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Engine running, the voltmeter should read 14.3 volts cold and this will drop to 13.3 volts when the alternator gets to its operating temperature. You can tell the state of charge in your battery by measuring the battery voltage at the battery terminals under no load. 12.0 volts and below, discharged, charge your battery up overnight with a battery charger. 12.9 volts and above, fully charged and linear in between. Tell any parts store what year vette you have and they can supply you with an alternator. You can also take your alternator to most parts stores for a free test.