Originally Posted by TomKat
One is marked g so must be 0 v but no clue on the others!
I'm not sure about that! I have a vague recollection from investigating options for mine that some Yam gauges mark the "signal" connection G.
Yamaha 85hp tach - need help for wiring
It looks like you might want:
LEFT PIN UNCONNECTED
NEXT TWO PINS JOIN AND CONNECT TO 12V (I think your engine has a 6 pole alternative but Rainmain (aka DHD) will probably be able to confirm)
CONNECT PIN ON RIGHT TO SIGNAL
CONNECT BODY OF DEVICE TO GROUND.