Chevrolet Repair: charging

Question
I bought a used 2007 Chevy 1500 Silverado about 4 months ago and everything was fine. It was a hot day (90 deg) yesterday and I got into slow traffic and heard my AC fan slow down. Then I noticed the voltmeter was low. During the rest of the trip home the voltmeter went up and down a couple times. When I got home I used my DMM. The battery voltage was 12.49V when I started the truck it was fine over 14V after running a bit it dropped as low as 12.8V but at times can go over 14 and back down. I took it to a shop and was told by one person that it is computer controlled and that it could be either the alternator or computer. They advised me to go to the parts store where they put a tester on it and said the alternator is good but the voltmeter in the truck showed that during the test. They could not run the test when it went down because the truck would have to be restarted and it would check out good. Both places hesitated to say what was bad and that an alternator is about $200. Any idea on what is wrong or how to find out what is wrong?

Answer
   I have been repairing charging systems for decades now. It used to be that if a system lacked charge, that about 90 percent of the time the culprit was an alternator, maybe 5 percent the regulator. This was when most auto makers kept the alternator and regulator separate. Now days their integrated, so that in a way, it simplifies things. 95 percent of the time you will be removing and replacing the alternator with a re-manufactured, or new one.Do the basics; load test and check specific gravity of your battery.Check the belt for tension and wear.Look over the wires that attach to the alternator as well as the battery terminals. Make sure there isn't anything loose or corroded. Check fuses with a test light and replace as needed.After you have done this ,and all seems within limits, it's time to change out that alternator.
   Or if you just don't want to deal with having to do all of this, just get the alternator and stick it in.Just make sure of the drive belt quality and determine if it is good before you do it. The A/C runs off the same belt, and taking into consideration your story, that belt should be scrutinized until you are satisfied that it is not the cause of the problem. It may be slipping, so check the tension. If you were to pull on this  belt with a constant force of 70 to 80 lbs., and try to push it in the middle , that deflection is about what you should be getting with the belt being tensioned by the tensioner. If there is more deflection , this would mean it's too loose, and it could then be that the belt has stretched or worn to the point that the tensioner has no throw left to accommodate the extra slack ,or the tensioner is bad.    good luck