I'm assuming you have the MFS30B, EF type, right? Depends on which tach you have, how it's set, and how it's wired.
Until recently, there were only 2 Tohatsu tachs that were setup for that motor: 3N8-72640-1 with the oil warning lamp, intended for EP type (remote) motors, and 3B7-72530-2 without the oil lamp, intended for tiller-steered motors. Lately, you can get the newer 3AC726400M (black face), and 3AD726400M (white face) units. I believe these both have oil lamps. The tachs also have night lighting. The meter is operated by the alternator output pulse... should be a white wire coming from the harness on the motor. The other harness wire, yellow, is the ground reference.
If you have a tach that has the oil warning lamp, you need to confirm that you did not accidentally connect to lamp wires on the tach. If you connected the tach output from the motor to either the night lighting circuit or the oil warning lamp, the tach meter won't work. Another thing to pay attention to is that the tach must be set for 12-pole operation for use with the MFS30. There is a setting switch on the back of the meter. If that's set incorrectly, the tach won't operate correctly... But that usually results in the tach working, but giving an incorrect reading. Another possibility is that you have the white and yellow wires reversed. That would result in the tach always showing Zero RPM no matter what.
