Re: OMC Tachometer and Evinrude 40hp.
+12V and ground are obtained internal to the control box. +12V is via the ignition switch. Ground and the tach line passes from the three-pin connector, into the harness, back to the connector at the engine and from the mating connector on the engine to the terminal block on the engine. If everything looks original at the terminal block (no disconnected wires) check the plug terminals to make sure those are not damaged or corroded. If you have any gauges on the boat that come to life when the ignition is in the RUN position +12V and ground are making their way to the control box. The fact that the engine turns over with the ignition in the START position also indicates you have power and ground at the control box. Absence of +12V and ground at the tach connector would then mean the problem lies in the box. Lack of a tach signal could be due a harness problem or the charging system (fried rectifier). Measure running voltage at the battery. As I mentioned, anything over 13V indicates its functional. But 12.6V or less means the battery is not charging. But that can be due to corroded or bad connections at the solenoid, battery, battery cables at the engine, or the terminal strip, or a bad rectifier.