I have the same problem on my lads bike and the unit i bought as a replacement was stated as working, so i am going to look at the feed signal for the tacho, which i think comes direct off the coil, if this is faulty then no new unit will work. most electronic tachos take a pulse feed from the HT lead as a signal across an induced magnetis field, hence the voltage to the tacho unit. I will let you know how i get on.
Fast4forty, February 2011