First I would like to say hello, and thank you for the pages of info that has helped me resolve other problems with my truck.

I bought this new, 2000 F 150, OEM Tow Package, with 7 way plug.

Recently we purchased a used boat, that came with a used trailer. All in very good shape.

I purchased a 7 to 4 pin adapter, I have cleaned the ground on trailer, cleaned the 7 way plug blades, appropriate fuses all look good, but my problem is still there.

With trailer on hitch, and wiring plugged in, I get left/right turns, brake lights, flashers, with brakes applied, both turn signals work.

Turn on headlights, all running lights work, both right and left tun signals work, apply brakes and both the tail lights go out all running lights remain on.

With headlights still on, and brakes applied, and the tail lights out, if you use the turn signals they will both work as they should.

Take you foot off the brake and the tail lights come back on.

So everything works right, except for tail lights with headlights on , and brakes applied???

Thx MB

 

Best is to repeat the test with a meter or test light at the 4 pin side of the 7 to 4 pin adapter, and if needed at the 7 pin adapter on the truck.

If I had to take a huge SWAG, the tests with a meter or test light at the trailer tow adapter are going to work correctly, and there is an issue with the bulb socket in the stop / turn housing on the trailer.
That is a huge SWAG only from the description ( older boat trailer that the rear lights have seen better days ).

 

Will do, and thank you very much for the advice. I will let ya know.

 

When both turn signals blink, it is usually a bad or poor ground. Make sure the ground in your truck is good.

 

They don't both blink, like at the same time, they work as they are supposed to.

 

Tested 7way plug and adapter tested fine.
4pin plug tested fine.
At the trailer tail lights, the ground wire, after pulling the tail light housing out, was bad.

Cleaned re-attached and BOOM all work perfectly.

I would have never thought to look at the tail light housing! Thanks MB

 

That's what I said it would be, I was just wrong where the bad ground was.