The basic cycle: how refrigerant moves through your AC system
Your car's air conditioning works by circulating a chemical refrigerant through a closed loop of metal tubing and components. The refrigerant changes between liquid and gas states as it moves through the system, and those state changes are what absorb heat from inside your car and release it outside. The whole process repeats continuously while the AC is running.
The cycle starts at the compressor, a pump driven by your engine's serpentine belt. The compressor takes low-pressure refrigerant gas and squeezes it into high-pressure gas, which heats up in the process. That hot, pressurized gas then flows to the condenser, a radiator-like component mounted in front of your car where air flowing over it cools the gas back down into liquid form. The liquid refrigerant is still under high pressure at this point.
Next, the high-pressure liquid flows through a receiver-drier (or accumulator, depending on your system design), which filters out moisture and debris that could damage the compressor. Then the refrigerant passes through an expansion valve or orifice tube, which is a tiny opening that drops the pressure suddenly. When pressure drops, the liquid refrigerant evaporates into a gas again, and that evaporation absorbs heat from the air around it.
That cold gas then flows through the evaporator, a heat exchanger mounted inside your dashboard. Warm air from inside the cabin passes over the evaporator's cold coils, loses its heat to the refrigerant, and comes out cold. A blower fan pushes that chilled air through your vents. The now-warm refrigerant gas returns to the compressor, and the cycle starts over.
Key Takeaways
- Refrigerant circulates through four main components—compressor, condenser, evaporator, and expansion valve—changing between liquid and gas to move heat out of your cabin.
- The compressor is powered by your engine's belt and is the only moving part that requires mechanical energy; everything else relies on pressure changes and heat transfer.
- The condenser releases heat outside the car by cooling pressurized refrigerant gas into liquid; the evaporator absorbs heat inside the car by letting refrigerant gas expand and cool.
- A receiver-drier or accumulator removes moisture and particles from the refrigerant to prevent compressor damage and system failure.
- The expansion valve or orifice tube is the boundary between high-pressure and low-pressure sides of the system and controls how much refrigerant enters the evaporator.
The compressor: the engine of your AC system
The compressor is the only part of your AC system that requires power from your engine. It is bolted to the engine block and connected by a serpentine belt that spins whenever the engine runs. When you turn on the AC, an electromagnetic clutch engages and locks the compressor's internal shaft to the belt, causing it to spin and pump refrigerant.
Inside the compressor, pistons or rotating vanes compress the refrigerant gas into a much smaller volume, raising both its pressure and temperature. This high-pressure gas is then forced out of the compressor and into the condenser. If the compressor fails, your AC system cannot function at all, because nothing else in the loop can move the refrigerant on its own.
The compressor clutch is designed to disengage when the evaporator gets too cold or when you turn off the AC, which prevents the compressor from running continuously and wasting fuel. Some systems also disengage the clutch if the refrigerant pressure drops too low, as a safety measure to prevent damage.
The condenser: releasing heat outside
The condenser sits in front of your radiator and looks similar to it. Hot, high-pressure refrigerant gas flows into the condenser from the compressor, and as outside air passes over the condenser's metal fins and tubes, the refrigerant cools down and condenses into liquid form. This is the same principle as a radiator cooling engine coolant, except the condenser is cooling refrigerant instead.
For the condenser to work efficiently, it needs good airflow. When your car is moving, the wind naturally pushes air through the condenser. When you are stopped in traffic or idling, an electric cooling fan kicks in to pull air through the condenser and keep the refrigerant cool enough to condense. If that fan fails or if the condenser becomes clogged with dirt and debris, the refrigerant will not cool properly, and your AC will blow warm air.
The liquid refrigerant leaving the condenser is still under high pressure—typically between 150 and 300 pounds per square inch, depending on outside temperature and system load. This high pressure is necessary to keep the refrigerant in liquid form until it reaches the expansion valve.
The evaporator: absorbing heat from inside your car
The evaporator is mounted inside your car's dashboard, usually behind the glove box or under the steering column. Low-pressure liquid refrigerant enters the evaporator and when ready begins to evaporate into a gas because the pressure is so low. Evaporation is an endothermic process, meaning it absorbs heat from its surroundings—in this case, the warm air inside your cabin.
A blower fan pulls air from inside your car and forces it over the evaporator's cold coils. As the warm cabin air passes over the evaporator, it transfers its heat to the cold refrigerant, cooling the air down to around 40 to 50 degrees Fahrenheit. That chilled air is then directed through your vents into the cabin. The refrigerant, now warmed by absorbing that heat, exits the evaporator as a gas and returns to the compressor.
The evaporator also removes moisture from the air as a side effect. When warm, humid air comes into contact with the cold evaporator coils, the moisture condenses on the coils just like dew forms on a cold window. This condensed water drains out of the system through a tube that runs underneath your car, which is why you often see a puddle of water under a parked car with the AC running.
The expansion valve and receiver-drier: controlling flow and protecting the system
The expansion valve (or orifice tube in some systems) is a small but critical component that sits between the condenser and the evaporator. Its job is to meter the flow of high-pressure liquid refrigerant into the low-pressure side of the system. As the refrigerant passes through the tiny opening in the expansion valve, its pressure drops dramatically, which causes it to evaporate rapidly.
