Why Car engine shake when AC is on at idle?

When the air conditioning system in a vehicle is turned on, it puts an extra load on the engine. This can cause the engine to vibrate or shake, especially at idle. This is because the air conditioning compressor is driven by the engine, and when it turns on, it draws power from the engine. When the compressor draws power from the engine it can sometimes take away enough power to cause it to momentarily bog down which cause the engine RPMs to drop. When this happens the engine will try and compensate for the drop in RPMs by increasing them thus creating the shaking affect. Additionally, the air conditioning system also cycles on and off to maintain the desired temperature in the cabin. This can also cause the engine to shake, as the load on the engine changes when the air conditioning system turns on and off. Another reason could be the idler pulley. The idler pulley helps keep the belt tight and in place. If the idler pulley is worn or damaged it can not keep the belt tight enough causing it slip. When the belt slips it create excessive vibration which is felt throughout the whole car.