Just pulled apart my tacho on my LX Torana today (SLR dash/gauges) as the tacho was reading too high with my MSD ignition box. Am very very curious to know how it works and would love to hear from anybody that could please shed some light on this. It is the original tacho and the circuit board on the back comprises a inductor (a iron cylinder with windings of enamelled copper wire inside), two signal diodes, a 7.5V Zener diode, two tantalum capacitors (1 for 6cyl select, other for 8cyl select) a 1W 330ohm resistor and a 2K ohm trimpot. Especially curious about what the inductor does in this circuit, and how such a simple circuit actually was working well with the tach output signal from the MSD (or -ve coil terminal in stock igntition system). All replies greatly appreciated.

Thanks,

Paul E