Q: Why does the AC stop cooling the car once the car speed exceeds about 80 Km/h?

The A/C thermal expansion valve (TXV) most likely is the cause of the A/C system freezing up at higher engine or vehicle speeds. The TXV controls the flow of refrigerant through the evaporator and the valve can stick open causing the system to not get cold after it opens or the valve will stick on an almost closed position and cause the orifice to freeze up and cannot control the correct amount of refrigerant flow through the evaporator. The A/C system should be performance tested and checked for pressures to make sure the system has enough refrigerant or the TXV is the cause of the problems. The TXV is mounted to the evaporator and can be replaced separate from the evaporator. If you would like to have this inspected, a certified technician from YourMechanic can come to your car’s location to diagnose the AC issue in person and pinpoint needed repairs.