What Causes Car AC to Not Cool at High Speed but Cool at Low Speed?
2 Answers
When a car's air conditioning fails to cool at high speeds, there are two main reasons. The details are as follows: 1. Insufficient Refrigerant: A lack or shortage of refrigerant in the car's AC system can cause it to not cool properly. The primary cause of insufficient refrigerant is usually minor leaks in the system. If there is not enough refrigerant in the AC system, the amount of refrigerant sprayed into the evaporator from the expansion valve will also decrease. Consequently, the heat absorbed during the evaporation of the refrigerant in the evaporator will be reduced, leading to a drop in cooling capacity. If the refrigerant is insufficient, it should be replenished. 2. Moisture Intrusion: The presence of moisture in the car's AC refrigeration system can result in insufficient cooling or no cooling at all. If the lack of cooling is due to moisture in the refrigeration system, the desiccant or dryer in the car's AC should be replaced. At the same time, the system should be evacuated and recharged with an appropriate amount of new refrigerant.
I ran into this issue recently too. I was driving my old car on the highway when the AC started blowing hot air and stopped cooling completely, but during low-speed traffic jams, it got so cold I needed a jacket. After some troubleshooting, I found it was a fan issue: at high speeds, the strong airflow made the fan lazy and it wouldn’t start, causing the AC to overheat and stop cooling. Another possibility is unstable refrigerant pressure or a minor leak in the system. Back then, I had the mechanic test the fan resistor, and replacing it fixed the problem. These kinds of issues are pretty common in middle-aged cars, especially with aging electronic components. I’d recommend regular checks on the AC fan motor and refrigerant levels—otherwise, you’ll be stuck in a heatwave on the highway, which can affect driving safety. Better to address it early and not delay.
