I had the same thing happen on a Yamaha f115. The tilt switch on the side of the motor was shorted, causing the motor to trim up. I seem to recall being able to unplug that switch and still have the throttle trim switch work until I was able to replace the bad switch.

 

Thank you for the tip. I want to take it out this weekend - but can only imagine the fun of it trimming up while I am running to and fro.

 

Trim and tilt switches can occasionally be problematic on any make or model of motor… To complicate matters it might be your wiring - and not the switch or the motor…. The first thing I’d do is contact your dealer and ask for a tech to walk you through how to turn it off - then back on to be able use the motor until you can get it in for service…

Remember as well that a trim motor left on - can burn out as well. If all else fails you can always dis-connect your battery if needed.

“Aren’t boats fun?”

 

While my wife is not happy about the garage now having a nice gaping hole in the wall... I am MUCH more worried about the trim motor burning up.
The time in the summer when it happened w my buddy- it had been running for over 5 min due to the issue.
I have no idea how long it did it the first time last week- but the second time I was able to disconnect it after about 2 min.

While it still works as of now- I am sure it was aged. I appreciate all the insight. Call to the dealer is next.

 

Shorted switch's or shorted relays can do it. Try wiggling some of the related wires to see if you can duplicate the problem. I always keep my batteries off when the boats are stored.