AC Only Works When Driving

AC Only Works When Driving: What’s Causing It?

You get in your car, turn the engine on, and then switch on the AC. Expecting a nice cool breeze sweeping your face. But hang on, the air turns out to be only very mildly cold. Then you drive along and now the air conditioning is working. Seems that your car’s AC only works when driving, why is this?

There are several possibilities on why this might have happened. We’ll talk you through the components inside your car’s air conditioning system, how it works, and why your AC only works when driving. And of course, we’ll list down the solutions depending on the problem you have.

AC Only Works When Driving: How The Car AC Works

Let us explain how the air conditioning system in your car works. Otherwise, you won’t be able to make heads or tails of what might be wrong and why your AC only works when driving.

It works largely similar to how home air conditioning works; they use the same components and refrigerants as a means to cool the air. The difference is in the power source and the type of refrigerant used. Anyway, here’s how the system works:

  1. Most car air conditioning system is powered by the engine via the drive belt. When your engine is running, it will also run the drive belt. This powers a number of electrical accessories, including the alternator, the headlights, and the AC’s compressor.
  2. The compressor will compress the refrigerant (in gas form). This transforms it into a high-temperature and high-pressure gas. It will then enter the condenser.
  3. The condenser then removes heat from the refrigerant and transforms it into a liquid. Afterward, it pushes the liquid into an expansion valve or sometimes called an orifice tube.
  4. At the expansion valve, the refrigerant returns to its original gas state. As the refrigerant does this, it will drop down in temperature. Afterward, it travels into a receiver/drier or accumulator.
  5. The receiver/drier or accumulator dries the refrigerant by removing moisture.
  6. Finally, the refrigerant gets to the evaporator. A blower fan will pull air from the outside and blow it through the evaporator core. This results in only cold air flowing beyond the evaporator, and cold air blowing through your car’s vents.

We recommend watching the video above to get a better understanding of how the system works. And it’s pretty much the same in every car. Although in electric cars, the compressor is powered by the battery, since there’s no internal combustion engine.

AC Only Works When Driving: Possible Causes

So, now you know how your car’s air conditioning system works. Some of these components may fail which could be the reason why your car’s AC only works when driving. Here are the possible causes:

1. Faulty Condenser Fan

The most likely culprit is the condenser fan. The condenser fan is responsible for dissipating heat from the condenser, similar to how the radiator fans work. If the fans work properly, it will reduce the refrigerant’s temperature as it passes the condenser.

Carmakers usually design them to turn on automatically once the engine reaches operating temperature, which is somewhere around 195°F to 220°F. If the fan doesn’t come on, then the refrigerant won’t drop to an ideal temperature by the time it reaches the evaporator.

But why does the AC still works when driving? This is because when you’re driving, air from outside of the car will still pass through the condenser. This helps to cool it down, even without the fan. But since the fan isn’t working, there’s no air to cool down the condenser when you’re stationary. Hence why the AC only blows cool air when the vehicle is moving.

2. Low Refrigerant Level

The AC system is a closed-loop system, meaning it’s not supposed to lose the fluid – in this case the refrigerant – in it. However, leakages may occur in the system. This could be because of cracks in the AC system line, or a leak from the condenser.

Condensers are prone to damages from road debris since they usually sit at the front of the car. This is especially true if the car doesn’t have any under guard. Even if it does, the seals and tubes of the condenser can wear down, but in most cars, it will take somewhere between 10 – 15 years for this to happen.

In any case, leaks mean your car’s air conditioning system will lose refrigerant over time. Once it’s below the minimum level, the compressor will struggle to pump the refrigerant in the system, especially when idling. But as the car builds up speed, it can pump again as normal.

3. Clogged Condenser Or Compressor

Since the air conditioning system is a closed-loop system, it’s unlikely for debris to get into it. However, blockages can still happen inside the system. This is often due to a worn compressor where the internal parts are breaking down.

When internal parts in the compressor break away, it will disperse this debris throughout the system, causing blockages in the condenser or compressor itself. And this means that the system will struggle to recirculate the refrigerant, causing your AC to be warm.

Also, the build-up of residue in the refrigerant may also cause blockages. The residue will turn into a sticky paste, causing circulation issues, as well as disturbing the heat dissipation process. Additionally, a blocked condenser can also cause short cycling. This is when the air conditioning system cycle on and off rapidly before it could reach optimal operation.

4. Other Faulty Parts

If you have blockages in the system, it will likely wear out the parts much quicker as well. When the parts in the air conditioning system are faulty, it will cause problems with your car’s air conditioning.

For example, the compressor clutch may not be cycling due to a broken wire in the clutch coil. If the compressor clutch doesn’t engage, then the compressor itself won’t be able to pump refrigerant throughout the system. In this case, you’ll probably feel the AC not working at all whether you’re idling or driving.

