Hot, compressed refrigerant from the compressor is pumped forward in the vehicle to the condenser that lies in front of the radiator. Here, the high-pressure gas is cooled via airflow. Next, the refrigerant temperature starts to drop, and the gas turns back to a liquid state.
Another common point of failure over time is the blower fan in the dash. If you find you have the problem that some fan speeds do not work, then you may have a vehicle that uses a resistor module and one or more of the resistors have failed. These modules are often located somewhere on or near the evaporator/fan housing.
Conversions
Should you have an older R12 A/C system, there are various means by which to update to the newer refrigerants. Some conversions can be done with a simple gas replacement. Others require changing hoses or other components. In any case, do some careful research as to which may work best for your application. There is a wide variance in system survivability and their ability to cool well.
Note also that it is illegal to vent refrigerants to the atmosphere, which is why you see service shops capture it in a recovery tank.
This isn't meant to give you a comprehensive guide to A/C systems, but to explain the basics and share some information that you may find helpful when doing some of your own work or evaluating proposed work from a shop. Hope you stay cool!
A filter/dryer is a small can-shaped device that contains a desiccant to remove moisture from the flow and remove it from the system. Some units will also have a small sight glass that allows you to watch the refrigerant flow. Bubbles present in the sight glass indicate a low charge condition. | 
Under the dash, the evaporator rests in an enclosure. Here, the expanding refrigerant gas drops in temperature and provides the cold source for the cooling of the blowing airflow. | 
A squirrel-cage fan tucked up under the dash spins and forces cold air out into the cab. The same fan is used for heater duties as well. Many blower motors use a small resistor module to provide low and medium fan speeds. Should you lose use of the lower speeds, there's a good bet that this resistor pack is the cause. |
The evaporator in its enclosure is hooked to ducting paths to the various A/C and heater vents in your rig. Airflow is controlled via moving flaps that divert the flow as needed. The flaps are controlled by cabled levers from the dash or from vacuum actuation using dash controls. These components can fail and disrupt the flap movement and not provide the desired airflow or temperature. | 
Near the input of the evaporator, you'll find an expansion valve (or similar restricting orifice). Its purpose is to control the flow of the expanding refrigerant to the evaporator. | 
This inline switch is used to monitor line pressure. Should the system pressure drop below some point where there is insufficient refrigerant for reliable operation of the compressor, this switch will signal the A/C controller to shut down compressor operation to avoid compressor damage. |
Somewhere along the hard lines or at the compressor, you'll find two Schrader-type valves. One will be in a low-pressure line and one in a high-pressure line. These are used to add refrigerant (low-pressure side only) and to connect gauges for monitoring system pressures. | 
Some systems utilize a thermistor (temperature-monitoring device) mounted to the evaporator. On humid days, a lot of condensed water will form in the evaporator core, and you'll see it draining from under your truck. This sensor checks evaporator temperatures and can cycle the compressor as needed to prevent the evaporator from freezing and forming ice which will block airflow to the vents. | 
Here is much of the evaporator portion removed from its housing. You can see the hard refrigerant lines that feed through the firewall into the engine compartment, the expansion valve, and low-pressure switch. |
To service an A/C system properly, it is best to use a quality set of gauges to allow you to monitor low- and high-pressure conditions. This can help you determine if the system has the right amount of refrigerant, if there is moisture in the system, and if the compressor and expansion valve are maintaining proper pressures for optimal cooling performance. | 
Along with pressure monitoring, a temperature reading in the ductwork will help determine how well the A/C is working. The difference in temperature between the incoming and exiting air can be checked against a performance chart to determine the level of cooling performance. | |