Anyone lucky getting it to work?

I tried the diode set up (what actually worked on a boa v6) and the tach fix (tach adapter) aswell the relais solution. No luck at all.

The revv counter actually works connected to one negative coil side (half speed).

Someone on www.volvoforums.org.uk is running wasted spark on their 940

http://www.volvoforums.org.uk/showthread.php?t=147900

And is he firing the original tacho?

I've yet to do this but I believe the simple solution is to retain the original logic signal and single channel amp to work in paralell with the micro and dual channel amp.

And how does this schematically look like? special parts used?

No special parts, I think the original circuit is wired to the original amp and the wasted spark pcb in paralell. Pretty much the original circuit to the tacho, but the coil is not needed.