Eagle & Spirit tach wiring

Green is coil (-).  Yellow is (+) 12v.  Run a ground to the metal mounting plate.  If the tach sits above zero at rest with no power connected, it is most likely damaged internally from the epoxy drying out.  Needs to be CAREFULLY disassembled and repaired.

The Eagle and Spirit tachometers are what is called by many an in line tachometer, not a triggered one.
It functionally fits between the Switched 12 volt source and power to the + terminal of the coil. A TSM will verify the exact wiring. Thus what it does is measures current to the coil when the engine is running.
At slow speeds, less current is used, at higher speeds or higher rpm's more current is used.
But rather than showing a current flow in terms of milliamps or something equivalent the display shows engine rpm. It is adjustable to be used in 4, 6 and 8 cylinder engines via a potentiometer located inside the housing.
Usually the first thing some one does is hooks it up as if it is a triggered tachometer which blows it up. That generally ends it's usefulness.
At least that has been my experience.