Hi Chuck,
The voltage gauge indicates Voltage in the entire circuit. When the Aternator is charging the system you should see about 13 ~ 14 Volts register on the gage.
If you have a volt meter, put it across the battery and take a read, then start the car and read again, the voltage at the battery should be the same as at the dash voltmeter

If the voltage doesn't change, with the car running disconnect the alternator plug and then reconnect and see if there is any difference.
It's not uncommon for those old 3 wire plugs to have bad contacts, or even wire break inside the plug itself
Bud