Why does the car air conditioner not cool at high speed but cool at low speed?
3 Answers
When the car is running at high speed, the air conditioner does not cool, there are two reasons. The specific introduction is as follows: 1. Insufficient refrigerant: Insufficient or too little refrigerant in the car air conditioner can cause the air conditioner not to cool. The main reason for insufficient refrigerant is usually a slight leakage in the system. If there is insufficient refrigerant in the air conditioning system, the amount of refrigerant sprayed into the evaporator from the expansion valve will also decrease, and the heat absorbed during evaporation in the evaporator will consequently decrease, leading to reduced cooling capacity. If the refrigerant is insufficient, it should be replenished. 2. Moisture intrusion: The presence of moisture in the car air conditioning refrigeration system can cause insufficient cooling or no cooling. If the lack of cooling is due to moisture in the refrigeration system, then the desiccant or dryer in the car air conditioner should be replaced. At the same time, the system should be evacuated and recharged with an appropriate amount of new refrigerant.
I've repaired many cars with similar issues where the AC fails to cool at high speeds but works at low speeds. This is usually related to changes in system pressure. For example, insufficient refrigerant or a leak can cause the compressor to barely squeeze out cold air at low speeds, but at high speeds, the engine RPM increases, and the system pressure can't keep up, causing the AC to fail. Worn-out compressors are also a common cause—they may hold up at low speeds, but at high speeds, insufficient internal lubrication or belt slippage directly impacts cooling performance. Another issue could be poor condenser heat dissipation. At low speeds, natural airflow might suffice, but at high speeds, if the condenser surface is clogged or dirty, heat dissipation efficiency drops, and temperatures won't decrease. A faulty expansion valve may also disrupt refrigerant flow at high RPMs. I recommend not pushing it—driving at high speeds with a non-functional AC can be distracting and affect safety. Get it checked by a professional shop for refrigerant levels, leaks, and condenser cleaning. Regular AC maintenance, like recharging refrigerant before summer each year, can prevent problems, saving both money and hassle.
Last time I was on the highway, the AC suddenly stopped working, but it was fine when driving in the city—it was really uncomfortable. Later, the mechanic found a severe refrigerant leak; the system pressure collapsed at high speeds, causing the AC to fail. It could also be that the compressor's belt loosened or internal parts failed under high RPM, making it unable to keep up. Or the fan motor might have issues—at low speeds, airflow cooling works fine, but at high speeds, uneven cooling makes the AC worse. Don’t ignore this—driving on the highway without AC can lead to heat exhaustion and increase the risk of fatigue. A quick fix is to open windows for ventilation or check if the AC filter is clogged and clean it. If that doesn’t work, head to a repair shop ASAP—don’t wait until it completely breaks down. During regular maintenance, remind the mechanic to inspect AC lines and components—this habit can prevent many unexpected issues.
