The measurements you took mean nothing as far as determining whether or not the tach is bad. Yes -- you proved it has power (12 volts to ground). The measurement from +12 volt to the sense line means nothing. The sense signal is a series of pulses from the ignition system which is what the tachometer "counts". Therefore, the voltage on that line is an AC signal, not a DC signal. The wire on that terminal originates at the engine coil. With the engine off and one end of that line disconnected, measure continuity from one end of that wire to the other. If it is not broken (open circuit) then you have verified it is likely a tach issue. But before replacing the tach, look at the back of it. You will find a calibration dial. GENTLY rotate that dial back and forth several times to "burnish" the internal contacts. Set the dial back to where it was and now start the engine. If the tach works, do your happy dance. If not, shop for a new tach. If you determine the sense wire is indeed "open" you will either need to run a new one or locate the break and repair it.
