Introduction

• All automotive air conditioning systems are comprised of at least 12 essential components, which will always be present. The way the system operates is also the same--with the exception of automatic climate control as opposed to manually operated fans and temperature (or climate control).
  
  In a manually operated system, the driver turns the air conditioning on or off and adjusts the fan speed and temperature manually. In an automated system, the driver needs only to turn the system on and set the temperature desired. The system automatically sets the fan to maintain the car's temperature.
  
  

Freon

• Modern air-conditioning systems use a fluid called Freon. The designation for this particular Freon is R-134. R-134 replaced R-12 when it was discovered that chlorofluorocarbons contained within the R-12 were harmful to the environment and had a negative effect on our ozone layer.
  
  R-134 is not perfect, but it is much less harmful. However, it still cannot be released into the atmosphere under penalty of law.
  
  The purpose of Freon is to absorb heat from the vehicle's interior and transfer it outside the vehicle. A wide variety of chemicals could be used to cool by heat absorption--even water. The trick is to compress the fluid to a point where it boils or has a higher temperature than the outside air. Heat always moves to cooler temperature, so if the temperature of the water is at 220 degrees Fahrenheit or higher it transfers its heat to the atmosphere. The cooler water is left under pressure and sprayed through a small hole, which then turns it into a mist.
  
  It will be very cold compared to the outside temperature, quickly absorbing the surrounding heat and carrying it away.
  
  Herein lies the problem. Water takes way too much compression to bring its temperature up enough to be practical. This is where these different liquids come into play.
  
  Freons have extremely low boiling points and it takes little compression to raise their temperatures. With an extremely low boiling point, as soon as the Freon absorbs the heat in the vehicle, it boils and turns into a gaseous state, like steam. It can easily be cooled enough to turn back into a liquid suitable for the air conditioning system.
  
  

The A/C Cycle

• The air-conditioning cycle starts at the compressor. The compressor's purpose is to compress the Freon to bring it to a higher temperature than the outside air.
  
  The compressor operates with the use of a magnetic clutch to engage and disengage. There is a metal disc attached to the shaft of the compressor and an electromagnet behind it that turns with the drive belt. When the compressor is off, the plate attached to the shaft does not move and the drive belt simply rotates on the bearing around it.
  
  When the compressor is turned on, the electromagnet engages and the plate on the shaft is attracted to the magnet, which then causes it to spin. The Freon moves while under pressure and at a high temperature from the compressor to the condenser.
  
  The condenser is a radiator in front of the engine's radiator. The high-pressure, hot Freon travels through the condenser, releasing its heat into the atmosphere, thereby cooling the gas and transforming it once again to a liquid.
  
  

• Air must always pass through the condenser, either by the vehicle moving or by a separate fan when the car is still.
  
  When the car comes to rest, the fan must operate to draw air through the condenser or the Freon will build up pressure and the air conditioning system will cease to function.
  
  In some cases, the pressure can build high enough to blow a hose. The Freon releases its heat and flows downward through the condenser and turns to a liquid under pressure. It then travels from the condenser, through a hose to a metering valve.
  
  

• En route to the metering valve, the Freon will encounter a high-side switch. This switch monitors the pressure within the system. If the high-side switch notes the pressure is too high, it will shut off the compressor. This should happen if the fan fails or too much Freon is in the system.
  
  At the metering valve (which could be an orifice tube, throttling valve, liquid line, H block on Chrysler or a proportioning valve, depending on manufacturer), the liquid is pushed through a small opening and turned into a mist. This mist is extremely cold and travels into the evaporator within the car.
  
  The evaporator is housed in a box under the dash. The gas moving through the evaporator is also very cold and absorbs the heat from the vehicle. The evaporator passes air by one of two methods.
  
  One is from the outside vent by the windshield wipers. This is good to replace the air in the vehicle but is much less effective than the second method.
  
  The other method is to recirculate the air, which is picked up from inside the vehicle. This method cools much better.
  
  

Conclusion and Other Issues

• The bottom of the evaporator case has a tube running through the firewall to allow the condensation to run out on the ground rather than the floor of the vehicle.
  
  After absorbing the heat from the vehicle, the Freon is now mostly a gas again and travels out to an accumulator on the firewall.
  
  This is a bottle that has two functions. One is to remove all the moisture that might be in the system. This is done with the use of a desiccant. Moisture will freeze in the system and cause the system to function poorly.
  
  Its other duty is to trap any Freon that has not yet turned to a gas as it passes through the evaporator. It will hold the Freon long enough to allow it to turn to a gaseous state because the next stop is back to the compressor to begin the cycle again.
  
  The compressor cannot compress any liquid, only gas. A liquid would destroy the reed valves in the compressor.
  
  

• The air-conditioning system has a certain amount of critical refrigerant oil in it that circulates through the system.
  
  The amount is usually listed on the vehicle near the radiator or on the passenger side fender well. Without the proper amount of oil, the compressor will fail quickly. Too much oil can clog the valve and also cause the system to fail, so the right amount of oil is necessary.
  
  Any time there is a leak in the system, the oil comes out first so oil must be added every time there is a repair.