There are several reasons why a car’s air conditioner may not cool down when you stop and cool down only when you’re moving. 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 condenser, other times there are TWO separate ones. The reason a faulty cooling fan can affect air conditioning so drastically is the fact that the heat from the refrigerant (Freon) is normally cooled as it passes through the condenser. So even if the condenser fan is NOT working, it may not affect the AC while the car is running. moving at highway speeds. This is because air passes through the condenser when driving on the road, so the fan is not needed. When the car is stopped, 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 slowly to sufficiently cool the condenser. To check the motor, a test light can be used to verify that it is receiving power and grounding to the electrical outlet. If power and ground are present and the motor does not run, the motor has an open circuit. When the cooling fan motor is worn, it can sometimes be temporarily turned on by gently tapping the electric fan motor with a small hammer or wrench. If the fan spins when this is done, the motor will need to be replaced. This is just another way to check that it’s getting the power it needs to function. Also keep in mind that a cooling fan can come on at any time (some even come on with the engine off), so be careful not to put your hand in the path of the blades! Note that if the cooling fan motor has seized, it is likely that the fuse has blown as well. Therefore, if there is no power to the fan and the motor is locked up, a fuse will most likely need to be replaced when replacing the fan motor.

Other Causes of Car Air Conditioning Not Cooling When Idle or Stopped   

1. The car could be overheating, caused by something other than the cooling fan.
2. Heat transfer from the radiator to the condenser can throw off efficiency if the car overheats.
3. The AC compressor may not be pumping enough at lower RPMs (revolutions per minute).
4. An expansion valve may not be regulating the refrigerant correctly.
5. The condenser fins could be bent or the condenser could be clogged with foreign debris.

There are special condenser fin combs to straighten the condenser fins. But in my experience, bent condenser fins are not that common of a problem. A more common problem if you drive in the countryside is pollen accumulated over time between extremes. Roadside debris, such as a plastic bag or piece of paper clogging up part of the condenser and reducing performance, can happen anywhere. It’s funny to me that during my time as an auto technician, many customers worried that the air conditioner wasn’t cooling, but didn’t notice that the car was overheating, even if the temp gauge was on!