my firebird was running at 10 volts at the battery and the gauge was needled out low inside. i tore off the altenator and brought it into oreillys, it showed no voltage on the bench and you would have to keep fielding it to make it charge and it wouldn't energize, i didn't want to play with tearing it apart when i found out the diode trio was $18 and the regulator was $12, so i let the local rebuilder fix it, he replaced the diode trio but left the regulator, today when i put it on the battery was completely discharged, so i jumped it and let it run for 20 minutes, the altenator got crazy hot, it ran at 13 volts at idle but would jump up to 15 volts at 2000rpm, its normal for the altenator to get hot like that, but is it normal for the voltage to climb that high in these cars?

89 lo3
100 amp altenator?

 

Yep, sounds pretty normal. Usually it should run at mid 14 volts most of the time, but at idle if you're over 12.5 then it's charging - so you're fine. I wouldn't trust the gauge in the dash too far either. (Or are you using a voltmeter?)

The battery was dead right? So your alternator was probably being loaded somewhat heavily, so it'd get warm.
From now on when your battery is still 90% charged when you start your car, and you're just pulling a trickle current to maintain the charge and run the ignition at idle - then it won't get that hot. Warm, but not "burn your hands hot".