Tac not working voltmeter went to 19 volts then rh@stayed ther just purchased 2004 gmc1500 sierra . tac@voltmeter siezed low. gm dealer did sweep volt showed16volts tac dead 2days later voltmeter went to19 then rh disconected battery shows12volts voltmeter stayed hard rh help 220000kms on truck canadian

Akternators for a 12 volt systum charge at 13.5 when running from 800rpm to max ??4000rpm and shouldent change as the alternator put out AC current and coverts to DC in the reg on alt but max of 13.5 is all you need at any time at all revs and the battery will not boil or over heat..
Make of truck dont make any differance and milage dosent either as its the alternator thats either faulty or regulator is.
Let me know how you get on Ron