another thought... did the new alternator come with a clutch pulley?

 

It didnt come with anything but the alternator

 

So, I'm assuming you moved the pulley from the stock alternator onto the new alternator? (stock alternator has a clutch pulley on it)

 

Ya I just left it on and put it on the new one

 

My car does the same thing. The aftermarket alternator I went with does not have a clutch pulley, so when you shift rapidly the crank (and thereby belt) speed drop, while the alternator shaft speed remains high. The squealing is the shaft slipping until the friction slows it down. Either find a clutch pulley that will fit your shaft and have it put on, or replace the alternator with one with a clutch. Or leave it as is and deal with it. It isn't gonna hurt anything. My belt's got about 50K on it doing that since the Alternator install. Doesn't so much as squeek otherwise.

 

Is the altenator pulley clutch seized?

 

I had recurring cold start squeaking issues. Tried everything, new idlers, thumprrr tensionser, etc. The only thing that seemed to work was going to a Gatorback Belt. Has not squeaked in over a year.
(BTW, its an auto, so no alt clutch)

 

The aftermarket alternator does not have a OWC or one way clutch
like the OEM alternator. Did you go with a www.PAperformance.com
alternator or an off brand from the stereo store?.
The moving mass of the alternator keeps going when the RPMs drop
and your belt is slipping (screeching) because it cannot slow down
the alternator fast enough...
A new serpentine belt will be in your near future.
Better carry a spare belt and the tools needed to replace it when it snaps.