Q: Car shakes with air conditioning (AC) on

The air conditioning compressor adds significant load to the engine. The compressor can be considered a small engine itself. Much in the way pistons compress air and fuel to power the vehicle, the internals of the a/c compressor compress refrigerant in the system to change the state of refrigerant from liquid to gas and back. If not for the engine control unit using things like the idle air control valve to increase the idle speed, the a/c compressor would put enough of a load on the engine to reduce the idle significantly, even make it stall. A number of components can cause the shaking you feel, but since it’s more pronounced when the a/c is on, I’ll focus there.

It is possible the a/c compressor may need replacement. A certified technician can inspect the compressor and monitor the pressure readings on the high and low sides of the system. If the pressure readings are not steady, it would indicate the compressor is worn internally and needs replacement. If the compressor is worn and the high and low side pressure readings are bouncing around, this can make the entire car shake when the compressor is running. Another possibility is weak/worn engine and/or transmission mounts. The mounts are under a lot of strain. They keep the engine and transmission in place to maintain correct suspension angles, keep things like wiring and hoses from constant flexing when the powertrain rocking back and forth, as well as isolate the passengers from vibration and noise. If the mounts are weak or any one of them are actually broken, more vibration and noise can be transmitted throughout the vehicle, especially with the added stress of the a/c compressor running.