Check the wires first
Before you guys spend the $$$ on a new tach, have you ruled out the possibility of burnt/corroded/cut wires? If you have duel engines, you can swap the plugs from the back of the tachs and start motor. if the troubled tach moves, you'd save a ton of money. One of my tachs didnt work, i bought a new one and it still didnt work. Traced the wires, nothing was wrong with them. One the magnito of you motor is an electrical connector. Pull the black sleeve back and inspect the wires. Pull the plug out and inspect the plug also. The pic below is the one on my boat, I have a new one (5 bucks from the jet ski cemetary). As you can see one of the black wires is fried and inside the connector, the pin from that wire is fried.