I need some insight on a trailer wiring issue I'm working on. I replaced the 7-pin wiring harness end on our trailer this weekend but can only get running lights on the trailer, when hooked up to a 2004 Expedition. When either turn signal is turned on, both lights blink on the trailer on the running light filament. I can get no brake lights on the trailer at all.

I've tested the connection on the truck with a test light and all posts on it work correctly. I've tested all wires going back on the trailer with a battery booster to make sure I'm getting power all the way back and all of them work correctly. All of the trailer towing fuses are tested and good.

I've hooked the trailer up to my F150 and everything works fine, when hooked to it. So, I'm assuming that it must be something in the Expedition. It's wired from the factory for towing. So, is it a possibility that one of the pins is loose in the truck's connection? I'm figuring that the trailer towing relays are good or I wouldn't have anything at all on the trailer. If one of the pins is loose or something on the truck side, would that cause the weird things on the trailer lights, like I'm getting?

 

Poor ground.

 

I also agree with what sounds like a poor ground. I would spray some wd40 into the connector and plug in and out 5-6 times to rub contacts to ensure good connection.

If this doesn't work, look under your Expy's rear right side. The ground for this connection is attached directly to the frame (easy to see).

It is however, a crimp connector and it may be necessary to remove and clean and /or replace.

 

You have either the plug or the receptacle wired backwards. To be more precise, you have it wired as a mirror image of what it should be.

 

I thought the same thing when first reading but he re-did the trailer plug and when he plugged it into another truck, it worked fine, thus eliminating the trailer plug as the possible issue.

He didn't indicate he did anything with his Expy.

 

Who says the other truck is wired correctly?

 

What's the chance you use another truck and the wiring is also incorrect ?
We don't know much about the other truck but the chances of two vehicles wired wrong but happen to have aligned terminals properly is about the same odds as Winning the Powerball.

Check your ground.