hello, i just purchased a 05 avalanche Z71, everything was fine untill i was driving at night when i noticed the voltage guage was going from 12.5 to around 15 or 16 volts, all my lights were dimming and getting brighter, i took it back to the dealer and they tested the alternator which they said was good and producing 84 amps of current, then they had the battery on charge all day and that tested good. i'm no stranger to vehicles and electrical so after i picked the truck up and it started doing it again, so i checked the output of the alternator with my multimeter and i was getting a reading between 13.6-13.9 volts (always changing) with a few spikes of 15.5volts. i'm looking mainly for second opinions befor i head to the dealer in the morning but to me it sounds like the regulator in the alternator is shot...just a lil extra info, the alternator is a 140 AMP unit.