[SOLVED] AC Only Works When Driving and Blows Warm Air When I Stop

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A common yet critical issue many car owners face is when the AC only works while the car is in motion. If you’re experiencing this problem, this article will help you understand the possible root causes of malfunctions in your car’s HVAC system. 

We will also explore troubleshooting methods and practical solutions to ensure your car remains cool and comfortable, whether you’re cruising on the highway or idling in a traffic jam.


Understanding the Air Conditioning process in Cars

The cabin climate becomes cool as refrigerant absorbs the heat from the cabin through a cycle of compression and expansion. The compressor pumps refrigerant throughout the AC system. Unlike domestic AC systems at home, your AC in cars doesn’t get a continuous and stable supply of power. Instead, it gets powered by the drive system of the car’s engine. 

This implies the performance of the AC depends on the operational state of the engine. 

Instead of getting into more details, let’s move on to the next section. 


Knowing when to ask for help with your car’s AC 

Your car’s AC might slow down a bit when the wheels aren’t moving. Usually, these dips are negligible, thanks to the refrigerants that still hold some cooling capacity. ACs malfunctioning to a state of cooling action that sends nothing but warm air is a sign of worry. 

If the AC completely stops the cooling process when the car is in an idle state. Don’t ignore this warning sign – early detection can reduce repair costs and extend the life cycle of crucial AC components.


Solving Your Car’s AC Problem – Blows Warm Air when you stop the car

If you have tested and found that your car’s AC starts working normally once you rev up the engine, it means there is no major issue. However, if the AC vents blow only hot air, it’s time to inspect the AC system. You will require basic tools to check for any leakages; performing this test with safety gear is advised. We recommend you follow the guidelines. 

1. Perform Basic Car AC service

Clean and flush the condenser and radiator; if the problem still persists, the condenser requires in-depth inspection and, in some cases, replacement. The condenser is made of parallelly arranged coils and is prone to dust and debris accumulation over time. This affects the heat dissipation process of the condenser, thereby affecting the optimum circulation of the refrigerant. 

Old and worn-out condensers leak their debris inside the coil, which is carried by the high-pressure refrigerant to other parts of the AC system. Under such conditions, the entire AC system needs to be cleaned or replaced.

2. Inspect the Refrigerant for Impurities and Moisture

Impurities are signs of heavy contamination inside the AC system. Handling refrigerants can be risky due to their environmental impacts; some countries have laws making their discharge illegal. It’s essential to handle its operation using the right tools without allowing it to get leaked. If you do get to visually inspect the refrigerant with the right tools, check for the presence of moisture or black residue.

Black Teflon or Aluminum chips present in the refrigerant sample indicate extreme friction. This happens when the compressor is going through a malfunctioned state called the “First Phase of Seizing.” In case you find such traces, make sure to use a safe recovery method to remove the refrigerant. Dispose of it carefully. The next step is to flush the system to remove any remaining impurities and contaminants. This can be done using a proper flush solvent.

If you find any traces of separate bubbles, it confirms the presence of moisture inside the refrigerant. It can significantly slow down the cooling process as moisture turns into ice, stopping the flow of refrigerant. You can remove the moisture and air with the help of a vacuum pump. Recharge the AC system with the appropriate type and quantity of refrigerant. Lubricate the compressor and other moving parts with lubricating oil to cut friction. 

3. Inspecting the color of the refrigerant

You can visually inspect the color of the refrigerant to assess its coolant performance. A healthy refrigerant or medium should be transparent and colorless. A light green or yellowish hue shouldn’t bother you; the refrigerant comes with a UV dye agent to help trace leakage. A dark gray or black soot-like appearance of fluid indicates a problem in the compressor. The change in color is because of extreme friction in the compressor. It may have failed, or it must be in the process of failing. 

4. Check the Refrigerant level

Both low and high refrigerant levels will affect the cooling process of the AC. Leaking refrigerant is the number one cause of the cooling performance of the AC going down. In case the pressure is high, it’s usually because of a faulty condenser that fails to dissipate the heat. Because of high pressure and temperature, the refrigerant cannot go through the evaporator cooling stage. The result is hot gusts of air flowing through the AC dashboard vent.

The low level of refrigerant is likely due to leakage, while the compressor action during the drive mode compensates for the pressure loss. The cooling process comes to a standstill when the compressor’s pressure drops when the car is stationary. The refrigerant is unable to complete the cooling cycle under such conditions. 

5. Inspecting the Expansion Valve 

Usually, a faulty expansion valve will lead to intermittent cooling, build frost on the compressor, or show unusual pressure readings. If the expansion valve is blocked in the open position, it could lead to extreme pressure readings. A clogged or blocked valve can cause the same issue. Try replacing the valve to see if the pressure setting turns back to normal. 

6. Find AC Leaks in Your Car (UV Dye)

Start by inspecting the AC line, Condenser, and Compressor for any signs of leakage. One easy way is to look for dust accumulation along the lines as fluid leakage attracts dust. Most refrigerants have UV dye, which illuminates under UV light. A simple UV light flash can reveal traces of leakages. The AC filter sometimes gets clogged, affecting the refrigerant circulation. A clogged filter can significantly impact the AC’s performance.


7. Inspect the Cooling Fan

Your car has a pair or one cooling fan near the condenser to remove extra heat from the engine and AC system. Its function is crucial since maintaining the optimum engine temperature prevents overheating. 

When either of the cooling fans malfunctions, the car’s Engine Control Unit (ECU) breaks the relay to activate the clutch mechanism in the compressor. This halts the AC system, bringing the cooling process to a standstill. 

