F15 power trim goes up in reverse

[jisaF150]

My F15hp power tilt let the motor go up when i shift reverse .
Where is the bad thing? Trim goes normally up and down by switch. In reverse it goes up and back up the boat is impossible.

[99yam40]

bad check valve inside would be my guess, if the system is full of oil

[jisaF150]

Check valve?? There is no check valve in trim pump.
Could it be absorber valve that is in the trimram piston?

[99yam40]

some type of valve should hold but give if you hit something while under way.

[panasonic]

Would help if you supplied year and model of your F15...

[jisaF150]

Year 2001 15 PLHZ

[DennisG01]

It may not be labbled in a diagram as "check valve", but that is essentially what it is. And if you're going to dive that deep into repair, it's probably best to remove, clean and replace all of the seals/check balls/o-rings. At nearly a quarter century old, it's due - and has certainly had a good, long life. BUT... it may be wiser to just buy a new assembly (or find a rebuilt one). Rebuilding this is very tedious... VERY tedioius!

Before you do that, though - check your fluid level and check that the manual release screw is tight. Open and re-tighten.