There are several reasons why a cars AC may not cool when at a stop and blows cool only when moving. The most common reason is the cooling fan for the condenser is not working. It's important to know that many times the cooling fan is shared by the radiator and condenser, other times there are TWO separate ones. The reason why a bad cooling fan can affect the AC so drastically is the fact that heat from refrigerant (Freon) is normally cooled when passing through the condenser. So even if the condenser fan is NOT working, it may not affect the AC while the car is moving at highway speeds . This is because air is passed through the condenser when driving down the road so the fan is not needed. When the car is at a stop, the condenser is totally dependent on the cooling fan to cool it down. Sometimes the cooling fan may be working, but it may be moving too slowly to adequately cool the condenser. To check the motor, a test light can be used to verify that it is getting power and ground to the electrical plug in. If power and ground is present and the motor is not working, the motor has an open circuit. When the cooling fan motor is worn, it may be started sometimes temporarily by lightly tapping on the electric fan motor with a small hammer or wrench. If the fan turns at all when this is done, replacing the motor will be necessary. This is just another way to verify that it is receiving the power it needs to operate. Also keep in mind that a cooling fan may start at any time (some even come on with the engine off) so be careful not to stick your hand in the way of the blades! Note that if the cooling fan motor has designated, it's likely that the fuse has blown also. So if there's no power to the fan and the motor is locked up, a fuse will more than likely need to be replaced at the time of the fan motor replacement.
Other Causes of Car AC Not to Cool at an Idle or a Stop
- The car could have overheating – caused by something else other than the cooling fan.
- Heat transfer from the radiator to the condenser can alter efficiency, if the car is overheating.
- The AC compressor may not be pumping sufficiently at slower RPM's (revolutions per minute).
- An expansion valve may not be regulating the refrigerant correctly.
- Condenser fins could be bent or the condenser could be constructed by foreign debris.
There are special condenser fin combs to straighten condenser fins. But in my experience, bent condenser fins are not that much of a common problem. A more common problem if you drive in the country, is pollen accumulated over time in between the fins. Trash from the road like a plastic bag or piece of paper obstruction part of the condenser reducing performance can happen anywhere. It's funny to me that during my time as an auto technician, many customers came in worried that the AC was not cooling, but failed to notice that the car was overheating – even if the temperature gauge was pegged!