What, other than a bad diode in the throttle trim harness to the dash rockers, would cause a loss of up trim on the throttle and the drive trim rockers on the dash? Dedicated "trailer" rockers will raise drives, and "down" works on all drive trim controls, just no "up" on drive trim controls ...throttle or dash. What's up? Is it definitely a diode? Thanks

Nobody has had this happen...and found why?

Yep exact same thing happened to me. It was a wire on the trim limit switch.

Yea, its likely the trim limit switch is bad. They get water in them etc. The trailer button overides the limit switch.

check the limit potentiometer (switch) on the drive's
port side
1) it rotated into overtrim position, screws are loose...
re calibrate to proper over trim limit position

2) it is shorting internally due to water - replace

3) wires between potentiometer and through hull are cut - replace or solder, seal and shrink wrap wires

4) CHECK THIS:
bolt behind limit switch (potentiometer) has rotated loose, causing the limit swith to be in off (or over trim) position. check and retorque bolt to 150 ft-lbs.
Iin some cases (what i just fixed this year) the bolt may be stripped. in that case use a stainless helicoil kit to drill out , insert helicoil, and reinstall bolt and torque to spec.

let us know which was the case

good luck

i had a similar problem ended up being the switch in the throttle handle, have someone push the button and check with a test light or continuity tester red=+, blue= up , green= down if you don't get a reading the switch is bogus. Most likely it's your trim limit switch though of which also can beeasily checked .

Trim limit switch is a on/off switch.
Trim sender switch is a potentiometer.
They both look identical and are usually located on opposite sides of the gimbal.

New twist...all up trim switches work fine with either ignition "on". WTF Trim switches should have power at all times regardless. Down trim works with ignition OFF. GRRRRRRRRRR BTW...twin engine boat.