I got to figure out this last thing before I really start driving this thing around. Especially at night. Seems as though I might have a bad ground on my passenger side rear brake/turn signal. It works when the headlights are off (although dim), but it does not when they’re on. Hazzards are also really finicky. Also an issue with backup lights but I think that is a socket/Blub issue but not sure.

I was looking at diagrams at garys and looked all over but couldn’t find any grounds in the back. Not behind the tail housings anywhere on the bed and not under the driver side rear bumper. Frankly was having a tough time following the diagrams especially since I don’t know what I have back there. Also got a random small white wire tied into a blue not connected to anything that you can kind of make out in the picture below.

I have what looks like a trailer hook up and the wires are tied in to the license plate lights and both rear tails/back ups that ultimately go to a harness to run to the front. That plug has a crusty ground on the frame right next to it. Also tucked in the corner by the harness and rear fuel tank Is 3 sockets sitting there connected to nothing. Not sure what those are about.

I tried to have this issue fixed by a mechanic years ago when I got the truck and can’t remember if it was or not.

Any help would be great.

 

Since the main ground is common to both sides in the rear, the main ground is probably good if the other side works. More than likely you have a bulb contact problem in the socket, or the socket is bad itself. It could also be one of the connectors that bridges the gap between the two lights, or the couple of splices inbetween. Just to get you more familiar with reading the diagrams, we will hunt down the ground circuit below.

You can see in the diagram below, the ground to both turn/stop/marker lights in the rear is common to both lights, and goes to G701. See the note in the grey box, grounds are on page 5,6,7.

 

The first diagram below, you can see the famous G701. It's a very important ground for the whole truck.

 

On the page below you can see the description of the location of G701. It says "Behind Instrument Panel near right hand side of radio". Over to the right of that you can see the page number that has a picture of this ground, which is on page 67-3.

 

There is G701. So they run the ground for the rear lighting all the way up front to a common ground connection on the firewall.

 

I never noticed it before, but splice S805 must be one heck of a splice. It has 14 wires spliced together at that point. They don't list a picture, but say it's near the cigar lighter.

 

Note on the rear ground - some trucks had the ground at the lights behind the housings (my 81 F100 style side did) and others they say had a ground wire that ran all the way back up front.

Ok what I would do is follow the harness down the left frame rail from the firewall / floor board to the rear and it should end at the end of the frame rail with a plug that the rear tail light harness plugs into.
That should have 4 (right brake/turn, left brake/turn, running lights, back up lights) or 5 wires (ground). You may want to take this plug apart to make sure it is clean.

Now is that trailer plug setup spliced into the tail light harness or does it have a Tee that plugs into the frame harness and the tail light harness into it?
If spliced in I would remove all spliced wires and that plug and start over.
Etrailer.com http://www.etrailer.com/t1-1982_Ford...250,+F-350.htm
Has a Tee that will plug into the rear frame harness and is a flat 4 connector. 1 of my trucks came with this Tee and I plan to use it but make a little change.
My trailer has the 7 flat connector like you have on the bumper as my trailer also has electric brakes. I will need to run the brake wire down the frame back to the plug.
I will cut off the flat 4 and wire it into the 7 flat plug, they make adaptors to plug into the 7 flat and hook up a flat 4 connector that I will also carry.
Make sure you seal the back of the 7 flat plug/socket as water will get in and cause heck, that could be your issue?

So that should get you the trailer light connection with out splicing any wires to the trucks harnesses.
Now if they spliced into the tail light harness you may want to remove it and work on it on the bench.
Fix anything that is broken or looks iffy. A few coats of Liquid tape over any exposed wires may work to fix it. Or the Liquid tape, electric tape and then Liquid tape over that and electric tape the harness back up.
Now clean all the light sockets.

Does this harness have ground wires to the sockets? If not is there a broken black wire sticking out of the harness some where?
That could be the bed ground that connects thru the frame harness running back up front? Could put an eyelet on it and screw it to the bed.
Could also run a heavy 10 ga. ground wire from the bed to the frame. Your frame is grounded up in the engine bay I hope?
You could also "ring out" ohm each wire from sockets to the plug to make sure all is good before you install in back in the truck.
With that all checked out and installed see if the light still gives you issues. It should not if all was done right.

Because my flare side harness was hacked and think not useable I am going to use the style side tail light harness modded to fit the lights for the flare side and plugged into the trailer light Tee.
It is work to check this all out but should not be that hard to follow all the wires if you break it down to the frame harness / trailer harness if you want it / tail light harness.
Dave ----

 

My 81's did not have G701 or S805 behind the radio on the firewall.
In the engine bay I have grounds at each fender by the head lights that pick up the lights in that corner and I have 4 or 5 wires on the fire wall by the brake booster, don't know what they pick up?

Once I get the flare side bed together I will then see what I have to tail light wiring but I remember the style side had grounds at each tail light.
Dave ----

 

It LOOKS like the trailer plug is spliced in with the tail lamps but I’m gonna have to take it apart to see. I’ll probably scrap plug and clean up the wiring. I have no need currently for a trailer plug. I went ahead and ordered new sockets for the back up and brake lights. 3/4 look like a mess and are pretty loose as well. Thanks for the info...

Any idea what those 3 plugs that are connected to nothing that I have are all about?

 

Before you go pulling everything apart I would take a test light and check them plugs to see if any of them get power with the lights on, right turn, left turn, back up light.
Are there any relays by the brake booster area?
If looking down the frame is there another harness that runs to the rear that the plugs are part of?
With that hitch and trailer plug light socket I wonder if there could be a trailer tow package on the truck and the plugs are part of that?
Something like the Etrailer Tee for the trailer lights.

I will see if I have time to look for my style side trail light harness but I don't remember there being any other plugs.
I think it had only the 1 that plugs into the frame rail harness and the light bulb sockets.
I should also find what is left of the flare side harness to see how bad it is.
Dave ----

 

Not sure what what I was looking for by the brake booster but I have what looks like 3 unused harnesses on the driver side fender pictured below.

Also the other pic pic is a better look at the rear harness for lights. Looks like 4 wires and then a separate black wire connected to that grey harness. Haven’t ripped into it yet.

 

They are relays and from the guess of it trailer / 5th wheel light harness.

Do you have cab lights on the roof?
A relay could be for that also.

I am sure that area is where the relays are for the roof and trailer lights.
You are going to have to trace out where the relays feed and wire colors for us to tell.
I would love to get a trailer harness for my truck as it will be a backup to pull my car trailer.
Dave ----

 

No lights on the roof. Wish I did though!

 

Definitely depends on what year we are looking at. Here is a ground diagram for a 1981, you can see the rear lights are all grounded at G1004. It says that ground is located "on frame, forward of LH rear light assembly.

 

Here's a picture of G1004.