Car AC Not Blowing Cold Air When Idle: Fixed!

Do you find yourself sweating and sweltering inside your vehicle when your air conditioner isn’t blowing cold air while your engine is idling at a stoplight? Are you frustrated with the fact that your car’s air conditioner can’t keep up with the hot summer days? At idle, your engine cooling system should be well-maintained so that car AC works properly when the car is not moving. Fortunately, there are a few potential causes and solutions to help get your car AC blowing cold air again when idle.

So, why does car AC not blow cold air when idle? When a car AC isn’t blowing cold air at idle, it is typically due to a faulty cooling/condenser fan or clogged condenser core. This can impede airflow and cause the car AC to stop producing a cooling effect. You should also check the belt of the AC compressor. If the AC compressor belt slips, it will cause the compressor clutch to not engage all the time. This will cause the car AC not to work properly when the engine is idling. You also check the engine temperature from the temperature gauge on the dashboard. If engine overheats at idle, it will also cause car AC not to work properly.

How Does Car Air Conditioning System Work?

Car air conditioning system has the following main components:

  • Compressor
  • Condenser
  • Expansion device
  • Evaporator

Compressor

Car air conditioners operate on a simple principle: they use refrigerant gas to absorb heat from the air inside your vehicle and transfer it outside, cooling the air inside. The process begins when the air conditioner’s compressor compresses the refrigerant gas, which is a type of liquefied gas. This process creates heat and pressure that causes the gas to become a hot vapor. 

The better the refrigerant is compressed, the more cooling it produces when it expands in the evaporator.

The compressor is run by a magnetic clutch that generates a magnetic field and attracts the drive plate of the compressor with the pulley. There is a belt drive supplying energy continuously from the engine crankshaft pulley to the AC compressor pulley. You can also read my guide on AC compressor clutch engages intermittently to learn more.

Condenser

Next, the hot vapor passes through the condenser, a series of metal coils that release the heat to the outside air and cool the vapor back into a liquid. The liquid then gets sent to the evaporator, which is made up of metal coils that absorb the heat from the air inside your car. As the refrigerant liquid absorbs the heat from the air, it evaporates into a gas and is sent back to the compressor to start all over again. 

Now, here is an important point that several people confuse between condenser and radiator. The condenser and radiator are two different components. The radiator is a component of a cooling system while the condenser is a component of an air conditioning system that condenses the hot, high-pressure vaporized refrigerant that comes from the compressor into a liquid form.

Similarly, people often confuse radiator fan and condenser fan. The radiator fan has nothing to do with the air conditioning system. The condenser fan is located on the passenger side and the radiator fan is located on the driver’s side. The condenser fan, which is also called a cooling fan, blows air across the condenser while the car is stopped or at low speeds. The speed of a condenser fan is controlled by the engine control module.

Expansion Device

After condenser, the refrigerant passes through the expansion device where liquid refrigerant expands to vapor, and temperature and pressure are lowered to a great extent to allow greater heat transfer between hot ambient air and refrigerant.

Evaporator

After the expansion valve, the refrigerant passes through the evaporator, where it absorbs heat from the air. As the refrigerant liquid absorbs the heat from the air, it evaporates into a gas and is sent back to the compressor to start all over again.

Car AC Not Blowing Cold Air When Idling: Causes

Here are the following causes of a car AC not working properly when idle:

1. Faulty Condenser Fan

The condenser fan is responsible for blowing air over the condenser to help the refrigerant cool down. If the fan isn’t working, then the air won’t be cooled and the AC won’t blow cold air when the engine is idling.

When the car is moving at a high speed, the air passing through the condenser is enough to cool the refrigerant and make it cold. However, when the car is idle, there is no air movement and the condenser fan is the only tool for cooling the refrigerant.

If the fan is not working properly, it will not be able to circulate the air and the refrigerant will not stay cold. Therefore, when the car is idle, the cold air from the AC will not blow. 

To check if the condenser fan is running, make sure that the AC compressor is also running. This is because the control system of the car conditioning system is designed in a way that when a compressor is turned off, the condenser fan is also turned off.

If your car has a climate control system (AUTO mode), turn it off by putting the AC in full-blast mode, and make sure the AC compressor is running.

In this condition, if the condenser fan is not running, try to rev up the engine to around 2000 RPM at idle. If the condenser fan does not run, it needs to be replaced. But, if at that stage, the condenser fan runs at a slow speed, the car AC system has either a low refrigerant charge or the refrigerant pressure sensor is bad.

