# Truck to trailer wiring question



## katonca (Aug 10, 2004)

Hi folks, here's my problem: I have a 97 GMC Sierra 1/2 ton truck and need a trailer wire harness. The truck has 5 wires as follows (bare wires no harness): 1 light green, 1 dark green, 1 yellow, 1 brown/black, 1 *thick* white wire. It needs to connect to a trailer with 4 wires as follows (in a harness connecter): 1 brown/black, 1 white, 1 green and 1 yellow. How would these wires match up and where would the extra wire from the truck go? My guess from what I read is, that the thick white wire from the truck would ground to the truck...

Anyone have ideas? THANKS :up:


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## hewee (Oct 26, 2001)

Now days a lot of thing like that are pre-made for cars and trucks and you should even have a place to plugin somewhere. Ask your GM Dealer. 
GM Parts will be the place to get free answers. 

I know the one I added on the 97 Ford Ranger had a place in the back lights and I just unplug and added the trailer wire harness to it and then plug the other light on the trailer wire harness. No cutting into anything like older cars and trucks.

Ask U-haul too.


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## katonca (Aug 10, 2004)

Thanks hewee, never thought about asking GM parts I'll check them out.


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## ekim68 (Jul 8, 2003)

They have adapters that convert many different kinds of wiring at places like U-Haul, too...

(Whoops, Harry already said that...)


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## katonca (Aug 10, 2004)

I really dislike electrical stuff (cars and home). I've been known to mess up a car's electrical component or two.


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## K7M (Feb 27, 2000)

I know you marked it solved but for reference, the colors are as follows. 
Light green is back up, 
Dark green is right turn/brake
Yellow is left turn/brake
Brown is running lights
White is ground.
I would use the harness on the truck as these wire are fused seperate from the regular tail/signal light circuit on the truck.


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## katonca (Aug 10, 2004)

Thanks K7M that's the ticket I was looking for. I'll be buying a harness for the truck side. Right now they're just wire ends because they were never hooked-up before. Question, the gauge of the white wire (truck side) is pretty beefy, is that a concern when hooking it to a small(er) gauge wire of a harness?

Thanks


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## K7M (Feb 27, 2000)

The ground wire is that large because if you are wiring it for a 7 way connector you need the larger guage to carry the current for electric brakes and the 12 volt charge line. If you are just hooking up a 4 way flat for a utility trailer, the smaller size white will not be a problem.


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## hewee (Oct 26, 2001)

You're welcome and I see K7M is helping on the rest.


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## katonca (Aug 10, 2004)

I wanted to let you all know that I hooked it up yesterday and works like a champ. Thanks again folks :up:


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## hewee (Oct 26, 2001)

Great to hear.


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## Drabdr (Nov 26, 2007)

Opened per OP request.


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## katonca (Aug 10, 2004)

Thanks Drabdr for re-opening this.

My trailer lights are acting up. We hooked the truck to the trailer for a night pull. After setting for a while with the truck running we attempted to drive away & noticed the truck had no dash boards lights I checked the tail lights and they were off as well as the trailer had no running lights. Both the truck and trailer had turn signals, brake lights, and hazards.

Fast forward to today. I found a 20amp fuse had blown in the truck (can't remember which one though). I replaced the fuse and checked the truck side harness connections splice(s) to the truck itself and they were fine - no loose wires, nothing shorting against each other.

I hooked the truck to the trailer and still have no running lights on the trailer. Something to note - While the headlights are on (truck running) if I plug the trailer into the truck the truck tail light go dim and the harness gets VERY warm.

I didn't blow a fuse at this time because the truck has dashboard & tail lights. This is probably due to the fact I didn't leave the headlights on after I realized the trailer didn't have tail lights

Friday was the first time the trailer was used at night with running light turned on.

I have it connected up the same way as above.

Any thoughts on this? 

I'm going to go check on a few things....I'll be back

Thanks


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## Drabdr (Nov 26, 2007)

Check the ground. Make sure it is clean and well made to the chassis. Follow it and make sure it hasn't been damaged.


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## valis (Sep 24, 2004)

Big D! 

First off, let me just state that I know nothing about electricity except it works by magic, so I'm not going to even answer here.....just wanted to stop in and say howdy, and wish you a happy 4th.


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## katonca (Aug 10, 2004)

Thanks Drabdr I'll give that a look. Question concerning the ground. The ground wire coming from the truck (pig tail from the wire harness original to the truck) disappears into the rest of the wire harness of the truck and goes who knows where. Could I ground the trailer to the truck through the wire harness from the trailer itself? Right now, the two wire harnesses connect and the ground disappears into the rest of the truck's wiring.


