I'll be following this...I would also like to know how/where exactly these get wired up...I know some of these gauges had the sending unit mounted on the back of the gauge so I'd suspect I could mount it under the helm out of the weather.
The model 650 Tachometer came in along with its matching transmitter box.
If anyone is wondering how to wire up the older Model 650 square transmitter box on a Johnson/Evinrude V4 with Magneto Ignition I tested it out and have results.
M1 to Ground
M2 to Kill Switch/Mag Lead
+ to + on back of Tachometer
- to - on back of Tachometer
I disconnected the cog belt that drives the Mag and spun it with a drill for this test. I have yet to test it on the engine while it is running but when I do will update this.
I attached the ground lead to M1 after looking at a similar model 653 square box with the M1 labeled "GND" I believe this is the correct way to install it. I decide to switch M1 and M2 to test and the result showed it still showed rpm but the needle was very unstable. Switched back to M1 ground and M2 Mag lead and the rpm needle was very steady.