Serpentine Belt coming off...
 

I replaced the alternator on my 850 non-turbo and put everything back together, but the serpentine belt starts to scoot off the alternator to one side right after start-up.

I thought it might be the tensioner, so I replaced it, but it still does the same thing. Looking at it now, the idler pulley looks a little off of everything else, but I can't understand how that would have happened - it doesn't feel loose or anything. PLEASE help me out with any ideas. Thanks.

the pully might not be right on the alternator.... check the rib count, and also check if it is in the same position ( in out wise) as the old one...

belt will probably now be damaged as well.

harmonic balancer
 

this can also be caused by a loose or damaged harmonic balancer. I had this problem with my car. It is worth taking a look at.