Thanks for the reply. I have tried, yes. No luck I am afraid. I am just swamped with spare parts suppliers, and am not getting anywhere finding a datasheet of any kind. Dissecting the tachometer looks kinda difficult, unless I crack open the whole motor.
After a little more research it looks like maybe I should be rectifying the signal, then running it through an opp amp (like the 741) and a voltage regulator. Would something like that give me a pulse train that arduino could read?