulb. That is dielectric grease and helps keep corrosion out of the socket.

If they still don't work, check to make sure you have power going to the socket. If there is no power visually inspect the wires (with the power off) to ensure none of the wires are broken in the light itself and then trouble shoot backwards to the truck / tow vehicle. I would check at every wire connection (if someone replaced the lights there will be a splice where they wired in the light).

If you have power to the socket and the bulbs are good, you more than likely have a bad ground (white wire I believe).

Hope this helps.

[Whoaru99]

Quite often the bulbs are the ubiquitous 1157.

If you don't have a multimeter might be a good time to pick up one of those too. A decently functional example can be had for $15, give or take, and up.

[Craigums]

Actually took another look at it....The Blinkers DO WORK, The Tail lights on the trailer and brake lights are not working though.

It's a RangerTrail Trailer, just picked the boat up this spring.

[icefisker]

My bulbs were the 1157...I took the broken one into the local store and the clerk said, "we go through a lot of those in the summer".

[Jeremy airjer W]

Actually took another look at it....The Blinkers DO WORK, The Tail lights on the trailer and brake lights are not working though.

It's a RangerTrail Trailer, just picked the boat up this spring.

If the blinkers are working than we know the wiring and the bulbs are good since the brake uses the same wire and bulb filament. I would have to believe the issue is on the vehicle side (blown trailer brake fuse?).

The tail lights and the side marker lights (the yellow or orange lights) are powered by the brown wire. I would check for power out of the vehicle first. If there is power than I would assume there is a connection issue at the plug or a broke wire on the trailer.

[Craigums]

Ok dumb questions here:

1. If its a blown trailer brake fuse on the vehicle. Do you know were the fuses are located on a 2004 F-150. and Will I visually be able to tell if the fuse is blown? I imagine I can buy fuses at any hardware/automotive store....

2. You mention that the tail lights and side marker lights run on the same brown wire.... are you refering to the running lights on the side of the trailer? The running lights all turn on when I turn my headlights on.

[Tom7227]

Fuses can be located and the correct one identified by checking the owners manual. Most likely the fuse panel is on the underside of the dash left of the steering column. When you open it you will see a number of fuses, 15 or so. That's why you have to ID the location of the one you want to look at via the owners manual. You probably will need a needle nose pliers to pull the correct one. It will have two prongs and a plastic head of a certain color. Inside the plastic as you look at it from the side you will see a piece of metal that is shaped sort of like S If that is broken then the fuse is kaput. Buy some fuses that are the same color/number/size and replace the one that was dead. If things work then you're OK, if not you have more work to do.

Advice again - spend the money to buy a multi-meter. It will allow you to test for electricity as well as continuity - whether the wires are all intact. There most likely will be a short pamphlet with it explaining how it works. Spend some time learning how to run it and it will make things a lot easier.

You could have corroded connections, contacts in the bulb sockets, at the plug at the rear of your truck or on the end of the trailer. All sorts of things can booger up the wiring. It just takes some time and patience to track it all down.

Since you're going to the store to get a fuse get a packet of each and store them in the vehicle. Get the little plastic tool that helps pull them out. Get a tube of dielectric grease and use it according to instructions. Get some shrink tubing and connectors and when you make a connection use the tubing to seal it up. If you drop about $40-50 you can have a set of supplies that will last you for a long time and the ability to sort out those light problems yourself and without too much hassle.

[Craigums]

Thanks for dumbing it down Tom. I'm gonna mess with it after work. I'll let you guys know what I find

[KJMinion]

Craigums, I agree with what Tom said and he has some great advice. I had a very similar problem with the left blinker / brake light on my trailer when connected to my 2007 Ford F-150 and a new fuse resolved the issue. Not sure if it is the same on a 2004 Ford but on my 2007 Ford F-150, The fuse box where the trailer fuse was located is under the dash on the passenger side of the vehicle. (On the side where the passengers right foot would be). Often times there are spare fuses put in there but if you use them, I would suggest to pick up some extras to replace the spares.

After replacing the fuse, I got it working again but I also went through all of my wire plug-in connections on the trailer and cleaned them up and put dielectric grease on them and I haven't had any problems since then.

[Craigums]

Thanks KJMinion. Im really hoping its just a fuse. Looks like the RangerTrail Trailer lights have the bulbs enclosed and you need to buy a whole light kit if you need to bulbs replaced

[SpecialK]

Craigums sending a PM

[Tom7227]

I have an 02 F150 and that blasted plug that hangs below the rear bumper gets cruded up in no time flat. Of course it doesn't help that I about ripped it off on a cross country venture long ago. That's where I would start if I had things screwed up.

[Craigums]

Ok Guys I might be onto something.... Checked the bulbs and the filaments look good so I moved onto the fuses and I didn't see any fuses that looked burned up. On the next page in my owners manual they discussed "relays" Here is what is shows:

It says "R201" has something to do with the tow vehicle

Here is what mine looks like:

You can clearly see that "R201" is missing...However I tried to take "R202" and put it in the "R201" spot and it doesnt fit snug. It's like the metal in the outlet that would contact the spades on the relay are missing... IDK if im even on the right track.

I checked 3 fuses described:

1) Trailer tow back-up lamps relay (PCB1, Trailer tow parklamp relay (R201)

2) Trailer tow right turn/stop lamp

3) Trailer tow left turn/stop lamp.

Again my issue is the Brake lights (when I hit my brakes)don't work, and the back (red) trailer lights do come on when I turn my trucks headlights on.

[Whoaru99]

If the terminals are not in the fuse/relay block then those things probably don't apply to your vehicle.

The fuse and relay callouts in Owner's Manual typically cover all options, even those that any given vehicle may not be equipped with. I think what you're reading is for a factory tow option of some sort that your vehicle isn't equipped with.

Stop screwin' around. Go spend that $15-$20 to get a multimeter to start checking things rather than poke and hope...