The receiver-drier sits on the high-pressure side of the system, usually between the condenser and the expansion valve. It serves two purposes: it removes moisture from the refrigerant, and it filters out debris. Moisture in the system can react with refrigerant to form acids that corrode metal components and damage the compressor. Debris can clog the expansion valve or scratch the compressor's internal parts. The receiver-drier contains a desiccant material (usually silica gel) that absorbs moisture, and a mesh screen that traps particles.
Some newer systems use an accumulator instead of a receiver-drier. The accumulator sits on the low-pressure side of the system and serves a similar moisture-removal function, but it also acts as a temporary storage tank for excess refrigerant, which helps stabilize system pressure and prevents liquid refrigerant from entering the compressor (which would damage it, since compressors are designed to pump gas, not liquid).
High-pressure and low-pressure sides: why the system is divided
Your AC system is divided into two halves by the expansion valve: the high-pressure side and the low-pressure side. Understanding this division helps explain why certain components are where they are and why the system works the way it does.
The high-pressure side includes the compressor, condenser, and receiver-drier. Refrigerant on this side is under pressure between 150 and 300 psi (pounds per square inch), depending on outside temperature and how hard the system is working. The high pressure keeps the refrigerant in liquid form, which is necessary for the condenser to cool it efficiently. If pressure drops on the high-pressure side, the refrigerant will not condense properly, and your AC will blow warm air.
The low-pressure side includes the expansion valve, evaporator, and the return line to the compressor. Refrigerant on this side is under pressure between 20 and 40 psi. The low pressure allows the refrigerant to evaporate, which is what absorbs heat from your cabin. If pressure rises on the low-pressure side, the refrigerant will not evaporate efficiently, and again, your AC will blow warm air or not cool at all.
Common AC problems and what causes them
A refrigerant leak is the most common reason an AC system stops cooling. Refrigerant can leak from any connection or component in the system, but the most frequent leak points are the compressor shaft seal, the condenser, and the hose connections. A small leak may cause the system to cool slowly; a large leak will cause it to blow warm air when ready. Leaks cannot be repaired by adding more refrigerant—the leak itself must be found and sealed, and then the system must be recharged with the correct amount of refrigerant.
A failed compressor clutch will prevent the compressor from engaging, so the refrigerant will not circulate and the AC will not cool. The clutch is an electromagnetic device that engages when you turn on the AC; if it fails, you will hear the compressor spin but feel no cold air. Replacing the clutch usually requires removing the compressor from the engine.
A clogged condenser or a failed cooling fan will prevent the refrigerant from cooling properly on the high-pressure side. If the condenser is blocked by dirt, leaves, or debris, or if the electric cooling fan does not turn on, the refrigerant will remain too warm to condense into liquid, and your AC will blow warm air, especially when the car is idling or moving slowly.
A failed expansion valve or a clogged orifice tube will prevent the refrigerant from flowing into the evaporator at the correct rate. If the valve is stuck closed, no cold refrigerant reaches the evaporator, and you get no cooling. If the valve is stuck open, too much refrigerant floods the evaporator, which can cause the compressor to receive liquid refrigerant instead of gas, leading to compressor damage.
Frequently Asked Questions
Why does my AC blow cold air at first, then warm up after a few minutes?
This usually indicates a low refrigerant charge, often caused by a slow leak. The system cools initially because there is still enough refrigerant to circulate, but as the compressor runs, the remaining refrigerant spreads through the system and pressure drops. When pressure drops too low, the refrigerant does not evaporate efficiently in the evaporator, and cooling stops. A technician can locate the leak and recharge the system.
What does it mean if my AC compressor makes a loud noise?
A loud grinding, squealing, or knocking noise from the compressor usually indicates internal damage. The compressor may have failed bearings, worn pistons, or metal debris circulating inside it. Continuing to run the compressor in this condition will cause more damage and may scatter metal particles throughout the entire AC system. The compressor should be replaced, and the system should be flushed to remove debris.
Can I add refrigerant to my AC myself?
You can purchase DIY refrigerant kits at auto parts stores, but they are not recommended. These kits do not measure the exact amount of refrigerant your system needs, and overcharging or undercharging will reduce cooling performance and can damage the compressor. A professional technician uses a scale to measure the precise charge and can also identify and repair leaks before recharging.
Why is there water dripping under my car when the AC is on?
That water is condensation from the evaporator. As humid cabin air passes over the cold evaporator coils, moisture condenses on the coils and drains out through a tube underneath the car. This is normal and expected. If the drain tube becomes clogged, water can back up into the cabin and cause mold or mildew growth inside the dashboard.
How often should I have my AC system serviced?
Most manufacturers recommend an AC inspection every two years or when you notice reduced cooling performance. During a service, a technician checks refrigerant pressure, inspects hoses and connections for leaks, tests the compressor clutch, and verifies that the cooling fan operates correctly. Regular maintenance can catch small problems before they become expensive repairs.