* The AC compressor is putting a load on the engine. When the AC is on, the compressor is working to compress the refrigerant and circulate it through the system. This puts a load on the engine, which can cause it to vibrate.
* The engine is running at a lower RPM when idling. When you are idling, the engine is running at a lower RPM than when you are driving. This can cause the engine to be less stable and more prone to vibration.
* The AC condenser is located in front of the radiator. When the AC is on, the condenser is working to dissipate heat from the refrigerant. This can cause the air in front of the radiator to be heated up, which can reduce the efficiency of the cooling system. This can cause the engine to overheat and vibrate.
* The serpentine belt may be loose or damaged. The serpentine belt is responsible for driving the AC compressor, alternator, and water pump when idling. A loose or damaged belt can cause vibrations and shaking.
If your car is shaking when you are idling with the AC on, you should take it to a mechanic to have it diagnosed.
