I have a 1998 F150 that I bought last year. I have been towing a travel trailer, and everything is ducky, all lights and trailer brakes (7-pin, round plug). When I hitched up my boat trailer (using a 7-pin round to 4-pin flat plug adapter), only the tail lights work on the trailer: no brake lights or turn signals. Here is where it gets weird: testing at the terminals on the upstream side of the truck plug, I also get no power to the brake and turn circuits. Hooking up the travel trailer again, it still works. How can there be power when the travel trailer is connected, but not at the terminals and when the boat trailer is connected?
Thanks for any help!

 

Check your ground wire

 

Do not depend on just the trailer hitch to supply a ground. Use a real wire connection through the 4 way to the trailer FRAME.

 

So this testing on the up side was this on the adapter to trailer side and not working?
Testing the truck side on the round 7 pin I bet you get power, now plug in the adapter and bet you dont have power.
If that is the case you got a bad adapter.
You can try a different adapter or put a round 7 pin connection on the boat trailer.
Dave ----