Why your car AC stops blowing cold when you’re stopped

Learn The Reasons Why Your Car AC Stops Blowing Cold When You’re Stopped

There are several potential reasons for why your AC stops blowing cold, ranging from something as simple as a faulty radiator fan, to more complex repairs. Understanding the common causes of why your car AC  stops blowing cold can help you diagnose the problem and determine whether it’s something you can handle on your own or if professional help is needed.

Common Reasons Why Your Car AC Stops Blowing Cold Air When You’re Stopped

• Your radiator or condenser fans aren’t working properly
• The AC system is low on refrigerant charge
• Your Compressor Clutch isn’t working properly
• The condenser fins are blocked with crud
• A combination of some or all of the above issues

1. A bad condenser fan can cause the AC to blow warm air at stops

The condenser, located at the front of your car in front of the radiator, relies on airflow to dissipate heat from the compressed refrigerant. When your car is moving, air naturally flows through the condenser, helping it cool and condense the refrigerant effectively. However, when you’re stopped, this airflow is significantly reduced, making it harder for the condenser to do its job. As a result, the refrigerant doesn’t cool down as much, leading to warmer air from the AC.

When you’re stopped, the condenser/radiator fan is supposed to take over, pulling air across the condenser to remove the heat. If the fan isn’t working, it can’t cool the refrigerant, and your AC will stop working, resulting in warm air coming out of your ducts.

Single fan with multiple speeds or multiple fans—something’s not working properly

Some vehicles are equipped with a condenser fan and a separate radiator fan. In those vehicles, the condenser fan runs the entire time the AC is on, and the radiator fan kicks in when the engine reaches a specific temperature. Other vehicles have a single variable speed fan. If the condenser fan isn’t working or the variable speed fan doesn’t run or run at the proper speed when the AC is on, the refrigerant won’t condense properly, and you won’t get cabin cooling when the vehicle is stopped.

So, checking for the proper condenser/radiator fan operation is your first step. Start by running the engine and turning the AC setting to MAX. Then, check under the hood to see if the condenser/radiator fan is running at full speed. If not, check for any loose wires/connectors at the fan. Next, inspect the fan’s fuses. If the fan(s) don’t spin and the wiring and fuses are good, take the vehicle to a shop for further diagnosis.

2. A low refrigerant charge can cause warm air at stops

A low refrigerant level causes low cooling when driving and no cooling when stopped. The compressor spins slower at idle speeds and can’t circulate the lower refrigerant amount fast enough. When driving, the compressor spins faster and circulates the refrigerant faster, so it cools better.

3. A low engine coolant level can cause the AC to stop working at stops

At first glance, you may not think that engine coolant level has anything to do with your car’s AC performance, but it does. The AC condenser sits in front of the vehicle’s radiator. The condenser removes heat from the refrigerant, and that heated air then flows through the radiator fins, putting an added load on the engine’s cooling system. The radiator, engine coolant, and water pump can handle the extra heat in a properly operating cooling system. However, if the cooling system is low and the car is stopped, the water pump spins at a much slower speed, so it can’t circulate the coolant as fast, causing the coolant temperature to rise. When the ECM sees a rising coolant temperature, it can shut off the AC to reduce the heat load on the cooling system. Keeping the engine cool takes preference over keeping you cool. As soon as the vehicle starts moving and the water pump speeds up, it circulates more coolant, thereby reducing coolant temperature. So, the ECM turns on the AC again.

Check the engine coolant level in the coolant reservoir under the hood. If it’s low on coolant, top it off with the recommended coolant. Then check again in another week. If it’s low again, get your vehicle into a shop to locate and fix the coolant leak.

4. A bad compressor clutch can cause the AC to blow warm at stops

The compressor clutch is responsible for engaging the compressor shaft. The conditioning compressor won’t pump refrigerant if it fails or works intermittently. Compressor clutch problems are usually caused by a bad relay, a wide air gap, a corroded or frayed wiring harness/connector issue, or a bad clutch coil.

To check if your compressor clutch works, turn the AC to MAX and start the engine. The clutch and the pulley will spin both at the same time. In case only the pulley rotates, check the clutch connector for power. If it’s not getting power, check the relay and fuse.

5. Blocked or dirty condenser

Dust particles and road grime can accumulate on the

Clogged condenser fins can reduce airflow and cause your AC to blow warm air at stops

condenser fins and that reduces airflow and AC performance, especially when you’re stopped. To check for dirty condenser fins, simply shine a flashlight onto the front of the condenser coil. If you see dust, bug or dirt buildup, clean it with a strong spray from a garden hose.

The problem of your car AC blowing warm at stops but cold when driving is a common issue with several underlying causes. While some temperature fluctuation is normal, significant changes could indicate a problem with your AC system.

Regular maintenance and timely repairs can help ensure your car’s air conditioning performs optimally in all driving conditions, keeping you cool and comfortable throughout your journey. If you’re experiencing persistent issues with your car’s AC, don’t hesitate to seek professional assistance to diagnose and resolve the problem.

©, 2021 Rick Muscoplat