tach needle bouncing on my 1994 Catalina sailboat

by tom brown
(pensacola)

i ahve a 1994 Catalina 270 sailboat with 18 Hp Perkins diesel. my tach needle has been bouncing from 300-1200 range. i have replaced my number 2 battery, cleaned the ground wires on back of gauge (there are 2). i placed my volt meter on the neg side and the positive side of the gauge and there was a negative defection so i moved the volt meter black to red side of gauge and the red terminal of the volt meter to the black side of the gauge and it registered 11 volts. not sure where i pick up the input from the engine to send a signal so i will assume it is from the generator. all wires from generator look good.any ideas????