What Causes High RPM When the AC is On?
4 Answers
High RPM when the AC is on can be caused by the following reasons: 1. Engine control logic error, unable to receive the AC activation signal: When the system cannot determine whether the AC is on, the Engine Control Unit (ECU) mistakenly interprets it as a temporary high load and attempts to compensate for idle resistance by increasing RPM. When the engine RPM drops, it cannot handle the load from the AC compressor, causing it to increase RPM again. This back-and-forth results in unstable idle. 2. AC control circuit error, unable to send the AC activation signal: Similar to the first point, since the system cannot determine the AC's status, the ECU continuously adjusts the idle RPM, leading to unstable idle. 3. AC circuit malfunction, causing intermittent AC operation: When the AC is off, the AC compressor belt runs without load, so it does not affect the engine load. However, intermittent operation of the AC compressor forces the engine to continuously adjust idle RPM, resulting in unstable idle.
I totally understand the issue of high RPM when the AC is on. Last summer during traffic congestion, I clearly felt the car shaking badly. Actually, after the AC compressor kicks in, the engine has to work harder to compensate for the power loss. It's normal for the RPM to rise by 100-200, but if it surges above 300, the problem might be insufficient power or cooling system issues. My old car had severe carbon buildup in the throttle body, and after cleaning it, the RPM stabilized. Another time, I found the alternator belt was loose and slipping, and replacing it solved the problem. Also, check if the air filter is clogged, as this can reduce air intake and cause unstable RPM. Lastly, a reminder to everyone: if you experience continuous high RPM accompanied by strange noises, get it checked immediately. Don't be like me, pushing it until breaking down halfway.
With over a decade of car repair experience, I've encountered numerous cases of abnormal RPM increase when the AC is turned on. The core principle is that the engine needs to additionally drive the AC compressor, and the ECU automatically raises the RPM to maintain power. However, if the increase is excessive, there are typically four common issues: First, a clogged AC high-pressure valve or excessive refrigerant can double the compressor's load. Second, engine carbon buildup affects throttle response. Third, a vacuum tube leak disrupts airflow velocity. Fourth, an aging battery causes unstable power supply. I once handled a case where a faulty coolant temperature sensor caused the ECU to misjudge cooling demands and drastically increase RPM. I recommend car owners prioritize checking radiator cleanliness, as dirt can impair AC condensation efficiency, forcing the engine to compensate with extra effort.
As a female car owner, I initially thought it was normal for the engine RPM to increase when the air conditioning was turned on. Later, the technician explained that a small increase of around 100 RPM is normal for small-displacement cars when the AC is on, but a sudden surge to 1500 RPM like I experienced was abnormal. The cause could be abnormal pressure in the air conditioning system or low battery voltage. Another time during maintenance, carbon buildup was found in the throttle body, and cleaning it had an immediate effect. It's recommended to regularly check the refrigerant level, as too much or too little can affect the compressor. Also, pay attention to whether the cooling fan is functioning properly, as poor heat dissipation in summer traffic jams can put extra strain on the engine. I've since developed the habit of letting the engine run for a few minutes before turning on the AC to reduce the impact of sudden load.
