This is a common issue with age on our cars. If it is not the issue that Hot Air mentions above, then most likely the volts light circuit which receives power from the brown wire off back of the alt is the culprit. Happened to my car as well. Our cars only have that one brown wire to the dash and if that goes, the battery won't charge.
I found this through a search to fix my own "no charging" problem, which it did. I am not sure of the author, he can be found through a search here. Anyhow below is what I followed and it worked dandy. I did the parts store plug he mentions, but you can simply buy the Casper's "Field Fix" harness as a plug n' play option.
Fixing no-charge battery problem:
The best alternator upgrade is to re-wire the 4 terminal connector to the alternator. Stock it has the single brown wire that goes to the dash light. Lose that and the alternator stops charging. So, grab another pigtail that has at least three wires/connections: the S, F, and L terminals (letters are molded into the connector body).
The L terminal stays the same, it is the brown wire going to the dash light.
The F terminal, wire it to a PNK/BLK wire that is hot at key-on. Can get it from the EGR solenoid or wastegate connector (among other areas).
The S terminal, grab either of the large feed wires that used to go to the fan delay relay, or the PowerMaster. These wires go directly to the starter power and then on to the battery.
Powering the F terminal will cause the alternator to charge even if the dash lamp feed is lost. This corrects the most common cause of no charge from the alternator.
Connecting the S terminal will cause the alternator to sense the vehicle voltage at the starter power feed. This makes up for the voltage drop between the alternator and that point. Will typically see at least a 0.5 volt increase in overall system voltage.
I did this on both of my turbo cars a few years ago. On cold startup my voltage is in the 14.2-14.5V range. After the car is warm the voltage stays in the 13.7-13.8V range no matter how many electrical accessories are running. I also moved the power feed lines off of the starter and moved them to a terminal block mounted on the firewall. I then ran a #8 gauge wire from the alternator to the terminal block and another #4 gauge wire from the terminal block to the battery. This eliminated the wire running under the front of the motor from the alternator to the battery.
Wiring the F (Field) terminal to IGN+ switched is the same as Casper's field fix. The V-plus volt booster is entirely different from what I posted.
Wiring the S (Sense) terminal of the alternator changes the point in the electrical system where the voltage regulator senses the voltage. Since the voltage at the starter is going to be lower then what the alternator otherwise sees, causes it to bump up the voltage.
This produces a higher and more stable voltage across the entire vehicle.
The changes I listed were also done with other GM vehicles, by GM.
