I trouble shot this thing yesterday till I was drunk & frustrated.
Both drives will trim down with the button on the throttle, but only one goes up. The same for the switches on the dash. If I hold the trailer switch, the drives travel both directions.
12v to all places there is to be 12v. I messed with the trim limit switches, bypassed them & still nothing.
WTF?
Any ideas?

Solenoid? There is a solenoid for down as well as up. It looks like a starter solenoid for an older ford. Bypass it by jumping the 2 posts and see if it works.

My guess is its in the trim limit switch. Probably has a bad wire somewhere or the puck is bad.

I bypassed the trim limit switch on the other motor & it worked, but on the affected one, no-go.
The affected drive will go up when pressing the trailer switch.

still sounds like something in the limit
switch circuit.

did you unplug the trim sender wires ( that come
out of the transom assy) and plug the wires from the harness
( the wires that connect to the sender wires) together ?

yes I unplugged the sender wires & looped the leads that connect to them. The problem stayed.

There is also a replacible fuse right by the selenoid. It is a little stip of alum that is bolted under a cover.

Would a blown fuse keep the pump from traveling in one direction?

I agree, just can't figure out why I can't bypass it on that particular engine, but can on the other.

The way it's set up, yes.