What causes engine to squeal when turning ac on and alternator not to charge

How long since spark plugs changed?
air conditioner works well? Maybe compressor going bad causing excessive load on engine bad electric clutch on compressor drawing down charging system .
Alternator ok?
battery condition? 13.6v DC or more

Can you provide more details, type of vehicle, only acts up with a/c on, if you turn a/c off and turn the defrost on, sometimes the manufacturer will have the a/c compressor cycle, does the vehicle still act up?

If the belt is not turning, there will be a seized pulley. Possible alternator.. power steering.... water pump... tensioner.... idler pulley ... AC compressor seized. The belt will need to be removed and all the pulleys spun by hand to isolate tone that may be seized

Are you certain that it's the clutch assembly?
Check the idler pulley. Poor tension in the idler pulley spring will cause the belt to squeal AT THE ALTERNATOR, and it may sound like it's coming from the AC.
The AC pulley typically "freewheels" when the AC is not on and the engine is running.
Have someone turn the AC on and off while the engine is running and watch the operation of the AC clutch. If it's working properly, the clutch should "click" and the back part of the pulley should start turning or try to turn.
If the squealing noise continues when the clutch is freewheeling, it's probably the idler pulley. If the AC Pulley won't start freewheeling, it's probably the clutch.
To remove the clutch system,

• disengage the idler pulley and remove the serpentine belt from one of the pulleys.
• rent, purchase or borrow a puller from a local auto parts retailer (Auto Zone and O'Reilly auto parts are both helpful)
• some of the clutch systems have retaining bolts, some are just pressed in, figure out what you have before removing the clutch assembly using the puller.
• The puller is also designed to help put the NEW clutch assembly back on the shaft.

The idler pulley assembly is a lot easier to replace.

sounds like a/c compressor starting to fail. hope this helps.

sounds like the belt needs tightening or replacing so yes you are indeed correct about the squeal, as for the rumble depends on how bad the rumble is but it shouldnt be caused by either you mentioned.

a bad belt will cause this concern

When the AC compressor kicks on, an electrical load is placed on the alternator and the drive belt that turns the alternator pulley, if not tight enough, can cause the belt to squeal. Replacing the belt should help with that noise if the belt is stretched from wear. The screeching noise can be from a bad idler pulley, a bad belt tensioner pulley or a bad alternator pulley or a failing clutch on a very old ac compressor and rarely a water pump pulley. To check pulleys remove the drive belt and turn them individually to see if they can be tilted up down or sideways with play in the "bearing" of the pulley. Also, you may wish to visually inspect the clutch on the ac and look for melted black rubber around the bolts that secure the clutch to the compressor (stinks,smokes and melts these rubber areas if ac compressor clutch fails).

Before anything is done to the AC, have the throttle body cleaned. A dirty throttle body will cause engine RPM to drop or even stall.
What makes you think the compressor is seized? Does the compressor pulley spin when AC is engaged? If compressor was seized the pulley would not move and belt would squeal.

if this happens while the defrost of ac is turned on, it could be a seized ac compressor which will cause the serpentine belt to squeal and drag making the engine sometimes stall. if it happens with the ac off and the selector on say the floor, or dash vents, the issue could be a bad alternator and battery. this is confirmed when the blower motor engages and the lights dim and then the truck goes dead.