AC Only Works 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.

Other common reasons include faulty condenser fans or a stuck AC compressor clutch. I’ll further elaborate on all these in a bit.

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 Blowing Cold Air Only While Driving?

There could be several possible reasons behind your car AC blowing warm air when idling. The most common is simply that you’re just a little low on refrigerant. Topping it off can get things frosty again. This can either be caused by a leak or simply by not topping it up for years.

Less often, the condenser may not be getting enough airflow to change the refrigerant from a gas to a liquid. Without this change of state, the refrigerant can’t evaporate and cool. In this case, faulty condenser fans may lead to the lack of proper ventilation that the condenser needs to cool down the gas.

What to Do If the AC Only Works When the Car is Moving?

The idea of having to drive all summer without the AC working properly is simply outrageous for most drivers. So, I’ll fill you in on the main steps you can take in the situation where the AC only works when driving to easily fix the situation:

Check & Repair or Replace AC Condenser 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.

It’s not so expensive to replace the fans, and you can check out this explainer article on how the air conditioning system works in a car to get more insight into the components you are dealing with.

What to Do 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, which is why your AC is only working when driving.

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 Up Your AC Refrigerant

When handling refrigerants you should have some safety glasses and heavy gloves for personal protection. The 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 for you to use the right type of refrigerant for your car. The 134a is used by most cars, and anything else may damage your system. However, for newer cars (around 2020 onwards), it’s possible that the newer R1234yf is used. To be sure, check the underside of your hood, there should be a sticker there indicating the type of refrigerant 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 a further diagnosis will require specialized equipment.

Fixing a Stuck AC Compressor Clutch

In case the refrigerant level is sufficient and the fans turned out to be working upon inspection, a faulty compressor clutch could be the culprit. You may hear loud noises when turning up the blower. These are often signs that your AC compressor clutch is nearly out.

It’s a vital component of the AC system as the clutch is what powers the compressor by transferring the engine’s output onto it. From this point, the compressor pressurizes the refrigerant, turning it into cold air at the end of the cycle. If your AC only blows cold when driving, it could be due to the air passing through the system, which is why the clutch issue manifests as you come to a stop.

Usually, this problem will affect the AC system’s performance as a whole, so it’s a less likely reason behind this issue. Even so, I’d advise you to look into it in case everything checks out with the refrigerant level and the fans. It’s best to have a qualified mechanic check it out if you are inexperienced to look into it yourself.

Costs of Repairing AC that Only Works When Driving

If the lack of refrigerant is the reason why your air conditioner only works when the car is moving, topping it up can save the day. I’ve already laid out the method, and doing it yourself can save you some cash as it costs around $20 to $50 to do it yourself.

Of course, this means you’ll have to approach the matter seriously by taking into account the proper handling of the refrigerant. Plus, you may need a DIY refrigerant fill-up kit, so that may add 20 bucks more to the cost. Letting a professional technician do it for you costs around $150 to $200 depending on the car’s model. Still, this one is a better option if you want your system to be checked for refrigerant leaks.

I’d advise you to move on by topping up the refrigerant level every two years on average, or as needed. If it takes replacing a faulty condenser fan, it could be a fix in the range of $100 to $400, based on whether it takes replacing the fan or completely replacing the blower motor.

Finally, the compressor clutch repair can be the most expensive one and it could lead to a repair bill of around $500 on average.

Final Piece of Advice – Proceed with Care

Perhaps going through this guide has affected your mood for the worse knowing that you’ll have to pay for AC system repairs to get the cold air flowing again. However, it may not be an expensive fix at all, especially if you resort to the DIY way of topping up the refrigerant.

What I have to say again, in that case, is to please be careful when handling the belts, fans, and pulleys. Also, make sure to use proper gear or at least grab a pair of protective glasses when dealing with the refrigerant. This way you can stay safe and get your AC to blow cold air when idle in no time.

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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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