The tach on my 75 Mark Twain literally fell apart. I bought one on Ebay and the instructions say for 4 cycle engines. The tach I bought is a "Faria" TC9615A with a square face. On the back of the "Faria" has BAT, GND, SIG, light and a dial with 1 - 2 - 3 - 4 settings. The old Tach only has +POS, -NEG and light hookup. I have a 1975 - 115 hp Merc. Will the "Faria" tach work and if so, how should I hook it up and which of the 1 - 2 - 3 - 4 setting should i put it on?
