AC Only Works (Blows Cold Air) When Driving – Reasons & Fixes!

Updated: | Author: Steve Freling | Affiliate links may be present.

In the hot summer months, air conditioning is an absolute must in your car! When the car stops moving is when you need it most, but what if that’s when your AC starts blowing warm air? I’ve got some tips for you, and getting your vents frosty again may be easier than you think.

Your AC only working when the car is moving could be due to a lack of enough refrigerant. Low refrigerant levels will cause a lack of AC system pressure when the engine is idling. This can result in warm air when the car is stopped. Alternatively, a faulty engine fan may keep your condenser from getting the air it needs to convert your refrigerant to a liquid for cooling.

How Your Car AC Works

The air conditioning systems in most cars use a refrigerant called “R-134a”. Older cars used R12, and some newer models are using a new kind that’s called “R1234yf”.

We’ll only be discussing 134a here. 1234yf refrigerant is much more heavily regulated, and R12 hasn’t been used in decades.

Refrigerant is a gas at room temperature, and very volatile. Since evaporation is a cooling process, AC systems can take advantage of a refrigerant’s properties to create cool air.

An AC system is one big loop for your refrigerant to circulate through. It starts with your AC compressor.

Your engine spins the compressor, which pressurizes the refrigerant to a high-pressure gas state. The refrigerant then passes through a special radiator called a condenser, where it condenses into a high-pressure liquid.

In this part of the system, the refrigerant is actually very hot. Don’t worry though, this is how the system works. It uses the transfer from a very high-pressure high-temperature state to a very sudden low pressure state to create the cooling effect.

The liquid refrigerant now enters what’s called an expansion valve, or in some cases, a similar device called an orifice tube. The valve limits how much refrigerant comes through, resulting in a sudden and dramatic pressure loss. With such a great loss of pressure, the refrigerant evaporates very quickly.

Science tells us that evaporation is a cooling process, so as the refrigerant evaporates into a gas state it gets very cold. It does this immediately after the expansion valve, in another special radiator aptly called the evaporator.

The entire evaporator gets very cold when this happens. Here is where your fan comes in, and blows air through the evaporator and into your car’s interior. The refrigerant leaves the evaporator as a low pressure gas, and the process starts all over again.

So, Why is My AC Only Cold While Driving?

There are two main reasons. The most common is simply that you’re just a little low on refrigerant. Topping it off can get things frosty again.

Less often, the condenser may not be getting enough air flow to change the refrigerant from a gas to a liquid. Without this change of state, the refrigerant can’t evaporate and cool.

Check Your Fans

PLEASE be careful when checking your fans! You can be seriously hurt by them. Make sure you keep tools, loose clothing and of course any parts of your body AWAY from the fans!

The condenser I talked about earlier needs air flowing over it to work properly. When the car is moving it’s getting plenty of air and the system works. When the car is stopped, the radiator fan is what takes over to keep air moving through the condenser.

If your radiator fan isn’t working, you can bet that your AC performance won’t be too great when you’re stuck in traffic. Worse still, this fan keeps your coolant temperature in check. If it’s not working, it could cause your engine to overheat too.

Mechanical fans are very easy to check. If the car is stopped and the engine is running, it should be spinning. If not, there may be a problem with the clutch that connects it to the engine.

Electric type fans are pretty simple to look at too. With the engine running and the car stopped, turn your AC system on. The electric fan (or at least ONE of them if your car has two) should be running. The car knows you want the AC so it tries to turn the fan on.

If none of your electric fans are running, you may need a fan, a fan resistor, or there may be a wiring problem.

There is the possibility too in either case that damage to the fan has caused it to not move air like it should. Look for bent or broken fan blades. If you find any, the fan should probably be replaced.

If You Have Low Refrigerant

If your fans all appear to be in good working order, the likely cause of your AC troubles is low refrigerant. But how can the AC work at all if the refrigerant is low?

If the system is low, it will have to work a little harder to get the pressures needed for a lot of cooling. While you drive down the road with the engine between 1500 and 2500 RPM, it’s no problem. Idling at a red light, however, is probably not turning the compressor fast enough to pressurize the system.

In the case of the AC only blowing warm when the car is stopped, your system is probably only slightly low. If it was very low, or empty, it likely wouldn’t work at all. It’s easy enough to top off the system, you just need a can of refrigerant and a dispenser.

Topping Off Your AC

When handling refrigerants you should have some safety glasses and heavy gloves for personal protection. Refrigerant evaporates incredibly fast and can cause frostbite.

It’s also very harmful if inhaled, so make sure you’re working outside or in a well ventilated area.

Be advised, you MUST check laws and regulations where you live when it comes to handling refrigerants. It is illegal to vent refrigerant into the air, and you may need special licenses or certifications to handle it.

It’s also very important you make sure of the type of refrigerant your car uses. 134a is used by most cars, and anything else may damage your system.  Check the underside of your hood, there should be a sticker there indicating the type your car uses.

Once you’ve got your refrigerant and dispenser, be sure to read all instructions carefully before attempting to top off the system. The basic idea will usually be the same though.

The dispenser should have a pressure gauge and a corresponding table to show you roughly what pressure readings a full system will have. Make sure the AC system is on, and follow the instructions it came with to properly top it off.

If your pressure readings are normal according to the table, don’t add any refrigerant. Overloading the system can cause damage that can be expensive to repair.

Modern AC systems have become more complex as time goes on. If you’ve checked everything I’ve mentioned here and it all looks good, there may be a deeper problem causing your issue. Have a well known shop check it out, as further diagnosis will require specialized equipment.

Proceed With Care

It can’t be stated enough, please make sure you take appropriate safety precautions when working on your AC system. Be very careful around all moving parts, including belts, fans and pulleys.

Make sure you have the proper gear, too. Safety glasses and thick gloves are a must, but it’s not a bad idea to have a basic respirator too if working in a garage. Stay safe, and enjoy your cold AC!

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About Steve Freling

Steve has worked for more than 20 years as an automotive mechanic, and later run his own repair shop for both cars and motorcycles. He's a maintenance freak, and generally pretty good at troubleshooting!

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