Why does the car's idle speed drop when the air conditioning is turned on?
4 Answers
The reasons for unstable idle speed when the air conditioning is turned on are as follows: Engine control logic error: When the system cannot determine whether the air conditioning is turned on, the engine control unit (ECU) mistakenly interprets it as a temporary high load and attempts to compensate for the resistance at idle by increasing the 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 manifests as unstable idle speed; Air conditioning control circuit error: Similar to the first point, because the system cannot determine the air conditioning's operating status, the ECU continuously adjusts the idle speed, resulting in unstable idle speed; Air conditioning circuit malfunction: 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, leading to unstable idle speed.
I've encountered a similar situation where the idle speed drops when the AC is on, likely because the compressor activation increases the engine load. Normally, the engine maintains a stable idle speed, but once the AC is turned on, the compressor starts working, drawing power and causing the idle control system to lag in adjustment. The issue might lie with the idle control valve, which is responsible for maintaining speed balance. If it's contaminated with oil or worn out, its response slows down. Alternatively, inaccurate sensor readings could prevent the electronic control unit from promptly increasing the RPM to compensate. Additionally, a clogged air filter restricting airflow can also make the engine struggle. I recommend first testing whether the idle speed returns to normal when the AC is off during idle. Then, check the cleanliness of the idle control motor or visit a professional shop for a diagnostic scan. This can help avoid long-term issues affecting fuel efficiency and safety. Remember, unstable idle speeds can lead to stalling at intersections, which is quite dangerous, so don't skip regular maintenance.
Last week while driving on the highway, I noticed the tachometer dropping sharply after turning on the AC - from 800 RPM down to 500, almost stalling the engine. As an experienced driver, I suspect engine overload - the compressor consumes power for cooling. Possible causes: poorly adjusted idle control components (like throttle valve carbon buildup or loose plugs) or unstable alternator output causing voltage fluctuations that disrupt idle. Quick fix? Pop the hood after parking, listen for abnormal compressor noises, then unplug and reconnect the MAF sensor to reset it. If the issue persists, don't ignore it - I once delayed repairs and ended up spending big on an idle control valve replacement. AC-induced RPM drop is an early warning; never let your car limp along with underlying issues.
It's quite rewarding to troubleshoot the issue of engine RPM drop when the AC is turned on by yourself. Once the compressor kicks in, the sudden increase in power consumption and engine load can easily cause RPM fluctuations. Start with simple checks: first, clean the idle control valve, as carbon buildup can cause it to stick; then inspect if the air filter is too clogged—replacing it is inexpensive. Check for loose battery terminals, as unstable voltage can affect regulation. Online tutorials show how to measure compressor resistance with a multimeter—normal readings should be in the tens of ohms. For electrical faults like blown fuses, simply replace them. Learning to diagnose such minor issues can save money, but don't push your luck—seek professional help immediately if severe vibration occurs.
