You're certainly having a bit of a puzzle, aren't you Mac?

So, the brake controller shows fault when trailer is plugged in? Look at condition of wires at or near both plugs, then trace the brake activation wires from the wheels back up to the tongue looking for problems. Those wires are exposed over much of their run and could be damaged.

But if you can check the wire connections at the back of your 7-pin plug on the tow vehicle for possible shorts - you might find the problem there. I'm suspicious of this problem happening right after you successfully got 12V power to the battery charging circuit.

- Jack

There's always something!
Not plugged in...just showing short on controller as brake pedal pushed in truck.
ALL pins show they are working fine.The harness is solid from front to back of truck.The truck's digagnostics show nothing out of whack...and Ford's diagnostic panel is pretty extensive.
If it's moisture,it could be a while before it gets dried out...forecast is nasty for the next week.

Wish I had noticed a couple days ago if the short showed.Only saw it today as I was showing eldest how to set the controller...it's a bit out of the way for viewing normally.
Trailer brakes worked great....the only addition was the relay.And the rain.

Just had it in my head that as I was testing the pins in the truck end that I could have slipped and crossed a couple...so I disconnected and reconnected the controller...kinda hoping for a re-boot,y'know? Only pulled it apart for a couple secs tho.
No such luck!

Well, the good news is you've isolated the problem to the truck.

The bad news is it's probably more difficult to find the problem there.

- Jack

There is voltage to all pins.... Just checked elec brake pin,and the voltage changes progressively when brake pedal is depressed.... Not by a lot...about 2.6 volts. Am told this is about right.controller is a cheapie too.
I have decided not to worry about it for now.... Doesn't seem to be draining battery,nothing shows on diagnostics... Everything works.

If the controller reacts to deceleration (applies brakes more aggressively during a quick stop rather than a gradual stop) it won't apply much power when your truck is stationary, no matter how hard you press the brake pedal. It will have something like a "pendulum" inside that swings forward as the truck slows down which causes more voltage to be applied to the trailer brakes. You can adjust the pendulum swing with an adjustment on the controller.

As you said, 2.6V is probably about right with everything at a standstill.

- Jack

The controller is very new-fangled...fully electronic,the sort that can be mounted any which way...so no 'pendulum' or mercury switches.
This is not to say I know anything about them of course.
Rather,the point I am trying to prove to myself is,with equally limited knowledge of electricl systems,whenever one runs up against a short,something will not be working.Or battery goes pfffft.
Neither is the case here.All variables work.
Faulty fault?
Time will tell.
Maybe.

Mac,
Check out this thread also it may have some more answers for helping you out.
https://www.hilotrailerforum.com/f28/...html#post13784

As pointed out in that thread by poprichie,
"This is something that I didn't know awhile back, that you can damage the Prodigy if you pull the cable out on the brake away switch with the controller plugged into the trailer. I then read the instructions and it does say that.
You may have a bad connection to the Prodigy. To test the trailer brake wiring and magnets, pull the emergency break away switch cable, with the trailer unplugged from the tow vehicle, try to pull the trailer, as this will apply a full 12 volts to the brake magnets. If all 4 wheels slide or nearly slide the trailer brakes are ok. replace the break away plug into the switch as soon as possible, it left on the magnets will over heat."

Hope this possibly helps

Seems the quart of water drained from my 7-way on the truck solved the short problem.
Go fig eh?
Darn thing holds water nicely....too bad it's a one-way flow.
Crap design.

Great! You can either replace it or probably waterproof it with the paint on "Liquid Tape" you can probably find in a hardware or automotive store. Perhaps it would be best to do both. I THINK the replacements have rubber boots where the wires enter to help protect things.

- Jack

Thanx Jack!
Thing is,it's brand new...is the better model too.
Dielctric grease,and lots of it!
That'll learn it;)
12v from truck charges trailer too.
Finaly,things be goin' my way.

Strange, I've not heard of the problem you experienced before this, but I can see how it can happen. You'd think it would be common, since that is such a "standard" design. Anyway, good job troubleshooting!

- Jack