I have a 2000 5.7L Firebird. Battery was not charging so had the alternator checked. Still good, but replaced it and the battery at the same time. Still won't charge. Checked cables, grounds, fuses, etc. Any suggestions?
Does the battery warning light turn on when the engine is not running? If not, there is a break in the wire going to the generator. That is the turn-on signal for the built-in voltage regulator.
How do you know it isn't charging? Battery light on? Fails to crank? If you have an inexpensive digital voltmeter, measure battery voltage while the engine is idling. It must be between 13.75 and 14.75 volts.
It is real common to go through four to six GM generators in the life of the vehicle. What more and more professionals are finding out is to reduce the repeat failures, install a new battery at the same time, as you did. As they age, their "internal resistance" goes up and they lose their ability to absorb the huge voltage spikes produced by this terrible generator design. The old battery will work fine in pre-1987 vehicles that had the much more reliable generator that was used since the early 1970s.