How Air Conditioning Works

Your car's air conditioner is a vapor-compression cooler. It takes the heat inside the car to the outside. It is able to do this two ways: when liquid boils or condenses.

When liquid becomes a gas (by reaching a boiling point), heat is absorbed; when the gas condenses, the same amount of heat is released. The amount of force on a liquid or gas affects its temperature and the temperature at which it boils or condenses.

For example, if you apply 20 pounds per square inch (psi) of pressure to 211 degrees Fahrenheit water, it will warm to 257 degrees Fahrenheit and stay on the verge of boiling. Because temperature and pressure are directly related, changing one or the other as needed allows gases to be changed to liquids (and vice-versa) with easily.

Your car's air conditioner moves heat when a liquid become a gas inside your car, absorbing heat and returning the gas to liquid outside the car where it releases the heat. The liquid happens to be refrigerant-12 (often called R-12 or Freon, the DuPont trademark).The liquid, a refrigerant, R-12 also known Freon) is used because of its expedient boiling point (19 degrees Fahrenheit at 20 psi).

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Tips for Best Performance

Most car owners have very little understanding of how their car's air conditioner works or what to do when it doesn't-and with good reason. Opening the pressurized system should really be left to the pros, who have the expertise and appropriate tools needed. But that doesn't mean there you can't tackle a few common challenges. It helps for to have a basic understanding of the trouble, so check out the keys mentioned below to help you unlock some common AC issues.

The Simple Parts of Your AC

Your car's air conditioner has five crucial parts.

1. Compressor The compressor takes low-pressure, cool refrigerant and compresses it, making it hot. Two to six pistons similar to the ones in your engine compress it. A V-belt from the engine drives the unit, and a clutch separates it when the air conditioner is off. (Some systems cycle the compressor on and off to control the temperature inside the car. In these, the clutch is wired to the on/off switch and a thermostat.) You can easily locate the compressor when you look for a relatively large object with a pulley and fan belt, two rubber hoses about an inch in diameter, and valves on top that resemble the ones on your tires. When you locate it, trace the one wire going to the compressor. That's the compressor clutch wire. There should be a fuse inline close to it. You might want to take note of its rating, and purchase a spare to keep in your glove compartment.

2. Condenser The condenser removes heat from the high-pressure refrigerant gas, causing the gas to condense and form a liquid. This occurs at a relatively high temperature because by now the pressure is high. Look in front of the car's cooling-system radiator, and you'll probably find another radiator-looking thing. You can confirm that it's the condenser (rather than a transmission or oil cooler) by tracing a hose connected to its top back to the compressor. Called the "high side," the section between the compressor and the expansion valve could experience pressure as high as 270 psi. Watch for deterioration of hoses on the “high side.” They usually don't last as long as those on the "low side," though both sets require inspection.

3. Receiver-drier Once the refrigerant becomes a liquid, rests in the receiver until it's needed. It passes through a filter to remove dirt and through desiccant to remove water. (It's vital the refrigerant remains dry. It forms hydrochloric acid as it's mixed with water, which could corrode the metal pans of the system.) There's a good chance the receiver-drier will be near the condenser. Follow the hose out of the bottom of the condenser to locate it. You'll know you've found the right can-with-hoses when you locate one that has a roughly half-inch viewing window near its inlet. This is called the sight glass. When the air conditioner is working, you should see clear liquid inside. If it's milky, you've got water in the system; if it's frothy or bubbly, there's air inside it. IF you discover any of these conditions, contact us at any of our 14 locations to have one of our qualified service technicians assist you.

4. Expansion valve This important component restricts refrigerant flow to maintain high pressure upstream between it and the compressor. As refrigerant sprays through the valve's opening into the evaporator, it transforms into a gas, and absorbs heat. The low pressure on the downstream side of the expansion valve causes it to evaporate. (Some systems use the expansion valve to control temperature inside the car by varying the amount of liquid refrigerant sprayed into the evaporator. In this case, the valve will be connected to a thermostat.) The expansion valve is sometimes hidden behind a panel. Follow the hose from the outlet of the receiver-drier, and see if you can find the expansion valve.

5. Evaporator The evaporator works in reverse of a radiator; absorbing heat rather than radiating it. It rests inside the passenger compartment behind and underneath the dash, so you may not be able to see it. A fan blows air over the evaporator's fins to help with heat absorption. You should be able to find the low-pressure hose from the evaporator snaking its way back through the fire wall to the other connection on the compressor.

Keys to Troubleshooting Your Car's A/C

• Challenge: Fan doesn't work.
  • Problem: Electrical problem.
  • Solution: Replace fuse, wire or fan motor.

• Challenge: Fan works but air isn't cool.
  • Problem: Compressor not operating.
  • Solution: A. Check for broken V-belt, replace if necessary.
  • Solution:B. If compressor spins but clutch doesn't engage, check fuses in main panel and in line with clutch; check for broken clutch wire.
  • Solution:C. If compressor will not turn, consult mechanic.

• Problem: Clogged system.
  • Solution: If frosting occurs at receiver-drier or expansion valve, suspect clogs at these points; consult a mechanic.

• Problem: Lack of refrigerant.
  • Solution: If condenser doesn't feel warm at the bottom, suspect lack of coolant or an ineffective compressor.
• Challenge: Fan works but air isn't cool enough.
  • Problem: V-belt slipping.

    • Solution: Adjust V-belt tension or replace V-belt. Problem: Clutch slipping. Solution: If the clutch and compressor are spinning at different speeds, consult a mechanic.

• Problem: Insufficient refrigerant; air or water contamination.
  • Solution: Look for bubbles or cloudy appearance in sight glass; consult mechanic.

• Problem: Fouled condenser or evaporator.
  • Solution: Check and clean as necessary.

• Problem: Partially plugged receiver-drier or expansion valve.
  • Remedy: If either component is cool, consult a mechanic.