On Friday the tach on my 6 month old JD 3320 went south as well. Operation seemed normal except RPM would not register over 2300. At first I thought maybe some linkage had slipped, but as time progressed the actual tach readings became erratic - e.g. at quick idle reading showed 0 RPM.
Called the JD store to set up for appointment. Service guy tells me "most guys don't pay any attention to tach anyway" I say "sure when I'm paying the bill maybe I won't either, but seeing it's under warranty it's getting fixed."
Also need them to look at the Rockshaft - is about 3/8" short of full lift to insert pins to hold backhoe in place. Also needs to go down about 1" further so I-Match can engage pins on BB when sitting on flat level floor.