I have a 1979 vette with 54K miles, had the vehicle for 15 years. Volt gauge reads at 18v always. I had changed the gauge, first from an aftermarket, then from a GM dealership. No change. I replaced the alternator from GM dealer, still no change. I checked the volts from the alternator and read 14.2v when engine running. I haven't had any electrical problems or dead battery. Any suggestions??