Why does the idle speed decrease instead of increasing when the air conditioner is turned on?
3 Answers
The reasons why the idle speed decreases instead of increasing when the air conditioner is turned on are as follows: The engine control logic is faulty and cannot receive the signal of the air conditioner being turned on: When it 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 offset the resistance at idle by increasing the speed. When the engine speed drops, it cannot bear the load of the air conditioner compressor and increases the speed again. This back-and-forth results in unstable idle speed. The air conditioner control circuit is faulty and cannot send the signal of the air conditioner being turned on: Similar to the first reason, because it cannot determine the status of the air conditioner, the ECU continuously adjusts the idle speed, resulting in unstable idle speed. A fault in the air conditioner circuit causes the air conditioner to work intermittently: When the air conditioner is not working, the air conditioner compressor belt is idling 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, resulting in unstable idle speed.
I've repaired many cars with similar issues where the idle speed drops when the AC is turned on. The key is to observe the extent of the drop. If it falls below 500 RPM accompanied by noticeable shaking, it's likely due to failed engine load compensation. First, check the idle air control valve or throttle body for carbon buildup – older cars are particularly prone to sticking, which affects air intake. Next, inspect the AC compressor clutch and related circuits, including whether the pressure sensor signals are abnormal. There was a real case where poor contact in the pressure switch caused the ECU to misjudge that the AC wasn't activated, failing to increase fuel supply. Also worth checking are vacuum lines, especially if the intake manifold has been modified – even a thumb-sized leak can cause RPM to plummet. I strongly recommend connecting an OBD scanner to check the throttle opening and AC signal status in the data stream – this is much more efficient than blindly replacing parts.
Last time I helped my neighbor with this issue, the tachometer would drop straight down when the A/C was turned on. The core problem actually lies in the logic judgment of the engine control unit, which should increase RPM to compensate for compressor load but failed to do so. We focused on checking three areas: The simplest is to unplug the throttle body connector and relearn idle speed; then use a diagnostic tool to check if the A/C request signal reaches the ECU; finally test the degree of intake pressure sensor value drift. Experience shows that 70% of cases in older cars are caused by sticking idle air control valves, especially those with throttle bodies that haven't been cleaned for 100,000 km. Here's a lesser-known fact: Abnormal blower motor resistors in certain models can indirectly affect idle stability - while rare, don't forget to check them as well.
