Why is My Car Losing Power when the AC is Turned ON?

A car loses power when the AC is turned ON because of the additional AC compressor load on the engine. A problem with the AC compressor such as clogged air filters or low coolant level can also trigger this problem.

  • Clogged air filters
  • Faulty AC compressor
  • Low coolant level


Clogged Air Filters

Combustive engines require oxygen to function properly. Dirty or clogged air filters limit the amount of oxygen or clean air being supplied to the engine, thereby making it lose power.


Faulty AC Compressor

AC compressors that are faulty can add more load to the engine. The horsepower of the car is shared between the compressor and the engine. When the compressor adds more than the normal load, the horsepower available for the acceleration is reduced, which makes the car lose power.


Low Coolant Level

If the head gasket in the engine has blown, then it is most likely due to the less coolant circulation through the AC compressor because of low level. . This can cause the engine to lose power as well as have reduced efficiency.


Car is losing power when AC is turned ON


How to Fix a Car that Shuts Off when AC is Turned On

A short circuit in the AC system can cause the car engine to stall or turn OFF when the AC is turned ON. This can be fixed by identifying the exact component causing the problem and taking the necessary action.


Tool Required

  • Fuse puller or tweezer
  • Safety gloves


You can fix the issue of the car turning OFF when you try to switch on the AC by following these simple steps:

  1. Use a fuse puller or tweezer to remove the AC fuse. This will most likely disconnect the AC unit from the engine.
  2. Unplug the AC compressor. This will reduce the load on the engine and thereby prevent it from shutting down.
  3. Replace the idle air control (IAC) from the car. Note the error code and refer to the manual to fix the issue.


Why Does My Car Accelerate Slowly with the Air Conditioner is On?

The throttle power is shared between the car’s engine as well as the air conditioning system. Since the car has less power for the engine when the AC is turned ON, the acceleration is generally sluggish.

You can fix this issue by following a few simple steps.


Tools needed

  • Level gauge


You can try one of these steps to try to fix your issue.

  1. Use the level gauge to check the level of the coolant in the car. If the coolant leaks, then the car might overheat when the AC is switched ON. This might cause the acceleration to be sluggish. You will need to repair the leak to fix this issue. If the level is low, you can top up to the desired level.
  2. Open the hood and check the cooling fan. If the fan is not working properly, then the acceleration might be slow. Check the fan for cracks or slow running. If you notice one or both of these problems, replacing the fan will take care of the issue.


Does AC Affect Engine Power?

Yes, the AC uses power from the engine, which reduces the power available for the drive.

  • The AC uses up more fuel when driving with the windows down. Hence, this affects the mileage of the engine.
  • The AC and the engine share the throttle power and hence the AC will reduce the acceleration capacity of the car.

Every car has an ideal fuel speed range. If you follow this speed range, you can reduce the loss of fuel efficiency.


Why does my Car Idle Hard with AC ON?

A faulty IAC valve or a faulty cooling fan can cause the car to idle hard when you switch ON the AC.

Some of the common causes for the car idling in a rough manner are described below.

  • If the IAC valve is dirty or broken, then the car can idle hard.
  • If the throttle body is dirty, then the car might idle roughly.
  • A broken cooling fan or a condenser fan can also cause this problem.

You can check for carbon buildup on the throttle body or the IAC valve. Getting this can help clean out most of the dirt and resolve this issue. If the IAC valve is clogged or broken, it must be cleaned. On the other hand, the valve must be replaced if it’s broken.


How much Horsepower do you Lose with AC On?

Around 10% horsepower would be lost if the AC is switched ON. The exact loss depends on your driving habits and the external climate conditions.

  • Most of the time, you don’t need full horsepower while driving. If you use around 20HP for your drive with the AC ON, there may be 10% reduction of horsepower.
  • If the external climate is extremely bad, then you might use around 4 HP of power. This can cause a little more reduction in the power.

Although switching ON the AC might cause you to lose power, it doesn’t impact your driving too much. A reduction of power by 10-20% will often not be felt by the driver.


Does Turning Off AC gives you More Power?

Yes, turning off the AC can give you more power. Since the AC and the engine share the same power, switching OFF the AC for a while can make you notice the car moving faster.

  • If your car is of an older model, then you might notice that the car slows down while going up an incline if the AC is switched ON. In such cases, you can turn OFF the AC for a while to increase the power.
  • If the engine capacity is quite large and the car is of a recent model, then you might not notice any difference even if the AC is switched ON

If you have an older car, then you can turn OFF the AC when you want an increased speed like when you are driving on the highway or driving up an incline. Under normal road circumstances, this might not be necessary.


Is it Bad to Start the Car with AC On?

It’s not recommended to start the car with the AC switched ON. However, this depends on the model of the car and its engine. In low-end cars, it’s not advisable to switch OFF the engine before switching OFF the AC.

  • If the clutch isn’t engaged, then the AC would take its power from the battery. This can lead to a serious drain on the battery.
  • If the AC is not switched OFF before turning OFF the engine, it could lead to hazardous chemicals getting generated when the car is locked, especially if the car is kept under the sun.

It’s always safer to turn OFF the AC and other electrical components like the audio system and headlights before switching OFF the engine. This can result in better battery life as well as protect the cooling system and pulley attached to the AC system.


Why does My RPM Go Up and Down While AC is On?

The RPM goes up and down while the AC is ON because of low coolant level in the AC system. The compressor cycle will become more frequent due to the low level of coolant thus raising and lowering the RPM quite often.

  • Check the coolant level in your AC system. If the level is low, it’s better to top up the coolant fluid. This can help resolve the issue.
  • Check the AC compressor. If the compressor has an issue or is failing, then it might not be able to handle the additional load properly. Replacing the compressor can help fix this problem.

The AC adds additional load to the engine. This can cause the engine to miscalculate the power required and give it more power during idling. The latest engines do not have this issue.


Should I Turn Off AC Before Turning off the Car?

Yes, you should turn OFF the AC before turning OFF the car. An AC that is kept switched ON after the engine is turned OFF can cause a drain on the battery.

  • The AC releases toxic chemicals and/or gases when the car windows are raised and the engine is switched OFF. There is also no air circulation inside the car. When you get into the car after a long time, you are likely to be affected by the toxic components.
  • In recent models, the cars have been designed to auto shut OFF the AC and compressor if the engine is switched OFF. In such cases, it doesn’t matter if you don’t switch OFF the AC before turning OFF the car.

If you have an older car, it is better to turn OFF the AC and other electrical components before you switch OFF the car. If your car is of a newer make, then this isn’t a problem as the AC system is auto-regulated.


Does AC Affect the Speed of a Car?

No, the AC will not affect the speed of the car. Even though the engine power is shared between the car and the AC, the car has sufficient throttle power to manage both the speed and the AC system at the same time.

  • Modern engines are equipped with the ability to manage both the AC and engine systems in parallel.
  • An alternator or a battery can help give out power to the AC when the clutch is engaged.
  • The maximum speed of the car as well as the acceleration might decrease when the AC is switched ON. However, if you are driving at a constant speed, it will not be affected if you switch ON the AC.

If you notice that the speed of the car is affected when the AC is switched ON, you can get the car calibrated. This can help balance the load better. Some models have automatic calibration while most of the older models don’t.

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