Voltage running high with engine running can happen if the voltage regulator looses it's ability ro regulate. That can be a regulator failure or sometimes a bad connection in the wiring to the alt/regulator can cause it to charge too high.
If you're showing <10 volts while running, the alternator is not charging at all. Again, bad wiring could cause such. I believe there is a fusable link in the output wire from the alternator. I had a Subaru that the output wire broke off the back of the alternator. I';d start out checking all connections and fuseable links. A schematic and voltmeter would be very handy, provided one is familiar with both.
I have found it rather common for the brushes to wear out in older alternators and that would certainly stop any charging from happening.
