What Causes Fluctuating Idle Speed When the Car AC is On?

[DeZoe]

Here are the reasons for fluctuating idle speed when the car AC is on: 1. Engine control logic error: When unable to determine whether the AC is on, the engine control unit (ECU) may mistakenly interpret it as a temporary high load, attempting to compensate by increasing RPM to offset idle resistance. When engine speed drops, it cannot handle the AC compressor load, causing RPM to rise again. This back-and-forth results in unstable idle. 2. AC control circuit error: Similarly to the first point, because the system cannot determine AC activation status, the ECU continuously adjusts idle speed, manifesting as unstable idle. 3. AC circuit malfunction: When the AC is off, the compressor belt runs without load, thus not affecting engine load. The AC compressor's intermittent operation forces continuous idle speed adjustments, showing as unstable idle.