I need some help with my trailer light circuit on the 2001 F150. Just bought a used trailer, and discovered that I'm having some problems with the wiring on the truck. I live in the rust belt, so everything is rusty. I'm not particularly good with electrical, so bear with me.

I've hooked the trailer up to a different vehicle, and all the lights on the trailer function normally, so I know the problem is with my truck. The trailer lights are LED, and was recently redone.

I think the primary problem is with the ground, although I'm not convinced that the plug is working correctly. I found where the ground attaches to the inside of the frame rail. The whole area was coated with rust. I took that connection apart, cleaned it up, and put it back together. That seemed to help, but still didn't fix anything. When I tested the running light wire in the plug, I got 3 volts if I grounded to the plug. I got 12 volts if I grounded to the bumper. I jumped the plug to a wire connected to the bumper, and get 12 volts. I hooked up the trailer at that point, and the lights work. Mostly. The running lights all work. The brake lights work. The R turn signal doesn't work. The L signal does.

What's my next step? Take the plug apart? Replace the plug? Are there any other grounds I should be checking? The truck seems to run fine, I drive it about 250-300 miles per month, and it starts and runs normally every time. Occasionally I've noticed a slight dimming of the headlights as I pull up to a stop sign, but I've noticed that for a long time.

Thanks.

 

Correction...the R tail light of the trailer works as a running light, but not as a turn signal or a brake light. Again, this is with the ground jumped to the bumper, so I have some sort of ground issue too. Thoughts?

 

Do you just have a 4 pin plug or do you have both 4 and 7 pin plugs? If it's the 4 pin plug you are having trouble with I would purchase a 4 pin plug tester ( they are inexpensive) put some dielectric grease on the tester plug and plug and unplug it a few times. Then check if the test lights light up as they should on the tester. The contacts in the plug might be corroded to the point that they are not making good contact. If the test lights don't light up try and move the tester plug around while in the truck plug. If they still don't light up does the harness from the back of the plug go back and plug into the main harness coming to the back of your truck? If it does unplug there and test that plug. Try and isolate the trouble spot.

 

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In your case with what you described, I would pull the wiring harness out, it's easy. Runs from rear bumper to the driver side of the spare tire on the frame. I swapped my 4 pin out with a 7 way plug. Wasn't hard at all. In your case it would be a good idea to pull it out and inspect for damaged wires and repair as necessary.

 

 

Thanks for the words of wisdom. I'm planning on taking 2 hogs to the butcher on Tuesday, so I'm a bit short on time here unfortunately. I was able to get a new plug locally, but not a new wiring harness. I replaced the plug, and now the ground works, and I have lights without grounding to the bumper, but I still have a RR taillight that works with running lights, but not turn signals or a brake light. Will get a new harness ordered, and will report back. Thanks for the advice so far.

 

Yeah so you're still having either a connections or wiring issue. Often times when I hook up my trailers they all work except left turn signal but after I wiggle it and pull it in and out a few times it'll work perfect.