1976 115HP Johnson OB Tach Installation
 

I have a 76 115hp Johnson OB. Last weekend my tach melted down, not sure why, but i have to give it credit for lasting thirty seven years. I found a new Teleflex tach on ebay that said it was compatible with outboards (with alternators). My dead tach has only two wires. One is grounded inside the control unit. The other enters a wiring harness, leads back to the motor and I really can't tell where it goes from there. I checked it for voltage and it has none with the motor stopped, key on. My question is, if i hook up 12v and a ground to my new tach, is that grey wire what i want to use for a signal? I believe the tach reads alternator pulse. If that's so, wouldn't any hot wire provide a signal? How does all this nonsense work?