Why does the AC get cold when accelerating but not when idling?
2 Answers
Introduction to the reasons why the AC is not cold at idle but becomes cold when accelerating: 1. Belt slipping: Since the AC compressor is driven by the crankshaft via a belt. When the belt slips, the transmission efficiency of the AC compressor decreases, and the most obvious symptom is that the AC cooling effect is poor or even completely non-functional at engine idle or low RPM. Only when the accelerator is pressed and the engine speed increases does the cooling gradually recover. 2. AC compressor damage: The operation and idling of the AC compressor are controlled by its internal electromagnetic clutch. If the electromagnetic clutch malfunctions, it may cause the AC to not cool at idle. Similarly, when the engine speed increases, the cooling effect will slowly recover.
I've been driving for over a decade and encountered this AC issue before. The AC blows cold when accelerating, but hot air when idling, mainly because the engine RPM is too low at idle to provide sufficient power to drive the compressor, drastically reducing cooling efficiency. If there's a slight refrigerant leak or insufficient charge, the pressure can't keep up, and the AC system struggles. An aging compressor with excessive internal clearance can also cause this—it can't function properly at idle and only works marginally at higher RPMs. A loose or worn belt affects power transmission efficiency too. I remember once my car had a similar problem, and refilling the refrigerant fixed it, but severe cases may require checking the compressor internally. Additionally, a dirty or clogged radiator hinders heat exchange, making idle cooling even worse—regular cleaning helps.