In such instances, the dashboard AC vent in your car blows warm gusts of air instead of cold air. You can diagnose the faulty cooling fan by visually inspecting it and consider servicing it or replacing it with a new one. 

8. Inspecting the Pressure Switch

The pressure switch acts as a gate to allow the prescribed refrigerant pressure for an efficient cooling process. It protects the compressor from overheating and seizing due to pressure temperature build-up and lack of refrigerant circulation. 

In instances where refrigerant leakage causes low pressure, the pressure switch shuts down the compressor to protect it from overheating. Wherein, during high pressure, the switch performs the same function to cut off the compressor. This is performed to avoid the build-up of pressure to dangerous levels that may end up damaging the linings or other components. 

To diagnose a faulty pressure switch, you can use special tools to check if the low-pressure and high-pressure switches are working properly. The common method used is the bypassing strategy. The refrigerant is allowed to pass without the restriction of high-pressure and low-pressure switch to see if the AC system responds. If the cooling process resumes during the bypassing phase, it implies the pressure switches need to be changed.  


Troubleshooting Compressor Clutch: The Heart of the AC System

The compressor clutch is located near the engine and connected to the drive belt. It engages to pump refrigerant through the condenser. It’s important to maintain the compressor clutch in its optimal state to power the cooling process. It can malfunction for various reasons, leading to intermittent cooling or a point where the compressor shuts off. This leads to hot air blowing out of your AC vents. 

Common Reasons for Clutch Not Engaging 

You can locate the clutch on the side of the pulley wheel. It’s the most critical component of the AC system that engages when the AC switch in the dashboard is turned ON. Most cars come with indicator lights that signal a problem with the cooling process or the clutch. There are various conditions that impede the clutch from engaging. The scenarios are provided below.

1. Temperature Extremes

High temperature

A car sweltering under the sun for a long time can affect the functionalities of the AC system components. The compressor clutch can get hot, and to prevent overheating, the clutch may not engage until the temperature is down. 

Low Temperature

When the temperature is too low, the refrigerant loses its properties, which affects the circulation. This can also prevent the clutch from engaging.


2. Faulty magnetic clutch relay

The magnetic clutch relay calls a function from the AC switch button in the dashboard to the compression clutch. The clutch engages when the relay is complete. The cooling process of the AC system gets affected if the relay switch fails completely or partially.

 If the magnetic clutch relay fails completely, the compressor won’t receive the drive power from the engine, causing a complete stall to the AC system. If the relay switch is partially malfunctioning, it may lead to intermittent cooling wherein the cabin receives periods of hot air followed by brief bursts of cool air. If you hear a clicking sound while turning on the AC, this is because of the relay trying to engage but failing. The AC system halts under such conditions. 

Diagnosing the Magnetic Clutch Relay

Remove the magnetic clutch relay and insert the horn relay for testing. Switch on the AC system from your car’s dashboard, if the clutch engages, replace the magnetic clutch relay with a new one. Conduct a voltage test to measure the voltage at the relay’s terminal. If the voltage reading is lower than recommended, it implies faulty wiring or a problem with the control module. 

3. Faulty Electromagnetic Coil

You may hear a clunking noise before the AC in your car turns off. It possibly points towards faulty clutch engagement. There can be multiple reasons for this, but one common root cause is a faulty electromagnetic coil. Extreme temperature build-up in the engine area sometimes ends up damaging the electromagnetic property of the coil. 

The role of the coil is to engage the clutch with the pulley when AC is turned ON. A faulty coil may not initiate the clutch engagement, following which the compressor fails to receive power. The problem can escalate further if the car is stationary, as reduced airflow and heat dissipation increase the temperature. 

You can visibly inspect the engaging sequence of the coil. Have someone turn on your car’s  AC and check if the clutch is engaging. If the coil is healthy, it should be able to attract the clutch and engage with the pulley. If there is no clunking sound or visible difference in the engagement mechanism, the coil must have become faulty. 

Another common problem with coils is when they fail to receive enough voltage. Test the voltage using a millimeter; see if the reading matches the recommended level. If the reading is zero or deviates, the problem is with the coil and electric circuit.

You can use an onboard diagnostic system to retrieve error codes related to the AC system. This can make things clear although not all cars have this function

You must replace the faulty coil and other faulty electrical components. It’s also important to investigate the reason for increased temperature build-up in the car.  Increased temperature in the engine area could also be due to faulty fans, clogged condensers or blocked vents.


4. Clutch Wear

Prolonged usage and increased wear and tear can leave the clutch in a bad state. You can inspect by checking if the clutch turns freely when disengaged. If you feel that the clutch is not easily turning and stuck, this indicates a faulty clutch that’s arresting the engaging and disengaging mechanism with the engine drive. This can also affect the AC system. Consider replacing the clutch to see if the AC system resumes its normal operation. 


5. Faulty Drive Belt

The drive belt connects the engine to the compressor. As the engine revs, the crankshaft pulley rotates the drive belt. This powers the compressor, which pumps the refrigerants through the AC system. A faulty drive belt wouldn’t power the compressor and the AC system remains disengaged. 

An optimal tensioned drive belt provides effective power transfer and maximizes the cooling process of the AC system. A worn or frictionless slipping belt impedes power transfer, leading to malfunctioning of the AC system. Visually inspect the drive to look for cracks, glazing, or excessive wear indicating the need for replacement. 

A properly working AC requires optimal operation of all components during both idle and engine running conditions. If AC systems blow cold air only while driving, it implies concern. Finding the root cause can be exhaustive, but when done under experienced and expert supervision, this issue can be resolved. 


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