As mentioned, the condenser can wear down over time as well. There may be leaks from the seals or the tubes. Or it may also experience coil damage, which is the part responsible for removing heat.

The evaporator may also break over time. There may be damage to the outer seams of the evaporator core, which will cause a leak. And as we’ve learned, leaks will lead to low refrigerant levels which disrupt the circulation and can cause the air conditioning to be warm when idling.

The evaporator may also corrode over time, which will cause leaks as well. These three major components of the air conditioning system typically last for about 10 – 15 years. If your car is around that age, it’s to be expected to start experiencing these problems.

5. Overheating Engine

It’s also possible that the problem isn’t coming from the air conditioning system itself but from your engine. If your engine is overheating, this will raise the temperature within the engine bay, which is also where your air conditioning system is located.

The higher temperature will affect your car’s air conditioning since it can raise the temperature of the refrigerant and components of the AC system. All this basically means your AC system now has to remove excess heat that it wasn’t designed to in the first place.

AC Only Works When Driving

When the car starts moving, air will go through the engine bay. Helping to cool down both the engine and the AC components. This is why even if you have an overheating engine, the air conditioning may still work when you’re driving – thanks to the outside air going through the engine.

If you notice your temperature gauge sitting higher than usual, then you have an overheating engine. There are several possible causes, such as low coolant, coolant system leak, and a faulty water pump amongst other causes. We’ll get into this further later on.

AC Only Works When Driving: Repair Cost Estimates

So, now you know the parts inside the air conditioning system, and what might be causing a problem in your car. Now, let’s get into how to diagnose, and the repair cost estimates for each problem:

Condenser Fan Repair Cost

The condenser fan problem is probably the easiest and cheapest one to fix all the possible problems. When the fan doesn’t spin automatically, there are several possible causes:

  • Faulty temperature sensor.
  • Engine thermostat stuck open, which prevents the engine coolant from getting to the optimal temperature which triggers the condenser fan.
  • Faulty fan control module.
  • Blown fuse.
  • The fan is broken.

To diagnose this problem, turn on your AC and turn the blower to the MAX position. This will trigger the fan to spin, and if it does, then your fan and its control module are fine. This means the problem lies either with your temperature sensor or thermostat.

Keep in mind that when you have a temperature sensor or thermostat problem, you’re likely to notice engine temperature issues. The engine may either overheat, or it will struggle to get to operating temperature. We’ve written comprehensive articles about the two components and you can read them here:

If it doesn’t spin, then it’s either a blown fuse, a bad relay, or the fan motor is broken. Here’s how you can diagnose the problem further:

The video above shows how to diagnose the cooling (radiator) fans. But the method is the same, you just need to locate the condenser fan rather than the radiator fan.

Replacing a blown fuse or relay usually costs no more than $15. As for the condenser fan, it costs between $300 – $425 for most cars. Meanwhile, if your problem lies with the temperature sensor, it’ll cost about $150 to replace it. And an engine thermostat replacement usually costs between $150 – $230.

Low Refrigerant And Leaks Repair Cost

When you go to an auto repair shop for an air conditioning service, the first thing they will do is check the refrigerant level using a two-dial pressure gauge. If the level is low, they will recharge the refrigerant. This service usually costs between $150 – $300.

But as mentioned, the air conditioning system is a closed-loop system and it isn’t supposed to lose refrigerant – at least not by much. So when the refrigerant is low, it’s likely you have a leak in the system.

The problem is that air conditioning leaks can be difficult to identify, and repairing them is likely to be expensive. Try recalling when was the last time you had a refrigerant recharge. If it’s been less than a year, then it’s likely there’s a big leak and you should ask your mechanic to find the leak.

However, if the refrigerant lasts over a year, then the leak is relatively small. In this case, we recommend recharging the refrigerant yourself with a refrigerant that has a sealant solution. Take a look at this excellent guide video from ChrisFix:

The refrigerant costs about $32 for a 20oz can. It’s relatively easy to use and we recommend doing it yourself, so you don’t have to pay for the $150 – $300 service at an auto repair shop.

Replacement Cost For Air Conditioning Parts

As mentioned, major leaks can cause the AC system to lose refrigerant quickly. Broken components in the AC system can also cause the AC to only work when you’re driving. In this case, you’ll have to repair the AC system.

If there’s a leak in one of the lines, it will cost anywhere between $150 – $800 to repair the leak. The cost will depend heavily on how many leaks are present in the system, as well as your car’s make and model. However, if it turns out that one of the components is broken, you’re going to have to replace it. You might be able to repair it, but it’s not going to be worth it and you’re better off replacing the component altogether.

We start with the compressor. The part itself is going to cost somewhere between $200 – $900. While labor will cost another $200 – $500, depending on how difficult the job is. On average, expect a compressor replacement to cost you somewhere around $750.

