Why does the engine speed fluctuate when the air conditioning is turned on?
3 Answers
It is due to an error in the engine control logic. Here is a detailed explanation of why the engine speed fluctuates when the air conditioning is turned on: 1. Engine control logic error: Unable to receive the signal indicating the air conditioning is turned on. When the system cannot 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 idle resistance by increasing the speed. When the engine speed drops, it cannot handle the load from the air conditioning compressor, causing the speed to increase again. This results in unstable idle speed. 2. Air conditioning control circuit error: Unable to send the signal indicating the air conditioning is turned on. Similar to the first point, because the system cannot determine the air conditioning status, the ECU continuously adjusts the idle speed, leading to unstable idle. 3. Air conditioning circuit malfunction: Causes intermittent operation of the air conditioning. 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, resulting in unstable idle.
With over a decade of experience in automotive repair, I'd say this issue is most likely caused by a faulty idle control system. When the AC compressor engages, it increases engine load. If the idle control valve or sensors (like the idle air control motor) are stuck or dirty, it can cause RPM fluctuations and engine vibration. Common culprits include severe throttle body carbon buildup, slipping AC compressor clutch, vacuum line leaks, or aging wiring leading to unstable signals. My recommendation is to first inspect the idle control valve and throttle body cleanliness, then use a diagnostic scanner to read trouble codes for confirmation. Regular maintenance—including cleaning the intake system and inspecting sensors—can effectively prevent such issues and avoid safety hazards like stalling in severe cases.
I encountered a similar situation with my old car - the RPM would fluctuate wildly when the AC was turned on, which was really annoying. Later, I found out the idle speed motor was faulty, and it improved after I replaced it myself. It could also be due to insufficient engine power, where the system can't handle the increased load from the AC. Other common causes include excessive carbon buildup in the throttle body or excessively high AC system pressure, making it hard for the system to adjust. I recommend regularly checking if the RPM is stable during cold starts, promptly cleaning related components, or seeking professional inspection and repair to avoid damaging more parts over time.
