Changing a Belt Tensioner

Hey there.

One of my co-workers just had the AC replaced in her 2001 Cavalier. For whatever reason since then, the belt has begun slipping off. She took the car back to the mechanic and he informed her that the tensioner needed to be replaced and quoted about $150. Basically when the car is running, the tensioner shakes pretty bad, seperate from the normal engine shake. The belt's tight and doesn't have a lot of play in it when you press down on it. I'm used to my Ford where the tension was adjusted manually and my first thought was that the bolt actually holding the tensioner on was loose, but I didn't get a chance to check it not to mention it could be very different for a Chevy.

My first questions is, does this sound like a reasonable cause for her problem. The two times it's occured, she's been driving and the AC would cut off, then her lights, etc. I'm not sure why this wouldn't have come up prior to him replacing the AC though he apparently changed the belt, too.

My second question is, does anyone know if there's a special tool required to replace it or what's involved. I'd like to help her save some cash (part's only about $40) if I can do it myself and it is in fact the cause.

Thanks for your time...

First I would make sure the mechanic routed the belt properly. Then I would remove the belt and check the tensioner for any play. I am not sure if there are any special tools required for the repair tho. Sorry.