Why does the car's idle speed drop when the air conditioning is turned on?

[KathrynRose]

The reasons for unstable idle speed when the air conditioning is turned on are as follows: Engine control logic error: When the system cannot determine whether the air conditioning is turned on, the engine control unit (ECU) mistakenly interprets it as a temporary high load and attempts to compensate for the resistance at idle by increasing the speed. When the engine speed drops, it cannot handle the load of the air conditioning compressor and increases the speed again. This back-and-forth adjustment manifests as unstable idle speed; Air conditioning control circuit error: Similar to the first point, because the system cannot determine the air conditioning's operating status, the ECU continuously adjusts the idle speed, resulting in unstable idle speed; Air conditioning circuit malfunction: When the air conditioning is not working, the air conditioning compressor belt runs without load, so it does not affect the engine load. The intermittent operation of the air conditioning compressor causes the engine to continuously adjust the idle speed, leading to unstable idle speed.