So, some time back I had a u-haul trailer fry my wiring under the dash.

I decided because of this that I am going to rewire the rear 4-flat plug to trigger a pack of relays instead, powering the trailers lights/etc from my rear battery.

So I dug into the wires and instead of finding only four wires I have six, and they don't match any of the diagrams/books I have seen.

I'm thinking I will just get a 7-round and use it instead, since I can supply the aux power from the rear battery.

Question is: can anyone tell me what wire is what? I have:

Small white, brown, yellow, dk green, AND i found an additional LARGE white wire and small lt green wire. Does anyone know what these last two wires which where previously capped off are?

And... Is the large or small white wire really the ground? the 4-flat is connected to the small.

U-haul blamed my truck so I'm paranoid that there is a hack job somewhere.

Thoughts and answers welcome!

 

Could they be for electric brakes?? I would think you you would have a control box somewhere for that tho.

 

It is apparently the standard "U89 : WIRING HARNESS, CAR TRAILER" pigtail. There are no trailer brakes or anything.

What I have figured out ( I think, I haven't tested any of this yet.)

My year manual (found in a JY!) says brown, yellow, green and supposedly the large white are the standards for the 4-flat type of connector.

A random 1996 diagram I found, ( and the manual, but manual claims this is only in the 8 wire harness), says lt green is for backup lights.

Now here is what concerns me: The 4-flat connectors ground HAS been connected to the small white wire!

Now what I know of circuit design says that it is always better to have a larger gauge ground if several small gauge wires ground through it. That tells me the large white should be ground as the manual says.

However, the 1996 manual also states that the small white wire is "Auxiliary stoplight".

So what does the "Auxiliary stoplight" wire do? and how the heck have I pulled trailers like this? Is this why I fried my wires? my third brake light/high mount stop light doesn't work, is this related? what kind of damage could have been done?

Any info/diagrams on this "light gauge" white wire would be really helpfull along with any insight anyone has!

 

So... Yep... The big white wire is ground. The small white wire is attached to the CHMSL circuit.

This explains my u-haul debacle, my wiring was essentially short circuiting through the trailer and back into the truck through the hitch. I assume this; as it would explain why there was enough resistance to keep the fuse from blowing, yet let the wires get hot enough to melt. Not sure exactly, it was the black and orange wires at the brake switch that got fried. But it was a small white wire that turns hot (+) when brakes are applied (like the white wire at the CHMSL connector) that was terminated in a 4-flat connector as ground! Yikes.

 

I have a 99 blazer and i wired the plug up corectly but when i hit the brakes the passenger side brake light on the trailer gose out along with that side parking lights bulb is good please help

 

That fat white wire goes to a ground bolt by the fuel filler neck on the frame. Accessable by reaching in the drivers side rear wheel well after pulling that rubber shield out of the way. Reach back behind the fuel filler neck and you'll feel it. 10mm bolt, use a small extension and a ratchet. I think the other wire on that ground bolt goes to the fuel pump.

 

I've been reading up on this a bit myself... I have a '99 2 dr sport, the RPO codes say that I have a "U89WIRING HARNESS,5-WIRE TRAILER 5 WIRE TRLR HARN(U89)" 5 wire... but I just looked under there and I have 7 wires; big white one (I assume ground), a brown one (should be running lights), a yellow one, a dark green one (yellow/dark green should be turn/brake), and a blue wire and a light green wire....
from what I have been reading there should only be one green one and there should be a red one for backup lights, which I am assuming is the light green one because I also read that the blue is the standard colour for a brake controller.. they started putting the wiring for brake controllers into vehicles in the mid 90's apparently
I have found this page to be somewhat useful
Trailer Wiring & Electrical - Installation Guide | Hitch Info
but as I say GM hasn't seemed to use the standard colour configuration,so I am currently trying to figure out if anyone knows for sure....