In response to an e-mail...
I've installed Nology coils with diodes but now my tacho doesn't work.
If the engine runs properly you've installed the coils and diodes correctly, so don't worry about that part.
Check the connector on the ignitor box. Look for bent pins, crud, etc. and if that's all OK reconnect it.
Remove the rear cover from the tacho. You should have three wires connecting to the back of it. Check that they're not loose or corroded. The wire labeled '+' is the +12V power to the tach, the 'I' wire should be the ground and the 'S' wire is the signal from the igniter.
Fire up the bike and check for +12V between the '+' and 'I' wires. If you don't have about 12 volts there's a wiring problem.
Switch your meter to 'AC Volts' and check from the 'S' terminal to the 'I' terminal. You should read a low AC voltage. Rev the engine and see if the voltage changes -- it should increase up the scale. If it increases your tacho is probably bad, but if it doesn't change then you've either got a wiring problem or a bad igniter box.
I haven't done these tests myself and since my bike is torn down right now, I can't give you a voltage to look for. Just try it out and let us know what you find.