What Causes Car AC to Not Cool at High Speed but Cool at Low Speed?
3 Answers
Insufficient or low refrigerant in the car's air conditioning system is the primary reason for the AC not cooling at high speed but functioning at low speed. Below is a detailed explanation: 1. Insufficient Refrigerant: A lack or shortage of refrigerant in the car's AC system can lead to inadequate cooling. The most common cause of insufficient refrigerant is minor leaks in the system. If the refrigerant level is low, the amount sprayed into the evaporator from the expansion valve decreases, resulting in reduced heat absorption during evaporation. Consequently, the cooling capacity drops. If the refrigerant is insufficient, it should be replenished. However, when adding refrigerant, note that if filling from the low-pressure side, ensure the refrigerant bottle is not laid flat. If filling from the high-pressure side, do not start the engine. 2. Moisture in the AC Refrigeration System: The presence of moisture in the car's AC refrigeration system can cause insufficient cooling or complete failure. If moisture is the culprit, the desiccant or dryer in the car's AC should be replaced, followed by evacuating the system and refilling it with the appropriate amount of new refrigerant.
I recently encountered this issue and just resolved it. The core problem is actually the cooling system failing to meet high-speed demands. Let me explain the most common causes: the condenser gets clogged—when driving at high speeds, debris like leaves and insect remains block the cooling fins, preventing the refrigerant from dissipating heat, which causes the entire system to fail. Another possibility is insufficient refrigerant, which might work at low speeds but becomes inadequate when the compressor's load increases at higher speeds. Pay special attention to the cooling fan—if it doesn’t spin fast enough or stops working at high speeds, the engine bay temperature will spike, affecting the AC. My car had this exact issue, and replacing the fan resistor fixed it. Additionally, blocked refrigerant lines or a faulty expansion valve can disrupt the flow when the engine speed increases. These issues require professional diagnostic equipment to identify.
As an experienced driver, I've encountered this situation a few times. The AC blows cold air before getting on the highway, but starts blowing hot air at high speeds. Later, the technician identified two issues: first, the electric fan controller was faulty—it still worked at medium-low speeds but failed at high speeds due to unstable current; second, the condenser was clogged with poplar fluff like a blanket. The technician explained that at high speeds, the engine load increases, demanding better cooling, and a blocked condenser only makes things worse. Some cars also suffer from a sharp drop in cooling efficiency at high speeds due to compressor wear. Pay special attention to the refrigerant pressure readings during inspection—last time, it was the abnormal pressure gauge reading at high speed that helped pinpoint the problem.
