Hello everyone!

I'm currently trying to get my 1985 F250 into working order so i can use it as a reliable beater/work truck. When it came to me, the bed was completely rotten, and the wiring was in shambles. the bed has since been disposed of, and now its time to get the harness ready in preparation for another bed.

Here's Sanford, in all his glory, right now.




So, i will start out with saying that the factory plug from the frame rail to the bed was intact. i ended up getting one of these to wire up the 7 way plug for trailer towing with.


Curt Manufacturing, #55313

I installed this connector onto the factory frame plug, i wired the ground to a proper ground, and the trailer 4 way wires were operational. I can safely assume that, up to that point, we were getting decent electricity.

From there, the factory harness that ran to both tail lights was a butchered mess. someone had previously used Scotch lock connectors to crudely wire in a trailer harness in the late 80s, i assume, wires were cut, broken, held together with duct tape, the works. I tried to patch a few obvious things up, plugged it in, and all i got were very very dim marker lights.

No good.

So, i got out my handy dandy test light and decided to test the male end of the Curt T connector (4 spade end) and i made a crude drawing of what i found.



the top two spades would light up for the left and right turns, the bottom right would light up for marker, when i hit the brakes, only the left spade would light up. i couldnt get any sort of power out of the bottom left spade, my guess was that it would be a reverse light spade, but alas, no power there.

so, who can help me from here? who understands the gibberish i just said? do i need to build a new harness from my 85 plug on out? Do i need to somehow splice my 85 plug onto a 1995 bed harness that i have sitting around? help?!?!?

 

There is a post on trailer wiring that may help.

Now from talking about trailer & bed lights later years ran a ground wire back up front so could the "no power wire" be a ground?

So if I understand right nothing works right on the truck side were you plugged in the trailer harness.
You need to get that side working first.
There should be a plug on the firewall side by the foot well check for power there.
You will also find other wires for fuel senders and tank switching but should not be too hard to figure out.
Dave - - - -

 

We all can understand it better if you look behind the plug and see what the wire colors are. And then you can also look at the wiring diagrams and understand what you are looking at and what you should be testing for.

For instance, the lower right spade should have a brown wire going to it. That is the marker light circuit, you can verify that in the diagrams here in this link.

Exterior Lamps - ???Gary's Garagemahal

Look at page 71. I bet the plug you are probing is c121 in the diagram.

Look in this link on page 76.

Turn, Stop, & Hazard Lamps - ???Gary's Garagemahal

Look in the middle of the diagram, there is c121 again(c means connector). You will see orange/lightblue and lightgreen/orange wires, these are the turn/stops. Look on your connector and see if you have these color wires on the top left and top right on your plug.

 

That is incorrect. i have power coming from the factory plug, to the curt plug. the 4 way trailer wiring works fine. the diagram i drew was from the 'output' of the curt plug.

here, pictures speak louder than words, here is what works.



you will note the random read wire, that will heading to the brake controller i havent installed yet. you will also note i wired the license plate light through the connector, that is so that if i need to remove the bumper, its all through one plug.

what i am asking is where do i go from there? the factory harness is so butchered beyond this point ( to the tail lights ) that im not sure what to do.

i will review the diagrams left in this post, to see if i can sort some things out. thank you.

 

Ok so the new trailer wiring that goes to the trailer plug does work.
But the output side of that plug that that trucks tail light harness plugs into is not working.
If so I would say you got a bad T connection on that trailer harness.

Could pull it off for bench testing with an ohms meter.
Dave - - - -

 

no, the output side of the plug works. the harness past that is a total mess tho. im looking for someone to guide me from there on out.

 

The wiring diagrams (links posted above by Dave) should be a great help to you here, go compare what you have to the diagrams to help you figure stuff out.

 

Let try this again, hit wrong button and lot the post!


How much of the trucks tail light harness do you have?
It is only 4 wires out of the "T" and if you have enough of the tail light harness to add wires to pick up 4 different color spools of wire and add to it.
OR
Find another tail light harness.


I was lucky my parts truck has a good harness but is for a style side and I have a flare side but I will make it work. I also have that "T" trailer connector I will use to wire in a 7 pin trailer plug.
I also will add a electric brake and power wire to the plug.
Dave ----

 

What if you just get a rear light harness from a junkyard truck? They used that same connector pinout for....ever.

