Trailer Plug Wiring (factory tow package)

[1991 F350 Crew]

Hello Everyone, this is my first post and I'd like to start out by saying this is a great site. Here's my problem. I just finished wiring my semi-homemade car trailer only to find that the socket on my truck had no running (tail) lights. After I checked the basics I pulled the 7 way RV plug off. Checked for loose connections broken terminals etc. and all looked good. So I followed the factory looking harness from the socket under the truck. Here's what I found the harness goes into 2 factory plugs, there is another factory plug going to my LH taillight, and there is a 4 pronged male plug next to all of these plugs that isn't connected to anything. So far I have left the one that goes to my tail light connected because the taillights work fine. But the plug that wasn't connected has one hot terminal when I turn on my headlights (tail lights). I also noticed that my running lights on my dually fenders aren't getting power. Now in the center of the bed, on the harness that goes from left to right there is a female plug that looks like it could plug into the disconnected male that is with the previously mentioned connections. Only problem is there is no way to make them reach, I tried to jump them but nothing happened. The only thing that looks out of place under there is one lone wire that comes out of the taillight harness (that runs the width of the bed) that has those blue scotchloks (taps) to power the license plate lights. Has anyone come across any problems like that? Hope you can help.

 

[Pipeliner_86]

Sometimes you just have to start from scratch and rewire until you get things working the way you want. Make sure you have good grounds and start with the basics, source out the running lights wire, left and right blinkers, then brakes. You may have to cut out what previous owners have done. Again, make sure you have a reliable ground. If necesary, run one individual ground wire from neg. terminal on bat. back to a clean grounding point on the frame at the back of the truck.
Spend alot of time testing wires with a multimeter before you go to hacking away, reason I like to use the multimeter is you can test continuity when needed.

Goodluck.

 

[1991 F350 Crew]

I think I found the problem. The stinking relay under the hood.

 

[sassyrel]

ok--thats good---but tell ya what---get rid of them damn scotchlocks as soon as possible--they let dirt and water into the wires--and will corrode them backwards from the connector---people ought to be shot for using them!!!!!!!!!!!!!

 

[1991 F350 Crew]

I replaced the relay and still have nothing. The wires that go into the relay are solid brown, brown with white stripe, yellow, and black. When the headlight switch is on I have power to the brown wire only. Now I used a jumper wire from that solid brown to the brown with white stripe (running lights), and I have power back at my trailer socket and my dually fender lights work now I hooked my multimeter set to ohms and went from my battery to my brown wire with the white stripe and it zeroed out on all of the wires except for the yellow. NOW LISTEN TO THIS. I took out the #4 fuse and I still have all of my lights including the dash lights. HELP!

 

[Andylad13]

theres only one way to help you out. re-do it!! it sounds stupid but i can guarantee you from experience, on something as simple as this, it will take you less time to start fresh and do everything all new.

running wires in the dirt is something we have all done, and we have also done the route of looking for the problem first. that one never works out when talking about wires.

remember you could be looking for something you wouldnt even be able to see, like a broken wire internally.

 

[1991 F350 Crew]

Well I found most of my problem. Thanks to the Running Light Relay Modification tech. article posted by AGNEM. My plug going to my light switch was FRIED. Got the new pigtail from Ford ($20). And now all seems well. I did have to jump the yellow wire on my trailer pkg. relay though (couldn't figure out where or what it came from). Thank you all for the great info.