Tach problems

[Morris]

I have an RVC tach (the one that came in my '79) that is non-functioning with my EDIS set up. It was working with my previous electronic ignition system. I have tried several suggestions I have read on other forums, but nothing has worked so far. So far I have tried: connecting directly to the tach out line on the edis (many people report success with this method and an RVC tach); connecting to a circuit made of two diodes and a zener diode that connect to each coil bank ground; and a 680 ohm pull up resistor between the 12v in on the tach and the tach signal from the EDIS. The last one kind of works... it reads at idle but drops to 0 when I rev.

Any suggestions, thoughts, RVC wisdom or general electronics wisdom would be much appreciated.

 

[dklawson]

It sounds like you have tried everything I could suggest. If Sarastro (Steve) doesn't reply... I'd send him a PM and see if he has any suggestions.

 

[Morris]

Check this out:
https://www.team.net/TR8/tr8cca/wedgelab/other/tach/tach.htm

Everything you wanted to know about RVC tachs and more.

 

[Sarastro]

I was looking at this earlier and trying to think of something, but I suspect it might be a lost cause.

I don't know a whole lot specifically about the RVC tachs. They are voltage triggered, which is good, in my opinion, as far as it goes. The problem is that the conventional points ignition creates a big blast of voltage at the points, a pulse of about 150-200 Volts as I measured it once on my Bugeye, which lasts the better part of a millisecond. Electronic ignitions are very different. The pulse can be almost anything, but on my Crane it was about 400 volts but a series of two or three (can't remember exactly) very fast pulses. The tach, being designed for the points ignition, just might not be able to trigger from this.

Added to this is the fact that I've never seen a circuit diagram for an RVC; one book says they use integrated circuits that are no longer made and are not documented. So, it's difficult to come up with any ideas that might make it work with the new ignition.

I suspect you might be stuck for using a modern tach or you might try putting the "guts" of a modern one in an older tach. My solution was just to make new electronics for the older tach, but of course that requires some electronic knowledge that not everyone has. Even then, it wasn't easy to get the thing to trigger off the electronic ignition.

 

[Morris]

Any opinions on this circuit?

https://josepino.com/circuits/tachometer-circuit