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]

You are forgiven my son! Lets hope its that simple!!

[Nick Kuhn]

Just redid the wiring on my brothers Trail Blazer. First time we tried to tow a trailer the following occurred: lights were very dim unless the tail gate was left down. When signaling a turn, the entire dash would flash with the turn signal if a trailer was attached. The you left your headlights on tone was always on with the trailer connected.

I didn't even know where to start so I just about replaced everything on his vehicle (the guy who previously owned it evidently tried to extend the wiring to connect to a long tongue trailer, and his wiring was nightmare caliber). The immediate problems were nicks in the wire exposing copper and corroding. Where he spliced he didn't use any solder. Copious amounts of electrical tape seemed to be the solution to everything. Connecting the ground to something was apparently optional in his mind, I followed the wire to where I thought it was attached only to find it floating around.

So that's my trailer wiring nightmare. Next time you have a problem hope it's a little simpler than my brothers. Fixed it up well enough to determine the running light filament was broken on one of the bulbs.

[pinkfloyd4ever]

You are forgiven my son! Lets hope its that simple!!

its not.

The fuse never blows unless i am plugged into the trailer. I plan this wknd to start fresh and start at page 1. Any other suggestions for what makes the tail light fuse blow? Still may get 10 miles or 10 feet

Thanks

[Nick Kuhn]

The only way a fuse would blow are the following:

1) you are using the wrong size fuse.

or

2) somewhere along the connection there is a short to ground, which in a vehicles and trailers case is just about everywhere.

So your best bets are to make sure you don't have a colored wire to a white wire, or check the entire length of wire to make sure no copper is exposed.

[Jeremy airjer W]

3) To much load on the circuit! This could be added resistance (corrosion)on the truck or trailer side. Typically you would see dimmer lights but the resistance on the vehicle side could be low enough to not see a difference but just high enough that the extra load of the trailer is enough to blow the fuse.

In all likely hood is a fault on the trailer side (see number 2 above) but the possibilities are endless!!

[Slob_Samurai]

My running lights work, but my brake lights and blinkers dont work... Everything worked fine last year on my trailer.

Where should I start?

Would a bad fuse knock them all out?

A bad bulb knock them all them out?

My ground on the trailer is bolted to the trailer and covered with silicon, it that a proper way to protect it?

Thanks for any help...

[Nick Kuhn]

Running lights and brake lights are each a separate filament in the same bulb, it's possible one filament broke and the other didn't in a bulb. I'd start there.

[Jeremy airjer W]

Is it a factory trailer harness or an aftermarket?

Its pretty easy to check for power out of the truck plug as well. Turn on the four way flashers and make sure two pins are flashing. If they are its a problem with the trailer (burned out bulbs as mentioned above or broken wires or corroded/poor connections). If there not flashing than you can concentrate on the vehicle side (broken harness wires, Blown fuses, corroded/poor connections).

[pinkfloyd4ever]

3) To much load on the circuit! This could be added resistance (corrosion)on the truck or trailer side. Typically you would see dimmer lights but the resistance on the vehicle side could be low enough to not see a difference but just high enough that the extra load of the trailer is enough to blow the fuse.

In all likely hood is a fault on the trailer side (see number 2 above) but the possibilities are endless!!

meant to update on this- so I had bought some of those trailer clips to keep the wire flush to the trailer, found one that I put on backwards so the metal teeth were digging into the wires. Corrected that no problem since

[Smoker]

I had a problem with a fuse blowing all the time. It ended up being the bulb. The turning wire in the bulb broke and it was long enough to short out to the light part of the bulb. It had me puzzled for a long time and I decided to redo all the wiring and for some reason I thought I would check the bulb. Glad I did.

[Jeremy airjer W]

I had a bad bulb in an old plymouth one time that would back feed power and turn on the radio when you stepped on the brakes. That will throw you for a loop!

[Jae]

cant figure this one out i got brake lights and hazards but when i turn the left or right turn on the hazards coome on the trailer im baffled?!?! replaced my coroded 4 pin on the truck and new harnes on the trailer im so comfused

[Smoker]

One thing we all over look alot is the ground wire. If you don't have a good ground the lights will do some odd things. I have had a few times when I had to take the wire off and take a wire brush to where it fastens to and things work fine after that.

[jeffreyd]

had a firetruck with the engine in the back end. it was located in the "bad part of town" so it had an alarm system on it just like a car. One night the alarm goes off in the station on its own. traced that thing down for a week, found a bad winding in the ac compressor clutch that was sending power back thru the system. like to have stripped the whole harness out of the truck before finding it.

[walleyemaniac]

Does your vehicle have seperate bulbs for the brakes and turn signals? If so you need to purchase a converter to go from a four light system to a two light system. You can get these at just about any parts store relatively inexpensive. Sounds like your back feeding one curcuit into the other thru the trailer lights. Hope this helps.

[Jeremy airjer W]

If all you did was replace the harness than you need to double check your work. What happens when you turn on the brake lights and the tail lights at the same time?

[JacobMHD]

I have an '02 Explorer that is causing me some problems. The trailer light hook up has only worked once since I purchased it and I think the weather had something to do with it. The roads were really wet and it was raining. Any other time I have hooked up a trailer there is nothing there. I think I have checked the correct fuses, according to the owners manual anyway. They were fine. I did check the plug for power at all four terminals with the lights on and the flashers going. No power to the plug. I followed the wires to a connector tucked up in the back bumper on the passenger side and checked the connector terminals there as well. No power to these either. Because the lights worked during an early spring rain storm I am thinking that it may be a ground?? Any suggestions are greatly appreciated. I can take pictures if that makes it easier for the experts.