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## katonca (Aug 10, 2004)

Hi V :up:

I'm not much of an electrical fan either. Anything I can't see that could kill me, I try and stay away from. This is my oldest son's trailer so the old man is trying his best 

Have a safe 4th my friend...shoot off a bottle rocket for me.


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## Drabdr (Nov 26, 2007)

Oh, besides the ground, do you see any damaged wires? Is it reasonably possible that there is some bare wire getting up against the trailer chassis?


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## Drabdr (Nov 26, 2007)

katonca said:


> Thanks Drabdr I'll give that a look. Question concerning the ground. The ground wire coming from the truck (pig tail from the wire harness original to the truck) disappears into the rest of the wire harness of the truck and goes who knows where. Could I ground the trailer to the truck through the wire harness from the trailer itself? Right now, the two wire harnesses connect and the ground disappears into the rest of the truck's wiring.


I would focus mainly on where the ground wire connects to the trailer. Obviously it goes into the "electrical center" of the lights and all. Somewhere the green ground is connected to the truck/trailer/ etc. ; I would guess using a round wire connector/ terminal underneath a screw/bolt. See if you can see anything.


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## katonca (Aug 10, 2004)

I checked for loose wires, bare and nothing found. I believe the trouble may be at the ground connection to the trailer as you say. The area has been painted and I'm not sure if the contact is on bare metal or not. I have been trying to remove the screw from the trailer ground but it's a double square 8 pt screw head and I'm fresh out of those  . 

I'm going to try and remove the screw with a #2 square tomorrow and see if the ground was attached to a painted surface or not. Thanks for the help and I'll comment tomorrow as to what I've found.


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## katonca (Aug 10, 2004)

The trailer ground was not making good contact with the trailer (at all) so I removed the paint and connected it to the bare metal. I ran a separate ground wire to the truck body just to be safe. Still no good. One thing I failed to mention, when I turn on the headlights (of course, no running lights on trailer) the tail lights on the truck dim by about half the brightness. When I unplug the trailer harness from the truck the lights brighten...what does this imply?


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## Drabdr (Nov 26, 2007)

It sounds like a short to me. Generally fuses don't blow unless something goes wrong. Since this is a DC circuit, poor grounding can also increase issues.

Do you have a multimeter? With the trailer unplugged, can you verify the resistance from the trailer ground to each one of the trailer wires?


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## Drabdr (Nov 26, 2007)

Is the wire in the ground terminal (how it connects to the chassis) Tight and firm?


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## katonca (Aug 10, 2004)

Sorry for the delay in my response, I've been out for a while. The trailer ground is tight and clean now. I haven't tested with a multimeter all I have is a cheap "meter" that test for continuity. Would that show anything pertinent?

The ground on the truck is also clean and tight.


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## katonca (Aug 10, 2004)

I found that it may be necessary to install a converter for this truck wiring set-up. Seems they make a converter for older trucks to allow them to supply electricity to newer trailers with LED lights (my situation). I bought one today & will install tomorrow and let you know how it worked out.


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## Drabdr (Nov 26, 2007)

I did a poor job of following all the info; and forgot to clarify the question as to what changes had occurred with your rig.

Let me know about the converter.


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## katonca (Aug 10, 2004)

Sorry it's been so long to give an update. The converter didn't solve the problem. Still having the ground wire heat-up at the harness then blows the truck fuse. I'm thinking it's a short maybe caused by a wire touching metal in the roof area. We have 2 interior lights that run off of the truck to trailer wiring harness. There is continuity in all the other wires throughout the trailer except the interior lights. I haven't been able to check these light's connections yet because of a stripped 12 pt screw on the license plate light. I believe the license plate light is the last exterior light connection before it heads into the ceiling panel (could be this connection as well).


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## Drabdr (Nov 26, 2007)

I'm soo sorry I haven't responded. My apologies to you.

Question.... Does the "hot" wire to the truck heat up also?
The ground wire to the truck.... I assume it goes into a bundle and is difficult to track.

Ok.... I would disconnect the harness going to the truck.
On the trailer side, place your continuity meter across the hot and ground (on the trailer side; there is no voltage present).

The circuit should be open, or really really high ohms.

Next, I would start and pull the cabin bulb out. That should only give you the brake lights. See if it still heats up.
By process of elimination, you should be able to figure out which light is causing the issue.

Using a meter, check continuity for a short. You may have to 'jiggle' the wires to simulate travel.


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