I tried this a while back (about a year) but I couldn't get it to work. Had the zenner diode in reverse as per diagram and the two - pins of the coil along with their dioide. The only time the rev counter moved was when the engine cut out and the needle spiked a little. Otherwise it read 0 all the time. Tried 2 different sets of diodes, no change. Rev counter works but at half revs as expected. Any thoughts what I'm doing wrong?