The evaporator is equally expensive. Prices start at around $350 and can go as high as $500. While labor also costs somewhere around $300 – $500. In total, you’ll have to pay somewhere between $650 – $1,000 for an evaporator replacement.

Meanwhile, the condenser will cost around $450 – $950 to replace, including labor. These are just rough estimates and it may be cheaper or more expensive, depending on your car’s make and model.

Can I Fix Or Repair It Myself?

You can, there are tons of guides out there on how to replace the AC components in your car. However, we don’t recommend doing it yourself, whether to fix a leak or to replace major components. They’re quite difficult to do, and it’s easy to get it wrong. Unless you have the right tools and good mechanical skills, replacing AC components is probably best left for professionals to do.

AC Only Works When Driving: If Your Engine Is Overheating

If it turns out the problem is an overheating engine, then you’ll need to address either the cooling system or the engine’s internals. And you’ll need to do this quickly, as overheating can cause excessive damage to the engine which could lead to an expensive engine rebuild.

AC Only Works When Driving

The first thing you need to check is your coolant level. If it’s below the minimum level, then you have a leak in the system. Refill your coolant, and then let the engine run for a few minutes. And then see if there are any leaks in the engine bay.

If there’s no visible leak, then it’s possible you have a blown head gasket. If you have a blown head gasket, it will allow coolant to enter the engine’s cylinders and it will be burnt along with fuel, causing you to lose coolant rapidly. You will notice other symptoms as well, including engine misfires and thick white smoke coming out of the exhaust pipe.

However, if your coolant level is normal, then you probably have a faulty component in the cooling system. Your overheating problem may be caused by a bad radiator cap, broken radiator fan, faulty water pump, or a broken thermostat. We wrote a comprehensive guide on how to diagnose an overheating engine and you can read about it here.

How Do I Maintain My AC?

Once you’ve fixed the air conditioning in your car, you’re going to want to maintain it. Thankfully, air conditioning maintenance is relatively easy. Here are our tips on maintaining your car’s AC system:

  • Replace the cabin filter. It will cost between $15 – $50 to replace them. Check your owner’s manual on when to change your cabin filter, but usually, you’ll have to replace them every 30,000 miles.
  • Clean dust and debris from the air vents by wiping them or using a brush. This will prevent them from getting into the AC system which can clog and damage the system.
  • If you live in a cold area where it’s too cold to turn the air conditioning when you drive, turn it on for 10 minutes once a week. This helps to maintain gas pressure inside the system and keep the compressor working properly.
  • Run the defroster once a week for 5 – 10 minutes to prevent mildew and other unpleasant odors. This will also help to clear away excess moisture in the air conditioning system, which can damage it over time.
  • Service it regularly. Servicing your AC system once every two years is fine, especially if you don’t see any problems. But for better peace of mind, there’s nothing wrong with servicing your AC system once a year.

What If I Have A Heater Problem?

The heater is actually a part of the cooling system and not the AC system. Your car’s heater works by using a heater core, which looks like a miniature version of your car’s radiator. This sits behind your car’s firewall, so you won’t be able to see it unless you take apart your dashboard.

When you turn the heater on, your car’s cooling system will divert some of the hot coolants from your engine into the heater core, rather than back to the radiator. This raises the heater core’s temperature, and a blower fan will blow air through it. The resulting air becomes warm, which is then directed to your air conditioning vents.

AC Only Works When Driving

If your car’s heater isn’t immediately warm when you turn on the engine from a cold start, this is perfectly normal. This is because the coolant is still cold, which means it won’t heat up the heater core, resulting in ambient temperature air being blown through it.

But if the heater still doesn’t work even though the engine is at operating temperature, you likely have a leak somewhere. Either the heater core itself is leaking, or the entry pipe of the heater core is leaking. Either way, this results in the heater not working.

A heater core hose replacement usually costs around $180 – $210 including labor. Meanwhile, the heater core itself is quite expensive. A replacement job costs somewhere around $560 – $930 including labor. You can learn more about your car’s heater core in our repair guide here.

AC Only Works When Driving: In Conclusion

There are several possible causes of your AC only working when driving. The easiest ones to diagnose are the condenser fan and low refrigerant levels. We recommend taking a look at those two first if you have this problem.

If it’s the condenser fan isn’t working, then you’ll need to replace the fan – or the fuse/relay if it turns out that’s the cause. Meanwhile, low refrigerant levels mean you’ll have to recharge it, which costs around $150 – $300 at an auto repair shop.

If it is neither of the two, then you either have a clogged system, or a faulty component such as the compressor, condenser, and evaporator. In any case, you’re going to have to replace them. Repairing them is not going to be worth it, and you’re better off replacing them.

And finally, if your air conditioning system seems fine, then an overheating engine might be the cause. In this case, you’re going to have to troubleshoot the overheating problem and solve it immediately before it turns into an expensive engine rebuild. In any case, we hope this article has been helpful for you.

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