One of the main causes of a car stalling with the air conditioning on is a seized compressor. If there is squeaking, smoke, stagnation, and hot air coming out of the vents every time the air conditioner comes on, the compressor is probably locked up. If there is only a squeak from the belt but the car does not stop and the air conditioner gets cold, the belt or tensioner is most likely worn. The most likely cause of a car not idling high enough and sometimes stalling with the air conditioning on without any of the other issues mentioned can be an IAC (Idle Air Control) engine.
*Note that the compressor may be hot to the touch when you check it.
How to Check for a Blocked or Seized Compressor
- Try turning the clutch plate (at the compressor end).
- Using a socket on the nut that holds the clutch plate, see if it can be turned.
- Look for signs of clutch plate overheating; discoloration.
If the belt is loose, it may be due to a worn serpentine belt tensioner, but this alone will not make the car stop. A stuck IAC motor can cause the car to stall. An IAC motor usually builds up carbon which can cause it to seize. If the idle does not increase when the air conditioning is turned on, the engine may stall. Many times, if this is the problem, the IAC motor can be removed from the throttle body and cleaned with a brush and fuel injector spray and then reinstalled.
