Autosport Labs

I have a Fiat 128 with a Veglia Tacho in it and it runs off the -ve side of the std coil.
Having a few issues getting it to work correctly. At present I have taken two signal wires from the ford coil back to the tacho and built the little kit with the two diodes and a zender.Doesn't seem to matter what I do with this the tacho wont work. However if I connect both signal wires together with no diodes and connect that to the tacho it works perfectly and very accuratly. This completely stuffs up the spark though! .
The other thing I notice is the tacho gets 12.7 volts without the diodes and works but only get 11.6 volts with the diodes and zender. Could this have anything to do with it?
I have also tryed using the two diodes and omitting the zender.

Any help greatly appreciated

Hint: try putting a 100K reisistor in parallel with one of the diodes. That's what we use in our tachometer adapter board; we've seen it's needed for certain tachometer units.

Let us know what you find out!

Thanks for the hint Brent but no such luck. I now have the car runnng on the edis and the only way I can get the counter working is with one signal wire and this makes it run at 1/2 rpm.
Any other suggestions?
I have been keeping my eye out for another Tacho as well. Is there any particular spec I shoud be looking for to make things simple.