Car overheats when ac is on

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There can be 8 different reasons for why your car overheats when ac is on. These include a faulty cooling fan, its motor, or switch. This can also happen due to a faulty air conditioning AC compressor or condenser. Car engines can also overheat due to a faulty engine coolant pump, coolant temperature sensor, or just due to a low level of engine coolant.

Your car’s AC system relies completely on the engine for power. Any problem with the AC system, the power transferring system, or the cooling system will put extra load on the engine. So you should know why your car engine overheats when AC is on.

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Car overheats when ac is on

Car overheats when ac is on

Engine overheats when your car AC is on is not a usual problem. In most modern cars it can easily be detected when the check engine light with an engine overheating light comes on on the car infotainment system.

These lights can only alert drivers about the problem and cannot inform them about the main reason behind it. Our expertise and years of experience indicate that this can be due to any 8 reasons mentioned below.

1. Car cooling fans are broken or not working properly

Depending on the make and model of the car. There can be different names for a car cooling fan. It can be called a radiator cooling fan, a compressor cooling fan, or just a car cooling fan. Whatever the name is, these are all the same.

There can be a single or a set of these cooling fans in a car. In all cars, these cooling fans are present under the hood, in front of the car, and behind the condenser and radiator.

When this cooling fan is turned on by the car computer. It sucks air from the front of the car, over the condenser and then the radiator fins.

This might cause overheating due to the following reasons

  1. Its blades are broken
  2. Its blades are covered with dust and debris
  3. It came off from its motor shaft and is not rotating

If the car cooling fan is at fault then it can be detected by its unique noise. You can also very easily inspect them. 

Just open the hood and look at the front of the car, behind the radiator. You can even touch and shake them well by hand to check if they are properly fixed on the motor.

With the AC on, the car needs to draw extra air over its condenser to cool the refrigerant. With broken cooling fan blades, the refrigerant will not cool properly

This put an extra load on the car cooling system and thus the overheated light came on.

2. Car cooling fan motor is faulty or turned bad

Car cooling fans are powered using a 12 V DC motor. They are compact and are not really that powerful. Dust and debris accumulation can make them turn bad. 

They usually have small bearings to support their rotating shaft. If the bearing gets jammed due to dust, then the working of the motor can get compromised. 

Any extra load on the motor due to dust accumulating on fan blades can also make the cooling fan motor turn bad.

With a bad cooling fan motor, the cooling fan will not rotate at all or rotate very slowly. So you can overheat when ac is on.

You can easily check the motor by connecting it directly with the car battery. Both the DC motor and car battery have 12 Volts.

3. Your car cooling fan switch has turned bad

A switch installed on your car radiator is called the cooling fan switch. This switch monitors the heat from the radiator. It tells the car computer to turn the cooling fan on or off. 

If this switch does bad then the car cooling fan will not work. To check,

  1. Lift the car using a jack from the front. 
  2. Go under it and simply unscrew the switch from the radiator (there will be a rubber washer to remove it). 
  3. Start the engine.
  4. Go below the car again.
  5. Use a light or small torch to heat its end.
  6. If the fan starts to rotate within 10 seconds then the switch is ok. Otherwise, it is bad and needs replacement.

4. Your car’s AC compressor or compressor clutch is faulty

The car AC compressor is responsible for pressurizing the refrigerant. This makes it flow throughout the AC system. The compressor is run by the car engine through serpentine belt and a clutch mechanism. 

If this clutch mechanism turns bad, then engine power will not be delivered fully to the compressor. 

Other than this if there is a problem with the compressor itself like its bearing or any other internal part. Then it will not be able to deliver the required compression of refrigerant. 

Any problem with moving or rotating compressor parts will produce shaft metal grinding or squeezing sound. 

Other than the car overheating when the AC is on, your car’s AC performance will also be seriously affected by a failed compressor.

5. Your car condenser is covered with dust and debris

The car condenser is responsible for cooling the AC refrigerant before it cools the incoming air. The condenser cools the refrigerant by passing cool atmospheric air over the tubes containing the refrigerant.  

If there is a lot of dust and debris over the condenser tubes. Then the air passages of the condenser will mostly be blocked by that dust and debris. 

Due to this condenser will not be able to cool the refrigerant to the required temperature. This will put an extra load on the system. 

Blocked condensers will also block airflow for the radiator further increasing the load on the car engine.

If you are experiencing low cooling inside the car cabin with engine overheating. Then your condenser might be blocking air from passing over its radiator tubes. 

You can very easily clean the outer surface of condenser tubes using pressurized water and a brush.

6. Your car engine is low at coolant

There is a complete setup inside every car for cooling of its engine. The radiator is just a part of it. There is a fluid called coolant in car engine cooling systems. 

This fluid takes heat from the engine and transfers it to the outer atmosphere through the radiator. This does not happen properly if your car is low on coolant. This will surely make your car overheat when the AC is on.

Drain the old coolant and refill the engine cooling system with the new coolant.

7. Your car coolant pump has failed

If your car coolant level is not low and still your car engine overheats when AC is on. Then your car coolant pump is turning bad. 

The coolant pump is what makes coolant flow in its pipes. If you face any overheating issues without any air conditioning issues then it can be your car coolant pump.

8. Your car has a faulty engine coolant temperature sensor

If none of the above is responsible for the engine overheating when AC is on. Then surely it’s the car engine coolant temperature sensor or say the engine temperature sensor. 

The sensor monitors the engine temperature and displays a warning if it gets too high. With a bad sensor, your car will show a false check engine light or engine overheating light.

Changing the bad sensor is the only solution here.


Car overheats when ac is on and idle

If your car overheats when ac is on and idle. Then it is surely about low coolant level, failed coolant pump, blocked condenser or radiator. If not, the engine temperature sensor has turned bad.

The car only overheats when ac is on

If a car only overheats when ac is on then it’s surely related to the AC system putting extra load on the engine. This means your engine has enough cooling setup to cool itself without the AC system running. 

As soon as the AC system puts an extra load on it, it overheats. This can be due to a low coolant level, a blocked condenser or radiator, bad AC compressor or compressor clutch.

It’s not about the sensor as it only overheats when ac is on.

Last Words

Car overheats when ac is on? There can be eight different reasons for this. We have discussed each of them fully. 

Still, if you have any questions. Do ask them in the comments and we will reply to you ASAP.

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