Basically I identified the tach output from the ecu in the wiring harness. Then i used a 1k resistor and a random small signal npn transistor and drove its base with a function generator. I connected one end of the 1k to +12v , other to the collector , emitter to gnd and base to the sign gen via another resistor. I the connected the tach wie to the collector so it would swing from +12v to gnd. I might add that I obtained this information thanks to having the autodata cd on my computer.
As my car was a 4 cylinder it made sense for the tach to be a 2 pulse per rev of the crank and indeed this proved to be the case. Once i had this info i purchased a cheap npn industrial proximity sensor on ebay. I drilled the com end on the motor with a 50mm hole saw so as to expose the com end bearing. I epoxyed two little "buttons" made from some 6mm steel bar stock to the inner race of the bearing. Mounted the prox sensor on a little bracket so that the buttons swung past its face. Connected up a 1k resistor on the collector of the prox sensor output transistor as before and that was that.
This system has worked perfectly and feeds the dash tach and the controller rpm input (via a 4v7 zener). I do laugh sometimes when I see these rpm pickup systems for sale from some ev suppliers. Don't think my setup cost 10usd!
If anyone wants me to lookup a specific vehicle on autodata and provide pin data and signal levels for the tach i can do so.
Finally apologies to Freyguy for going off topic
edit: found a pic.