The tach is most likely driven by the alternator
on the engine
. If the solar
has got the batteries
to the full point and it keeping the voltage above the alternator
regulator's setpoint then the alternator will stop putting out power and the tach will stop reading (or read erratically).
As Sailmonkey says, you can turn on a load to pull down the voltage. Or ignore the tach, do you change throttle much once running? Or simply disconnect the solar
(either on the panel side or on the battery
side of any controller).