Re: Tachometer wiring?
There are four wires on a tach -- only one of them comes from the engine. The wires are as follows:
L = internal light connection -- connects to the light terminal on any other gauge or to the NAV side of the navigation light switch.
Ground = ground -- connects to the ground terminal on any other gauge or the ground lead in the engine harness.
+12 Volts = This connection is labeled "I" on most tachs and is connected to the "I" (+12 volt) terminal on any other gauge or the "A" (accessory) terminal on the ignition switch.
S = Send terminal -- Connects to the TACH signal from the engine harness. Depending on the year of the engine, there will be a three terminal plug below the ignitiion key on the control box. +12 volts, ground and S (send) are available there via an optional tach harness (available from the dealer or possibly here on IBoats).