I've heard that if you run the +ve to the coil(s) through the pick up on the back of the tacho it will work. I had problems with my tacho and converted it to run off a tach voltage signal. I wound a coil on some small black water pipe, fitted it over the pick up and drove it with a bc337 through a wire wound resistor. I went for (from memory) 100 turns of the coil and 60mA to get roughly the same field strength. I uploaded some pictures and would be happy to sketch up a wiring diagram. It works running off the tach output of my hei.