If your air conditioner blows warm air when you're at a stoplight and cold air when driving, there are a few possible reasons for this behavior:
Engine Load: When your car is at a stoplight, the engine is still running, but the RPM (Revolutions Per Minute) is lower compared to when you're driving. The air conditioning system's compressor, which is responsible for cooling the air, is driven by a belt connected to the engine. At lower RPMs, the compressor may not spin as fast, resulting in reduced cooling capacity and warmer air being blown into the cabin.
Air Flow: When the car is stationary, there is minimal airflow over the condenser, which is a part of the air conditioning system responsible for dissipating heat. As a result, the condenser's efficiency drops, and the air conditioning system may not be able to cool the refrigerant effectively. When you start driving, the airflow increases, allowing the condenser to work more efficiently and providing cooler air.
Fan Operation: Modern cars have electric fans that help cool the radiator and the condenser. When the car is at a stop, the fans may not be running at full speed, leading to less effective cooling of the condenser. Once the car starts moving, the fans may kick in at higher speeds, providing better cooling.
Low Refrigerant: If your car's air conditioning system is low on refrigerant (commonly known as "Freon"), it won't be able to cool the air properly. Refrigerant is responsible for absorbing heat and cooling the air before it enters the cabin. A low refrigerant level can lead to warmer air being blown out.
System Issues: There might be other issues with the air conditioning system, such as a faulty compressor clutch, clogged or restricted refrigerant lines, or problems with the expansion valve. These issues can affect the system's ability to cool the air consistently.
To resolve the problem, you should have your car's air conditioning system inspected and serviced by a qualified mechanic. They can diagnose the specific issue and perform any necessary repairs or maintenance to ensure that your air conditioner functions correctly, providing cold air regardless of whether you're at a stoplight or driving.