BEFORE YOU GO OUT AND SPEND ANY MONEY... what is your Tach doing??

My Tach is over 10 years old and working just fine, the days when it was acting odd included when I had it in the 3 bladed position when I was working on a two bladed prop, and then there was the day that the battery was failing (I got irratic readings).

Once I had the switch half way between the on and off settings [:@]

Bottom line here, double check the easy stuff before you pull out your wallet.