2. Bad Electric Connection Of Condenser Fan

Before replacing the condenser fan, you should also check the connectors of the condenser fan, condenser relay, and fuse. In older engines, there is a condenser relay that is activated as the AC compressor clutch is activated. So, you should check if the relay needs to be replaced. Next, you should also check fuse of condenser fan and AC compressor.

control circuit diagram of ac condenser fan

I found the following Youtube video helpful in understanding the electric circuit of car AC condenser fan.

To test condenser fan relay, you can watch this Youtube video:

After checking the relay and fuse, you should check the harness connector and terminals of the car AC condenser fan.

The following video is also good to test connector of the AC condenser fan:

3. Condenser Is Clogged

A clogged car AC condenser can prevent cold air from AC when idle. It’s important to visually check the condenser for obstructions like dirt, leaves, dead birds, etc. that covers the fins. 

The condenser has many small fins that allow air to pass through and absorb heat from the air. When the air is blocked by debris and pollens, the condenser is unable to cool the air and the AC is unable to produce cold air.

To make sure the condenser is clear of debris, use a garden hose to rinse the condenser and remove any particles that may have built up. It is also important to check for any dead birds or animals that may have become stuck in the condenser and block the airflow.

4. Low Refrigerant Charge

 A low refrigerant charge in a car AC system will prevent cold air from the AC when the engine is idle. The refrigerant is responsible for the cooling system’s ability to produce cold air, and it is under pressure from the compressor.

When the engine speed is increased, the pressure of the refrigerant is able to rise enough to become efficient at producing cold air. 

When there is a low refrigerant charge, the pressure of the refrigerant is insufficient to cool the air and the evaporator is unable to absorb the heat from the air. Therefore, the air will not be cooled and will remain hot, resulting in a lack of cold air from the AC when the engine is idle.

The low refrigerant charge in a car AC system can be attributed to a leak in the system. All air conditioning systems use a closed refrigerant loop that is pressurized to force the refrigerant to flow through the system. If there is a leak in the system, the pressurized refrigerant will be released, causing the refrigerant charge to become low.

The leaks in the car AC system can happen due to:

  • Faulty seal of condenser, evaporator and compressor
  • Damaged O-rings
  • Worn-out hose

To find out if the car AC system is low on refrigerant charge, you should measure high-side and low-side pressures in the car AC system. The optimal values of the high-side and low-side pressure of refrigerant depend on ambient temperature. You can find values from the R134-a ambient temperature and pressure chart. If the low and high-side pressures are too low, it means that the AC system is undercharged.

You can recharge the A/C system with a refrigerant that has a dye in it so you can trace all the hoses and connections when the system leaks down again. Since most cars use R134a refrigerant, which is heavier than air, always check 360° around all fittings for refrigerant leaks.

5. Faulty Pressure Switches Or Sensors

The high-pressure and low-pressure switches are located in the car AC system and are responsible for maintaining a particular pressure in the AC system.

The high-pressure switch normally closed until the high-side pressure reaches its limit or there is contamination i.e. water vapor and air in the refrigerant.

The low-pressure switch is a normally closed switch, which means that when the pressure is below the threshold, the switch will open and power will be cut to the compressor clutch and condenser fan.

The pressure switches send signals to the ECM to control the power supply to the condenser fan and compressor clutch. If the pressure switches are bad and the ECM, and doesn’t like what it sees, it may not even command the condenser fan to run.

To test the AC pressure switch:

  1. Turn the AC on to full-bast mode.
  2. Connect one end of the multimeter’s test leads to the wire of the AC pressure switch and the other to the ground.
  3. Turn the multimeter to the Ohm setting.
  4. Start the car and turn on the AC.
  5. The multimeter should read zero as the pressure switch is closed when the AC is on and an open circuit when the switch is open. If the multimeter reads differently, the switch may need to be replaced.

Note: Some pressure switches/sensors have three wires. One is a 5V wire, the second one is the signal wire from the ECM and the third is the ground wire. If there is something wrong with the values, you need to check the wiring.

Why Is Car AC Not Working Properly At Idle?

It is normal for some cars’ ACs not blowing cold air at idle because of low engine speed, and lesser airflow over the condenser. If your condenser fan is working properly, you will see that as you rev up your engine, your car AC will start blowing cold air at idle.

As your engine RPM increases, the AC compressor also runs at high speed, due to which it can compress the refrigerant to a higher discharge pressure. This also increases the performance of car air conditioning system.

Source: https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2469&context=iracc

A study was conducted to study the effect of refrigerant amount and compressor speed on the discharge pressure of the compressor. From the figure above, you can see that at higher compressor speeds, discharge pressure also increases.