I could NEVER make a living working on boats. The install of a tachometer took a looong time.  You get no wiring diagram with a new Mercury motor so you need to study a lot to understand where certain voltages are and how to get them.  As luck would have it they actually provided a connection for switched power inside of the ignition switch.  That was really nice since many of the functions are now digital.  Power for light in the gauge was obtained from an existing lighting circuit in the boat so it will only be on when needed and not glaring in my eyes all the time the motor is running.  The signal circuit came from one of the feed wires from the alternator to the rectifier.

After using the motor a few times I didn’t like the original location I chose for dealer installation of the ignition switch so I moved it to a more accessible location in the boat and installed the tach. Doing this meant running the 16 wire cable and the 4 wires required for the tach through the boat in an access channel behind the flotation and compartments.  Not an easy or quick task.  Took a lot of time but, now it is done. Everything works and proves the prop calculation was correct.  We are currently running slightly below the max rpm for the motor at full throttle and that will likely move up just a little as the motor breaks in more or we have a lighter load in the boat.

Fuel consumption is much better than the old motor. The 4stroke sounds different than the old 2 stroke, driving the desire for the tach.  Until you get familiar with the sound of a 4-stroke at 5500 rpm it sounds “way too fast”.   All is working fine.