I have a 2007 monte carlo ss. When I first got it the alternator was bad. The first alternator I bought was used and it last maybe 3 weeks. Then I bought a new one and it has been on the car a couple months and now I'm getting the service charging system warning. What can cause alternators to keep failing.

 

The 2006 and 2007 Monte has something the earlier Montes did not. There is a voltage sensor on a wire to the battery. I unfortunately don't know much about it, but I would recommend researching that to see if it is the true cause of your issues.