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John, I recall the waveform at the coil -terminal as a negative-going pulse of at least 100V peak with a lot of ringing. Pretty typical looking for a points or transistor ignition. The 914 tach needs this high-voltage ratty-looking waveform to trigger, since it has some R-C filtering on the input to deal with it. I think most of the ECUs put out a nice clean low-voltage (logic level) square wave and the old tach won't respond to it. There are some tach converters that are supposed to convert the low voltage signal to something close to the stock ignition. I built a tach calibrator a few years ago and used a transistor to switch the secondary of a small transformer (a common 120V - 12V step down transformer hooked in reverse as a step up transformer). This worked well to get a high voltage ringing waveform from a 5V logic level clock signal.
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