1988 F250 7.5 460 4wd. The truck had a 7 pin connector on the back, hanging by broken wires. Trying to clean up and do a good job. It has the factory Ford tow wiring. Whoever was in there seemed to do fair... But it is old. The connector at the rear of the truck was cut off and spliced with heat shrink. The splices and gone to crap over time. It already has a brake unit installed by the steering wheel.

Cleaned up best I could. I used a T style connector and that got me stop/tail/turn signals. (without splicing) The wires off the Ford harness (by rear bumper) are hard to identify and very short a couple inches before they loop over the frame. Looks like I have most identified. (The lighting wires are trimmed and sealed) Left me with the brake wire... Brake light on brake controller is dim. Need to get an extra hand to see if that is working correctly. Does not seem to be getting power when brake or controller is applied.

Now for the last wire.... UGH... it had 4 nice heat shrink splices in the last foot or so. Every one snapped in half when bent and blew out blue corrosion. Leaving me with a 1" piece of wire that is dead. Guessing it should be the 12 volt feed?? Anyone working on these old trucks knows , the wiring colors look almost the same being bleached out from their age. Where does the 12 volt power feed get power??? Looks like a yellow?? wire. From what I can find... There are 2 fusible links on the driver side, under the hood. (not passenger side by battery) They are by the EEC test port, in a mess of wires. One fusible link looks good. The other is missing with a corroded short piece of wire spliced in with a few turn of electric tape. (looks like a midnight hack) No power to it. Is this the power that feeds the trailer connector? The wire disappears in the loom. Where does it get power?

A lot of the trailer wiring threads on here are pretty old, and missing pictures. Need to keep digging for a good wiring diagram.

 

Strange, almost every thread on this topic is missing the wiring pictures. Maybe too old or got a board wipe at some time. Most of them mention look at this picture, not even a link.

 

What is your goal? back to factory, or just simple 4 wire trailer plug, or custom 7 wire?
By using the T connector into your tail lights, your by-passing the factory setup for trailer lights. The factory trailer lights are fused separately from the the truck lights. So, depending on your trailer, you may be overloading the circuit. Also, you now have factory trailer harness with dead ends still in the truck.

Yes, Yellow is the BATT+, used to power/charge battery on the trailer.
Blue would be the electric brake actuator from the brake controller.
Black with pink should be reverse lamps.

With all the bad corrosion at back of truck, probably the relays and connection under hood are also bad. Make sure you good ground, a bad ground may be what you are seeing with the brake contoller.

Post a picture of your under the hood wiring, then we can see if its factory or someone elses custom job.

I've had two trucks with factory trailer/tow package, one did not have the relays or wiring to back of truck, the other did.
On the one without, I had to wire in the blue wire from the brake controller into the main harness.

 

Goal? 7 wire trailer plug. One that is reliable and without a lot of rewiring or a hack job. I am quite familiar with electrical systems, but not very familiar with how Ford ran it's factory tow package wiring.

By passing factory tow wiring tail lights with a T connector?? Ah, I thought they were tied together. Yes I do have the dead ends in the back of the truck. Sealed, but I have spliced nothing so far and it is a quick T unplug to go back.

There was a 7 wire plug at the back already. Was hanging by the wires and in very poor unreliable shape. Looking to do a good rehab.

 

Originally a couple years ago when I got the truck the plug worked but not reliable. Mostly worked the trailer lights but not very well. Wanting to upgrade to new plug.

It looks like the truck has factory wiring from the front to the rear. As far as I can tell, I don't have any extra relays or fuses. Not sure where or what they would look like. The only fuses are the ones in the cab fuse box below the steering column. The 2 fusible links I found are next to the 2 relays. EEC and fuel pump relays, by the fender.

Now that I did some digging.... I found an 8" or so coiled blue wire in that area. Under the master cylinder. The end is covered in heat shrink. Makes me wonder if the brake controller ever worked.

I hate to split the loom from front to rear just to see where the wires go. Not my favorite either to run extra wires front to rear to have them hang or get pinched.

It is a reasonably clean truck for its age. Most of my towing is done with my other truck. Just want to get this one in good order.

The ground in back looks pretty good so I did not mess with it.

 

Will get some pictures later. I could not get 12 volt to the aux or brake wires. That's why I had trouble tracing them out.

Thanks for your help.

 

So, that blue wire, sounds like the issue I had. Wiring exists, but not fully connected, go figure.
From the inside of the cab, where the brake controller is, blue wire goes to firewall large plug(interior). What I found, was that the blue wire ended there. I had to re-pin a new pin, and connect to large plug on firewall(engine side).
That may/may not save your blue wire issue.

Now, under the hood, I believe there are two(2) plugs maybe three(3), that are interconnected, and wires run the length of truck. One set is for lights/turn/brake, and the other set is for batt+ auxiliary, electronic brake and reverse lights.

I think I have pictures on here somewhere....

 

This may or may not help you:
https://www.ford-trucks.com/forums/1...l#post19820316

Image shows the blue wire I had to add to the existing harness that came "factory tow package"


Blue wire I added from trailer brake in cab.

 

Looks like I am off base... I was going by memory.... The wire is brown with blue heat shrink.... It this even has anything to do with the rear trailer plug. This is under hood picture... Has a combination of my problems.

The coiled brown wire is at the top. The only relays I find are the ecm and fuel pump. Toward the lower right is the 2 fusible links... one is still there... The other (yellow) has a hacked wire into it replacing the link. May have nothing to do with the trailer plug. Is anything in this mess what I am looking for?? I have a couple more pictures.

Is the round connector with the coiled brown wire have anything to do with the wires to the rear?

 

These are what the wires looked like at the rear... They snapped like a pretzel.

 

Maybe these are the red and blue wires I am looking for?? I will have to follow them around. You can see part of the hacked fusible link at the bottom.

 

Well I just opened a can of worms....

BTW I just figured out more of the wiring diagram... Once I get a little more I will post it. No relays as far as I can tell.