Does anyone have a method or circuit diagram to allow the testing of a Tacho on the bench i.e. without an engine?

I've found a few 555 based circuits but all are 5v outputs

555 Timers will work up to 16V

see:https://www.fairchildsemi.com/ds/LM/LM555.pdf

It would be a very good way of checking it out.

I recently calibrated an old tacho using the 12v output from an old battery charger. I tapped the transformer output before the rectifier. The 50hz signal equated to 1500 revs. The tacho was a Cortina Mk2 item which senses pulses in the 12v coil feed wire. The technique may not work for a modern unit. Might be harmful even. But it might work??

Technically that would shove a sine wave to it... might work tho...

I have a 555 timer circuit to try later, if that fails i'll attack the battery charger.