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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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whistle

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Subtle...I like it!

Airjer, do you install electric brakes and wire for them as well? I may need to do this for my wheel house, I am weighing it within the week. The truck is wired, I would just need the electric brake hubs installed and the wiring done to the 7 pin.

Glockwinger

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I don't have any experience with electric brakes. I know its hard to believe but it's true.

Ok, I am officially crushed! cry How can our resident expert not be the expert? grin

I think I speak for many on this board that are VERY thankful for the knowledge you do have...and will let it slide this time whistle

-G

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So I just bought a Curt trailer hitch and TowReady T-one plug and play trailer wiring kit for my 01 Tundra. Everything installed very easily. Here's the problem. The Shorelander trailer has 5 flat end harness (the trailer has electric brakes). The TowReady harness harness is only 4 pole. Does anyone know which wire I tap into for the brake assist?

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Help, where would I look if when I hook up my boat trailer and with the lights off, both turn signals work. When I turn my headlights on and I do the left blinker it works, but when I use the right blinker both of them come on like my hazards are on? any help would be greatly apreciative!!!

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Sounds like a ground problem... is the white wire on the trailer side of the plug-in grounded out? How about on the tow vehicle?

It sounds like the ground is being made through the trailer ball, and when you slow down there's better contact between the coupler and trailer ball.

marine_man

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Does this have factory trailer wiring or after market?

Is it both blinkers that don't work?

What happens when you turn on the four ways?

Have you checked to see if there is power out of the plug on the truck side with the turn signals on?

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I wish to add my recent experience to this forum. My son and I are rebuilding a trailer that hasn't been used in years to get him started in fishing more independently. Installed new lights, and a harness, as these were many years old. We grounded each light to the frame, and grounded the WHITE wire from the trailer harness to the frame also. (Drilled and tapped a hole for each.) When we plugged in the trailer to my truck, the blinkers worked and the brake lights worked fine as long as the main running lights were turned off. When we turned on the running lights, the lights worked fine on the trailer as did the blinkers.

HOWEVER, when we pressed the brake pedal, both lights on the trailer went out. This lead to many laughs and head scratching, until I broke out the multimeter and checked things out. At the plug, we got 12v, but at the lights at the rear of the trailer we only got 6v! Further discovery revealed this trailer had a pivot point in the center of the trailer that had bad continuity in the frame. I installed a bonding jumper around the pivot point and now all lights work great!

Lesson learned. Check for a good ground!

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Thanks for sharing! That definitely fits in the "if its funky its a ground issue" category.

I had a trailer in the other day that none of the lights worked. The truck side was fine, the ground on the trailer side was clean and well bolted into the trailer, and the grounds for the lights where clean and secure as well.

As it turned out the trailer was essential two pieces that where bolted together. The spot where the two came together had rusted up pretty good. since the trailer plug ground was attached to one piece of the trailer and the light grounds where attached to the other there was no way they could find each other.

I ran a new ground wire from the plug to the lights and all was good.

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Ok, so I am going to try and figure this myself as I type it.

Boat trailer lights worked fine on my previous vehicle, this year on different vehicle 92 Chevy 1500, the fuse keeps blowing. May get 10 miles, may get 1 block, but the fuse will blow.

I want to assume then that the trailer is ok since the change has been the vehicle right? If I keep blowing a fuse I can only guess it is a grounding problem coming from the truck? Flat 4 connection.

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If there was added resistance in the ground circuit the lights on the trailer would be dim. You don't see bad grounds cause fuses to blow. The typical cause is added resistance on the power side or a direct short.

The first thing I would look at is how the trailer wiring on the vehicle is setup. If its "tapped" in a thorough inspection of the splices is a good start. If there is a plug and play trailer harness unplug it and take a good look at the connections. It is not uncommon for these to go bad and cause all kinds of problems. If everything on the vehicle side looks good than thoroughly check the trailer wiring. I assume its the running lights that blow so you can concentrate on the brown wire. There are plenty of opportunities for this wire to short out on a trailer!

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When the trailer lights are on they are on, no dimming out or anything. After looking over page 1, I see that I sinned. I used the blue splice connectors to make my connection. Looks like a good place to start repairing.

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