Why does the idle speed decrease instead of increase when the air conditioner is turned on?
2 Answers
The reasons why the idle speed decreases instead of increasing when the air conditioner is turned on are as follows: 1. Engine control logic error, unable to receive the signal of air conditioner activation: When the system cannot determine whether the air conditioner is turned 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 RPM. When the engine speed drops, it cannot handle the load from the air conditioner compressor and increases the RPM again. This back-and-forth adjustment manifests as unstable idle speed. 2. Air conditioner control circuit error, unable to send the air conditioner activation signal: Similar to the first point, because the system cannot determine the air conditioner's activation status, the ECU continuously adjusts the idle speed, resulting in unstable idle. 3. Air conditioner circuit malfunction, causing intermittent operation of the air conditioner: When the air conditioner is not working, the air conditioner compressor belt runs idle without load, so it does not affect the engine load. The intermittent operation of the air conditioner compressor causes the engine to continuously adjust the idle speed, leading to unstable idle.
I've been driving for over a decade and encountered similar situations. When the AC is on, the idle speed drops instead of rising, which is quite abnormal—normally, the idle speed should increase to compensate for the AC compressor's load. Possible causes include a clogged idle control valve stuck in a low position and unable to adjust the RPM higher, or a faulty mass airflow sensor providing incorrect intake measurements, leading the ECU to make wrong adjustments. Another possibility is a short circuit or loose connection in the AC system's signal line, preventing the engine control unit from receiving the command to increase speed. A vacuum hose leak could also cause a lean air-fuel mixture, resulting in unstable RPM. After the idle speed drops, the headlights dim, the AC cooling weakens, and the car might even stall at traffic lights, which is very dangerous. I recommend visiting a repair shop to check the sensors and wiring, and regularly cleaning the throttle body and idle control valve as preventive maintenance. Don’t underestimate this issue—prolonged neglect could increase fuel consumption and even damage the engine's lifespan.
