There are several reasons why a car’s air conditioner may not cool when it is stationary and only gets cold when it is in motion. The most common reason is that the condenser cooling fan is not working. It is important to know that many times the cooling fan is shared by the radiator and the condenser, other times there are TWO separate ones. The reason why a faulty cooling fan can affect air conditioning so drastically is the fact that the heat from the refrigerant (Freon) normally cools down as it passes through the condenser. So even if the condenser fan is NOT working, it may not affect AC while the car is moving at highway speeds. This is because air passes through the condenser when driving on the highway, so the fan is not needed. When the car is stationary, the condenser is totally dependent on the cooling fan to cool it down. Sometimes the cooling fan may be running, but it may be moving too slow to cool the condenser sufficiently. To check the motor, a test light can be used to verify that it is receiving power and ground to the electrical outlet. If there is power and ground and the motor does not run, the motor has an open circuit. When the cooling fan motor is worn out, it can sometimes be started temporarily by tapping the electric fan motor with a small hammer or wrench. If the fan spins at all when this is done, the motor will need to be replaced. This is just another way to verify that you are getting the power you need to function. Also note that a cooling fan can come on at any time (some even come on with the engine off), so be careful not to get your hand in the way of the blades! Note that if the cooling fan motor gets stuck, the fuse has probably blown as well. So if there is no power to the fan and the motor is blocked, it is very likely that a fuse will need to be replaced at the time of the fan motor replacement.
Other causes of car air conditioning not cooling while idling or at a stop
- The car could be overheating, caused by something other than the cooling fan.
- The transfer of heat from the radiator to the condenser can alter the efficiency if the car overheats.
- The AC compressor may not be pumping enough at lower RPM (revolutions per minute).
- An expansion valve may not regulate the refrigerant properly.
- The condenser fins could be bent or the condenser could become clogged with foreign debris.
There are special condenser fin combs to straighten the condenser fins. But in my experience, bent condenser fins are not such a common problem. A more common problem if you drive in the field is pollen that accumulates over time between the fins. Road debris such as a plastic bag or piece of paper clogging part of the condenser can reduce performance anywhere. It’s funny to me that during my time as an automotive technician, many customers came in concerned that the air conditioning was not cooling, but they did not realize that the car was overheating, even if the temperature gauge was fixed!