 

The only thing I remember having a post on was the later trucks ran a ground wire for the bed lights back up front where the early trucks had ground wires at each light.


I think this was done because over the years the ground thru the bed would go bad so Ford fixed it.


Easy to tell look up that trailer light "T" for different year trucks.
Dave ----

 

If you have a 7-wire round, that 4-flat isn't all you need back there, I'm finding this stuff out.

 

Oh I know that I would cut the 4 flat off use the wires and add a few others to a 7 flat pin round plug that my trailer and most travel trailers use.
If I come across a trailer that uses a 4 pin flat I will use an adaptor.


BTW my 02 Dodge came from the factory with trailer package and in the glove box was the plug to wire in the brake controller and the 7 flat to 4 flat
adaptor.


I have been around trailers from mid 70's wiring different cars, trucks & trailers so none of this is new to me.
Working on old stuff you don't have to watch (to much) where you run wires not to feed EMF to electronics.
I think that is why most trucks come with trailer setups, wires are run not to upset the systems.
Dave ----

 

Fuzz,

My current Tow Vehicle #1 (for right now, anyway, hopefully for the entire race season) is my friend's 1992 GMC Yukon with I'm guessing a Chevy 350 but it's also got a 4-spd OD tranny, 4WD, receiver hitch, and a transmission cooler - the OP originally towed a boat with it and it had some sort of crude 4-pin flat setup that I didn't totally disassemble to figure out, I'm hiring out the job to the local U-Haul deal tomorrow morning at 11.

My brother just bought a 20' tandem-axle trailer with electric brakes that uses the 7-pin round thing. The Yukon is kinda-sorta pre-wired for a brake controller by the PO but I couldn't verify one of the wires via an ohmmeter, the existing connections were green & corroded, and I found somebody used those internal-use-only, crimp-on, butt connectors back there - which I recognize as the absolute worst place to use such things.

This is my first serious introduction to this stuff; it's not terribly difficult to figure out but access to all the wires back there really sucks cuz of the gas tank & hitch... it's cold & crappy out today (almost snowing) and I got frustrated and decided to spend my time elsewhere, so I'm hiring it out.

BTW this is the trailer for the season (and I can even point out *my* truck (the baby-blue thing) in the background):

 

As you said sometimes it's best to farm the job out.

My 86 K5 had the light wires hanging near the hitch and the electric brake & power wires run down the frame rail with extra under hood and by hitch.
I had to buy th3 brake controller & plug out back then wire it all up.

My 02 Dodge was all wired out back.
Just had to buy the brake controller, wire the plug that came with the truck and plug it into the truck side plug and was done.

My Ford I have already mounted the hitch, need to buy a controller, should have a plug or 2 in my trailer electrical box kit, run wires not hard to do if you got the time.

I like the trailer with the front shield. Mine has a large tool box up front and a tire rack above it that can be removed if I have something really long.
Dave - - - -
​
Dave - - - -

 

so, an update on this project.

Today was the day that the bed was going on, So that meant it was go time for me to get this wired up.

First off, i found out that the 4th spade on my 4 spade plug was the backup lights. they werent working because the backup light wire isnt plugged into the trans ( and i still dont know where it plugs in to, someone help with this? its a 4 speed manual) i did make sure my harness was properly grounded, i ground an area of the rear most crossmember clean on both sides, and bolted a 3/8" bolt there for proper grounding.

I spent a solid hour sorting through all the wires of the "bed harness", i got everything operational with a power probe. left turn, right turn, reverse, marker, all good.

plug it in to the truck, and i have left turn, right turn, marker.... and only the left bulb hits for the brake light.

so, i first verified the bulb. i then verified the plug. then i inspected and tested wires all the way along the frame till i got to the firewall. then i put the bed on, anyways, saying " i'll ask the ford truck page tonight"

here i am.

heres what i can tell you.

wiring along frame to the bulb tested good.

the headlight switch assembly is operational, but seemed a bit weak. i have a new one on order, i was going to replace it when i did the wiper switch as well.

the turn signal switch ( in the column) seems to be pretty tired. its on my to do. it takes a bit of wiggling for the right turn to work.

and finally, i am totally useless reading wiring diagrams. please, someone explain it to me like im a 9 year old. "test ___ wire for ___ " i can do that.