It's hot enough to fry an egg on the sidewalk, and you jump in your truck with sweat dripping off your brow. Fire up your engine and push the magic button, and in a matter of minutes, you're bathed in a cool wind pouring from your dash. Sure, there are plenty of us that run sans tops or doors, but when summer hits, either our trail rigs or our tow rigs often have air conditioning.
Basic diagram showing the major components of an auto air-conditioning system.
So, how does that chill-making machine work? It all starts with the basis of a compressed gas in a closed system and the fact that an expanding gas drops in temperature. By utilizing these simple physics laws, we can build a system to cool us when outside temperatures soar.
The heart of the A/C system starts with an engine-driven compressor that utilizes an electrically actuated clutch to turn on and off. The compressor takes low-pressure refrigerant (Freon gas for older vehicles or R-134 for newer models) and pumps it to a high pressure at its outlet. This act drastically raises the temperature of the refrigerant. It's then pumped through lines to the condenser that sits in front of the radiator, where some cooling occurs, and the refrigerant starts to liquefy.
At this point, the refrigerant usually passes through a nearby filter/dryer (receiver) canister where desiccant is used to extract any moisture that may be present. Then the refrigerant enters lines running under the dash in the cab. By this time, the refrigerant has turned to a liquid state, and it passes through an expansion valve (small restrictor tube) that serves to meter the refrigerant flow into the evaporator at a controlled rate.
As the refrigerant enters the evaporator, it has the chance to expand back into a gaseous state, and when this happens, the temperature of the gas drops rapidly. This is the point where your blower fan pumps air across the fins of the evaporator, heat from the air is absorbed by the evaporator, and cool air is pumped into your cab.
From here, the refrigerant moves back toward the compressor and should be gaseous again before entering the low-pressure inlet, and the whole cycle continues. Additionally, light oil is moved through the system and used to lubricate the compressor internals. There are a few variants of how the compressor cycles and how the metering function at the evaporator inlet is performed, but this is generally how all vehicle A/C systems work.
This Ford compressor is typical of automotive units in use today, and this is where the cool incoming refrigerant is pressurized to several hundred psi. Also, oil circulates in the system to lubricate the moving parts within the compressor. | 
GM used this large A6 designated compressor for many years. This one utilizes a traditional V-belt for drive. Newer compressors, such as the R4, are larger in diameter but shorter in length and have serpentine-belt pulleys. | 
On the front of each compressor is an electrically actuated clutch assembly. The A/C control unit turns the clutch on and off based on whether the A/C is switched on by the driver and based on pressure build in the system. Typically, some part of the system also senses when the compressor is turned on and signals the carburetor or fuel injection to increase idle speed slightly to compensate for the added load of the running compressor. |