The tack is reading high and going higher. I know this is a voltage from the alternator that tracks the rpm. Is there a way to fix the alternator so voltage is correct? thanks
The correct voltage for the tach should be 8 volts AC at the "W" terminal. The tach uses one leg of the alternator unrectified thusly 8 volts AC, however when there is a problem with the tach it can indicate an alternator problem. Test your alternator DC output but also check for AC current an AC ripple across batteries can cause strange things.
The alternator more technically reads the frequency of the AC from the alternator. Some tachs have an adjustment, that's probably much less likely than the alternator to be the issue. Possibly "dirty" output from the alternator is being misread as a higher frequency by the tach? You can buy a rebuild kit for an alternator pretty cheap, and it's easy to change parts, most cases.
Is this the original tacho or an aftermarket item?