For those of you following an earlier thread which, I can no longer update.... I had a problem with a my 99 Silverado pickup, where the Battery Warning light would come on from time to time and only a few seconds at a time. This was after the alternator was replaced as a PM measure as the truck had 80k and I was going on a long trip. The instrument panel voltmeter still was showing 14.5 or so volts all the time the warning light was on. I replaced a 3 year old battery because a GM service tech told me that was all he could find wrong. Needless to say it did not solve the problem, he also told me he could not pull any codes.
So after weeks of putting up with the problem I went to a GM service center who had the proper ($3,700) scan tool, he told me I had a Alternator "L" wire fault showing. After about two weeks, and putting a monitor wire inside the truck cab so I could read the "L" wire going to the PCM on the road and underload with a DVM, I found the new AutoZone alternator to have a defective electronic circuit that switched (grounds) the "L": wire (on a fault), but the alternator tested fine. AZ replaced it under warranty, and after 700 plus road miles the Battery Warning light is no longer a problem. Thanks again to all those who replied to my other message.