tac wiring problem

[roydee]

I have a 1981 merc 115 6cyl. My tach is working upon inspection i notice one of the wires was not hooked up. I have a Medallion tac and also a repair maual but the color codes doesnt matchup to the book neither does the post on the tac. I have a ground post and a S post, and a dial. the dial is set to 4. The color wires i have seem to be is black, brown and tannish/white. The black is on ground and brown on the S post. the wire that isnt connect seems to be power when i tried to hook up it sparked and the wire seem to get a little hot. Can anyone help. thanks