Fix your car’s air conditioner and never over heat again. Here’s a short guide to how air conditioning (AC) works, why it might not work, and what you can do about it.
LEGAL WARNING! – Checking for refrigerant by poking something in the fitting and looking for venting is ILLEGAL as it releases refrigerant into the atmosphere. (Please don’t do this with R-12 especially!) Second, it’s also ILLEGAL to put more refrigerant into a system known to leak (because, again, you’re letting it vent to the atmosphere, albeit more slowly). [Note: The EPA web site disagrees with this statement.][It is NOT illegal to top off a leaking system unless there is a state or local law against it.]
This is why you may not find a repair facility that will simply top up the refrigerant, if they suspect that you need to repair a leak first. If you repair the leaks and failed components yourself, it is legal to recharge the system with 134 without a license, but not R-12. (However, getting a license to handle R-12 is easy online, and costs about $20.)
Realize that auto AC is basically a refrigerator in a weird layout. It's designed to move heat from one place (the inside of your car) to some other place (the outdoors). While a complete discussion of every specific model and component is well outside the scope of this article, this should give you a start on figuring out what the problem might be and either fixing it yourself or talking intelligently to someone you can pay to fix it.
Become familiar with the major components to auto air conditioning. The compressor, which compresses and circulates the refrigerant in the system. The refrigerant, (on modern cars, usually a substance called R-134a, older cars have r-12 freon which is becoming increasingly more expensive and hard to find, and also requires a license to handle) which carries the heat. The condenser, which changes the phase of the refrigerant from gas to liquid and expels heat removed from the car. The expansion valve (or orifice tube in some vehicles), which is somewhat of a nozzle and functions to simultaneously drop the pressure of the refrigerant liquid, meter its flow, and atomize it. The evaporator, which transfers heat to the refrigerant from the air blown across it, cooling your car. The receiver/dryer, which functions as a filter for the refrigerant/oil, removing moisture and other contaminants.
Understand the air conditioning process: The compressor puts the refrigerant under pressure and sends it to the condensing coils. In your car, these coils are generally in front of the radiator. Compressing a gas makes it quite hot. In the condenser, this added heat and the heat the refrigerant picked up in the evaporator is expelled to the air flowing across it from outside the car. When the refrigerant is cooled to its saturation temperature, it will change phase from a gas back into a liquid (this gives off a bundle of heat known as the "latent heat of vaporization"). The liquid then passes through the expansion valve to the evaporator, the coils inside of your car, where it loses pressure that was added to it in the compressor. This causes some of the liquid to change to a low-pressure gas as it cools the remaining liquid. This two-phase mixture enters the evaporator, and the liquid portion of the refrigerant absorbs the heat from the air across the coil and evaporates. Your car's blower circulates air across the cold evaporator and into the interior. The refrigerant goes back through the cycle again and again.
Check to see if all the R-134a leaks out (meaning there's nothing in the loop to carry away heat). Leaks are easy to spot but not easy to fix without pulling things apart. Most auto-supply stores carry a fluorescent dye that can be added to the system to check for leaks, and it will have instructions for use on the can. If there's a bad enough leak, the system will have no pressure in it at all. Find the low-side valve and with a gauge check the PSI level. DO NOT use anything else to poke in the valve to see if it will come out, THIS IS ILLEGAL (it's called venting).
Make sure the compressor is turning. Start the car, turn on the AC and look under the hood. The AC compressor is generally a pump-like thing off to one side with large rubber and steel hoses going to it. It will not have a filler cap on it, but will often have one or two things that look like the valve stems on a bike tire. The pulley on the front of the compressor exists as an outer pulley and an inner hub which turns when an electric clutch is engaged. If the AC is on and the blower is on, but the center of the pulley is not turning, then the compressor's clutch is not engaging. This could be a bad fuse, a wiring problem, a broken AC switch in your dash, or the system could be low on refrigerant (most systems have a low-pressure safety cutout that will disable the compressor if there isn't enough refrigerant in the system).
Look for other things that can go wrong: bad switches, bad fuses, broken wires, broken fan belt (preventing the pump from turning), or seal failure inside the compressor. Look for other things that can go wrong: bad switches, bad fuses, broken wires, broken fan belt (preventing the pump from turning), or seal failure inside the compressor.
Feel for any cooling at all. If the system cools, but not much, it could just be low pressure and you can top up the refrigerant. Most auto-supply stores will have a kit to refill a system, complete with instructions. Do not overfill! Adding more than the recommended amount of refrigerant will NOT improve performance but actually will decrease performance. In fact, the more expensive automated equipment found at nicer shops actually monitors cooling performance in real-time as it adds refrigerant. When the performance begins to decrease it removes refrigerant until the performance peaks again.