[Jonny H]

Bob is correct but there is also another factor in play. The older tachos are very particular about duty cycle, requiring the mark (on time) to be no more than around 5% of the space (off time).

Provided the duty is correct, you can drive an old tacho with a 12 volt signal. Believe it or not, you just disconnect the 12V feed from the back of the tacho and connect the signal wire to the 12V terminal instead. That's right, just GND and signal going to the 12V terminal. No connection on the I terminal.