[Jeremy airjer W]

Try running a ground wire directly from the ground of the battery and check the outputs again for power.

[wx_nut]

Ok, hopefully this is the right place to put my question.

Facts:

2007 Dodge Durango 4.7

Purchased from dealer with Tow Package (per the dealer's sheet) in December 2009.

Brought Durango home, hooked up my little Hitch-Cover light. It did not work. I called the dealer, and they told me to bring it in. I brought it in, and they told me it was a 'non-dealer part' and it would cost $95/hr to diagnose and fix. I called my sales guy, and he told me that was [PoorWordUsage], and he would figure it out with sales. I am still trying to get it figured out. The service dept. at the dealer suggested I check the ground, which I did. The ground was a little on the rusty side, so I did replace the ground, thinking that was the issue. After I replaced the ground, it still did not work. I checked with a voltmeter, and I am not getting any power to the pins.

After this, I checked the lights themselves. I noticed when I turned on my lights (from the Durango) a message quickly popped up on the console that says "NO FUSE". Thinking this was an issue, I opened up the fuse box in in the engine. There are two of them, and I opened up the one closest to the firewall. In there, I saw a spot missing for a 20A TOW LIGHT spot. I grabbed a 20A fuse and put it in there. I went back and tried the lights again - no go. I then opened up the second fusebox (the one just to the left of the other one) and checked in there. There are two more areas for fuses - one says "TOW RT TRAILER" and the other says "TOW LT TRAILER". Those appear to be more along the lines of circut breakers, and not fuses. Beyond this point, I am stumped.

Any suggestions anyone? If you need photos, let me know, and I'll add them in. Mods, if this is the wrong forum, please move it and accept my apologies.

[Jeremy airjer W]

My guess is there is an issue with the Integrated Power Module (formerly known as the fuse block) you're only option is to see the dealer and have them "reset" the module. Apparently if these se a short in the circuit they will shut down the circuit in order to protect the electronics. You know because fuses have worked pretty well until know?!?!?! Any ways if it can not be reset than it will have to be diagnosed.

[wx_nut]

airjer: Thank you for the information. I went to the dealer, and they had me go to a specialty shop and they took care of it. It turns out there was an issue (as suspected) with the IPM, and it was replaced, along with some of the trailer wiring, as they felt it was installed incorrectly by the factory. There was also a converter of some sort that was also failing, and that was also replaced.

All of this was covered under warranty, so I did not pay anything. Total charge if I would have to pay was $532.46. Thank goodness for the warranty!

Thanks again for your help.

[fisher_king2004]

I am about to start rewiring my trailer and am looking for some tips/advice. The trailer has a short in the running lights currently. It's an old trailer and I think replacing the wiring makes a lot of sense before the next short occurs. I've got all the wire/joints/etc. ready to go with some new LED tail lights. I have looked at the trailer and getting the wire through the first section of the tounge should be easy and I've got a good spot to attach the ground wire near the hitch. From there all three wires (yellow, brown, and green) go through a small hole in the frame on the right side. For the side running lights the brown wire comes out of a hole on each side and the running and turn signal wires exit the back on the right side and are an easy feed across to the left side for the rest of the wiring.

What should I try and do to get the wiring to the side lights? I can see using the existing harness to pull the new one to the back of the trailer but I'm a bit stumped on how to get the side running lights wired.

Suggestions? It's a shorelander trailer from around 1985.

[walleyemaniac]

2 ideas

#1 drill the hole out a little bigger so you can hook the wires out with a peice of baling wire or coat hanger and use a rubber wire grommet to seal the hole back up.

#2 this is what i did and a little more of a pain but I whanted to keep it factoy looking. I laid the trailer wires out along the outside of the frame, seperated the brown wire from the other two at every clearance light hole on the side. Then I took baling wire and ran it from the side hole up to the front of the frame. I put a hook in the baling wire and put it around the brown wire where I seperated it and as I pulled the new wires back thru the frame I pulled the baling wire out that hole and was able to pull the brown wire out to get at it.

When I connected the light wire I soddered the connectoin and put two peices of heat shrink tube over it. I was able to push the connection back thru the hole without wrecking the wires.

It was kind of a pain but it worked alright on my shorelander.

[Jeremy airjer W]

You can also run wire back up to the side makers from your splice in the rear. Using mechanics wire to fish it back to the hole. A little dielectric grease on all the connections will help keep them good to go for a long time.

[Lammboy]

Guys I could use some advice...

Recently on a trip my trailer plug from the trailer detatched from my truck and as it got dragged on the highway it eventually was hanging by one wire. It's a standard 4 flat prong plug. I bought another 4prong plug from autozone and spliced it back together with waterproof connectors on the cables. The lights work fine when I brake and use my signals but my parking lights now no longer work. I noticed that the plug I bought has 4 wires..your typical green, yellow, brown and white. However my trailer wiring also has 4 wires but there are 2 wires in each of the cables. Like a brown and green, yellow and green, green and green and then the one white for grounding. is there a special way I need to wire this so the parking lights work?