Re: Installing an RPM gauge
Most O/B have the tach sending wire standard. On Johnson, look at/inside the control box or under engine cover for the grey wire. Not sure what color Merc and others used. But no special sending units are needed.
What you do need to do is set the tach to match your engine's alternator pole number. Most are 4 or 5 or 6. There is a setting on the back of marine tachs to do just that. This is because the tachs are driven off of the alternator pulse, not the points/ignition. The instructions list almost all motors and the proper setting for that motor. If you search the Teleflex web site, it will have a document that lists the motors and settings there too.