Most likely your Trim Sender is bad, and probably the wiring to it at the transom. It is located on the starboard side of the gimbal ring. Mercruiser Service Manual #2 covers your 1976 MC 888 Ford motor and out drives.
Which drive do you have?
Service manual # 2 is still what you need. I think there are pleant of YouTubes on replacing the sender and limit switch. Its not easy, at leasy on alpha1 .
If the drive is original to the boat the trim sender might be a hydraulic cylinder thing on the inside of the transom.
It's the horizontal thing in the picture.
Check to see if the resistance changes as the drive moves up and down.
