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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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Hi guys,

I haven't checked everything out yet as it got too dark and late to work on it some more but thought I would put this out there and see if someone can come up with some advice before I get home from work tomorrow. I was hooking up the trailer tonight to test it for this weekend trip and the trailer lights would not work at all at first. Changed bulbs and the lights came on but no brakes and no turn signals on the trailer. Truck checks out fine (fuses, harness, etc). When I have the trailer hooked up to the Trailblazer the flashers on the vehicle freeze. Right signal does the same but the left signal works perfectly on the vehicle but again does not get back to the trailer. There seems to be power at the harness but I did not check out the trailer fixtures yet. Is this one of those weird grounding issues?

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Kind of sounds that way! The first thing I would do is check all the terminals on the vehicle with the trailer unplugged. Turn on the lights and four way flashers (this will check turn signal/brake function on both wires by yourself assuming you have a flat four connector on the trailer). The pin that is exposed is the ground. You can hook the test light up to this to start. Then check the other three connectors. One will have constant power and the other two will cause the test light to flash. If everything checks out on the vehicle side than you know for sure that its a problem on the trailer end.

Make sure you double check the bulbs that you installed to make sure they are installed correctly and that the buttons on the bottom are making good contact with the tabs in the socket.

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  marine_man said:
What colors are the wires?

You probably have two brown wires (one for each side of the trailer) that need to be spliced together and then into the 4 pin connector.

marine_man

Sorry, forgot I even asked. The colors are:

White

Br/Yellow

Yellow

Br/Green

Green

My wiring 4 pin connector has Yellow, green, black, white.

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Wallyeye & love2fish, did you ever figure out why your trailer lights were giving you problems?

I have had my trailer lights blow a fuse on damp mornings, but this only happens occasionally. If I connect the trailer lights to the tow vehicle and turn on the lights, the fuse wiil blow. If I connect the trailer lights to the tow vehicle and do not turn on the lights, the fuse will not blow and I'm still able to use the brakes, flashers and turn signals.

Never Enough Time.....

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I just got a used snomobile trailer and the lights would work then they wouldnt. I figured bad ground. Checked all wiring and it looked good but cleaned it up and re connected some areas. Nothing.

Well I wound up having to buy a new coupler for the trailer and 2" ball for my truck. Problem solved.

The coupler was so rusted it couldnt be cleaned and I just got a new ball because the old one was rusting to the point I had to cut it off. Once I got new metal to metal everything was good.

I hate trailer lights no matter if they are supposed to be waterproof or not.

I have 1 side light and they are the round ones with rubber around them. FLOW Trailer. How do you change the bulbs out? I tried twisting but nothing. I didnt want to break the lense.

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I don't think the bulbs themselves are serviceable on those. They are replaced as an assembly. The ones we get from Northern come with a new rubber seal thingy!

Also don't be afraid to put a little di-electric grease on that new connector! You will be amazed at how much longer and more reliable it will be.

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  Northlander said:
I just got a used snomobile trailer and the lights would work then they wouldnt. I figured bad ground. Checked all wiring and it looked good but cleaned it up and re connected some areas. Nothing.

Well I wound up having to buy a new coupler for the trailer and 2" ball for my truck. Problem solved.

The coupler was so rusted it couldnt be cleaned and I just got a new ball because the old one was rusting to the point I had to cut it off. Once I got new metal to metal everything was good.

I have 1 side light and they are the round ones with rubber around them. FLOW Trailer. How do you change the bulbs out? I tried twisting but nothing. I didnt want to break the lense.

Northlander, if replacing the coupler and ball made the lights work, it definitely is a ground problem. They may work now, but you havent corrected the actual problem. You still have a bad ground on either the truck or the trailer. A properly wired system should/will work regardless of it being hooked to the vehicle or not. Many/most times the ground wire on aluminum trailers is merely rived to the frame and becomes corroded. Simply drilling out the rivet, cleaning the surface with steel wool and reriveting will take care of it.

Also, The side lights are indeed replaced as a whole unit, not just a bulb.

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This is great info - thanks for sharing it! Not sure how to proceed with the problem my trailer has at the moment though.

First - a year ago one of the tires blew all the tread and trashed one of the fenders including one running light. That has not been fixed.

I always (except this one time of course) unplug the lights before backing into the water and have not had any problems but forgot to unplug them last weekend and now I have no running lights. Directionals and brake-lights work just fine - just the running lights are out. Have all the bulbs burned? a fuse blown? How should I proceed with figuring out how to make the repairs? All the lights worked on the way to the lake but not after being in the water.

Trailer is made by Eagle if that makes any difference. Plug is flat 4 that I use an adapter for to plug into the round truck receptacle.

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Make sure you have power at the plug at the vehicle. Check the bulbs and check for power at the sockets while you are looking at the bulbs. I would bet you blew a fuse, but it could be entirely possible for the bulbs to burn out. You are definitely missing power IMO!

Whats the year make and model of the tow vehicle?

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Thanks a lot guys. Great info. I went to Northern and got new light assemblies. I was able to just get the lens/light and slip that in the existing rubber gasket on the trailer. Old connection was so bad I wound up having to clip the old off and put a new on.

Looking fine now.

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  airjer said:
Check the trailer park fuse in the under hood fuse box on the drivers side. That would be a good start.

You nailed it dude! 15 amp fuse in fuse box right next to battery in engine compartment! Running lights are back in play! Thanks!

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My trailer lights have also given me trouble lately so I think I'm going to just bring it in to let a professional do it. I live in Roseville and am just wondering if anybody knows a place nearby that can do it for a reasonable price. Thanks!

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