It could be the tach is faulty. However, the fact that it zero'ed when turned on indicates it is not initially faulty. Please do the following for accurate troubleshooting.
1. Obviously there is 12VDC to the tach as it goes to zero when turned on. Ensure range selector is at 6P or 6
2. With engine running idle, measure voltage from the SENDER terminal to ground. There should be at least 5~7 VAC. If there is and the tach needle is still not moving, the tach is faulty or you have the wrong type of tach for your motor.
3. If there is no 5~7 VAC between SENDER and Ground, check voltage at the engine terminal compartment. Check voltage between grey signal wire to ground, if you do not have voltage still it indicates the regulator is not either properly grounded or the grey wire is not connected properly internally (regulator broke likely).
4. If no voltage between grey signal wire and ground, check voltage on each of the yellow wire to ground. In short, you should at least get 5~7 vac either on one of the yellow wires to ground or the grey signal wire to ground. And the same should be present at the tach SENDER and GROUND TERMINALS.
Note, if you do not get 5~7 vac on the grey signal wire to ground but do get it between one of the yellow wires to ground, then connect your tach signal wire to one of the yellow wires. Again there should be 5~7 vac on the tach's SENDER and ground.
