Why does the idle speed become unstable after turning on the air conditioning?

[RyanFitz]

Reasons for unstable idle speed after turning on the air conditioning are as follows: Engine control logic error: Unable to receive the signal indicating the air conditioning is turned on. When unable to determine whether the air conditioning is on, the Engine Control Unit (ECU) mistakenly assumes it is a temporary high load and attempts to compensate for the resistance at idle by increasing the engine 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 results in unstable idle speed. Air conditioning control circuit error: Unable to send the air conditioning activation signal. Similar to the first scenario, because the ECU cannot determine the air conditioning's status, it continuously adjusts the idle speed, leading to unstable idle. Air conditioning circuit malfunction: Causes intermittent operation of the air conditioning. When the air conditioning is off, the compressor belt runs without load, so it does not affect the engine load. The intermittent operation of the air conditioning compressor forces the engine to continuously adjust the idle speed, resulting in unstable idle.