I have a 1986 242LS with a 260 Mercruiser. Swapping it for a 2011 350MPI. Boat has a 9 pin plug and the engine is 10. What do I need to do to get these to communicate?

I believe it is something with ammeter/ volts or something. I don’t want to yank the pin until I’m sure.

Yes. The older Harness ran the ampmeter wiring. There is one pin that stops them from connecting. You have 2 options. Break that off and run a separate wire to power what it is. The option I would choose is to buy a 10 pin boat harness and replace the boat side plug with the proper one. Then everything stays simple and in the cannon plug

Thanks. At least I know I can make it work.