Chevy G20 Van

Clean the throttle body! GM issued a bulletin (more than once) that running the AC can cause an engine with a dirty throttle body to stall. Per GM: CONDITION: Engine may stall with the air-conditioner on and or while adding additional loads to the engine at idle. An example of an addition load would be turning the steering wheel left or right with the air-conditioning on. This type of stall usually occurs during idle or slow parking lot maneuvers. This concern may not set any diagnostic trouble codes. RECOMMENDATION: Clean the throttle body bore and the throttle valve plate using a clean shop towel with GM top engine cleaner, GM P/N 1052626 (Canadian P/N 993026) or AC-Delco Carburetor Tune-Up Conditioner, P/N X66-P, or an equivalent product. My .02: The PCV and EGR dump right into the throttle body and they can gunk it up bad, so clean or replace the PCV valve, all filters, use a MAF/MAP-approved cleaner for the MAF or MAP. If plugs/wires/coils look old, may also want to consider a tune up. That should take care of stalling when switching on AC.

Sounds like you have a bad A.C compressor clutch or compressor seizing up.