If its an industrial proxy there are 2 basic types, npn and pnp, the o/p is switched to ground or + rail.
Your tacho if an older one is probably looking for a switch to ground input, as in old fashioned points (or fets in ecu's).
If your proxy is a pnp o/p it might not work, try putting a pull down resistor 470 ohms or so from the o/p of the proxy to ground.
Edit: I have tried running telemecanique proxys from 12v, even though they say on the side 12v - 240v ac or dc they dont work from 12v, 24v was the least it'd work off.