Trim will only go half way down when I am in the water. But when I load it on the trailer and out of the water ,it works perfect?

Are you using the trailer trim controls on the outboard when on the trailer? Using the trim switch while on the trailer does it still not lower all the way?

Hmmm...I do use the trim on the motor when I am loaded, I can not remember if i have used the trim switches on the console when loaded?

Just had the original trim on my old '98 Merc. 175 EFI repaired at Fish and Ski (formerly Bear Marine) @ 1750 E. Lone Oak in Valley View on the N. side of Lake Ray Roberts. They did it for $50, which was fine for me as I know nothing about how to do that stuff. It would only go down and not up. They switched some wires around to see if contacts or a part was bad and then for the Twilight Zone incident everything started to work.

some motors have trim limiters it could be sending a mixed signal so to speak.

The reason I ask is because the trailer trim would not be affected by a faulty trim limiter/sender. So when your using the trim the motor thinks its at the down limit, but on your trailer switch it doesn't have an issue moving all the way down.

Try it out and let us know.

Edit: you could try adjusting the limit before taking it in somewhere, though I don't know how it would of come out of adjustment and probably needs to be replaced/inspected by someone who knows what they are talking about :P -
http://www.mercstuff.com/trim_limit.htm

Thanks for all yalls help!

You know an estimate price of what it takes to fix it if its a sensor?