Depending on make, model, and year of tractor tachometer and hour meter may be electronic and driven by input from tachometer. Many times if there is an alternator problem the tachometer operation is the first sign. I would test for charging voltage at battery with tractor running. If it's showing 14.0-14.2 volts then alternator and regulator is working properly. If that's okay then check wiring from alternator to tachometer. If that's okay and your hour meter is recording engine hours when it's running your tach might be defective. Wiring diagram can help you trace the circuits to check if needed.