Hi There,

Its been a very long time since my car used a standard coil and dizzy, did the standard tacho connect directly to the coil? (if not ignore the next bit!).

If it did (and some cars do) you can try removing the zener and replacing it with a resisor of between 330R and 1K, the zener is not actually there to limit voltage to 18V, it is there to block the 12V present at steady state. Further, the IN4004 diodes will not really conduct well into an open circuit (which you effectively have with the zener there).

An alternatve approach would mean adding a resistor of between 1 and 10K after the two IN4004 diodes to ground BEFORE the zener, this will allow current to flow through the IN4004's properly and should allow the zener to break down as you expect.

Your other option is to swap to an EFI rev counter and drive via the ecu OR drive the MFI rev counter via a relay coil from the ecu tach output, as far as I know this is how Karl does it and he has converted more RST's to MS than anyone else I can think of!

If you get stuck I can draw you some pictures.

Best of luck

Rob,