On any car, the air conditioning system is a complicated beast. This project is not intended to be a repair manual for your A/C system, but should serve more as a guide to how the system works and offer key points about topping off the refrigerant level.
Air conditioning systems work using the theory of thermodynamics, whereby heat flows from a warmer surface to a colder one. Let's begin by explaining in layman's terms how air conditioning works. Here is a list of the main system components and what they do.
Compressor: The main function of the compressor is to compress and pressurize the refrigerant in the system, and also to keep the refrigerant moving through the system when the A/C is turned on. The compressor is belt driven off the engine. The compressor takes in cold, low-pressure gaseous refrigerant and compresses it. As it compresses, it builds up heat. This now-pressurized hot gas is sent to the condenser.
Condenser: The condenser then receives the hot gaseous refrigerant from the compressor. Usually, condensers are placed in the front of a car, where it receives oncoming airflow from the radiator fan. The condenser then turns the hot gas into a liquid. This liquid (still under high pressure) is then sent to the receiver drier.
Receiver Drier: The receiver drier next receives the hot pressurized liquid refrigerant from the condenser. It is essentially nothing more than a desiccant tank. It removes moisture from inside the hot liquid refrigerant. Moisture in the lines is a potential problem, as refrigerant can eventually react with moisture and corrode the insides of the system. The process is completed when the hot liquid refrigerant then exits the receiver drier and passes through the expansion valve.
Expansion Valve: The expansion valve receives the hot liquid refrigerant. It allows the pressure of the liquid refrigerant to drop. The drop in pressure causes the liquid refrigerant to cool down. The result is cold liquid refrigerant, which is then sent to the evaporator.
Evaporator: The evaporator receives the cold liquid from the expansion valve. The evaporator is usually located inside the car under the dashboard. When you turn on the A/C fan, it channels air over the evaporator, cooling the air; this air is then blown through the vents of the car. As the cold liquid passes through, it evaporates from a liquid back into a gas, and is then routed back to the compressor to start the entire process again. Additionally, there are two more components to the system that control the operation when you turn the compressor on and off.
A/C Compressor Clutch: The A/C Compressor Clutch is used to engage or disengage the compressor when you press the A/C button on the dashboard. When the A/C is turned off, the belt that drives the compressor spins freely on the A/C Clutch, which is mounted on the front of the compressor. Now when you push the A/C button, it sends current to the clutch, which locks it and allows the compressor to begin turning.
High Pressure/Low Pressure Switches: These two switches are a built-in safety feature. When the system pressure is too low it interrupts the voltage going to the clutch on the compressor, which then disengages the drive belt from the compressor, and turns off the A/C. It works the same way when it senses there is too much pressure in the system.
So what can you do to maintain and protect the system? Simply operate the air conditioning system at least once a week if the outside temperature is above 50 degrees Fahrenheit. Doing this will circulate the refrigerant in the system and help keep the seals from drying out. Refrigerant leaks out of the system, which you can prevent simply by making sure that you run the system more frequently, causes most air conditioning system failures.
The biggest problem with A/C systems is a loss of refrigerant. Luckily, replacing and topping off refrigerant is a relatively easy process. Begin by starting the car, turn on the A/C system and fan to full blast, and let the car run for about three minutes. Following the instructions included with the kit, connect a new can of refrigerant to the hose/gauge assembly. Cans of refrigerant typically have two ways of adding it to the system. The most common is a trigger handle type assembly you simply pull to add refrigerant. The other method uses a valve you turn to open and add more refrigerant.
Be sure you shake the can for about 30 seconds and turn it upside down when you connect it to the gauge assembly. Connect the gauge assembly to the low-side port of your A/C system. Be sure to wear eye protection and heavy leather gloves when handling the coolant and gauge assembly! If coolant leaks out at any time, it can literally freeze a small patch of skin quite easily and give you frostbite. Just a word of caution here, do NOT add refrigerant to the high side port. Bad things will happen. This is why the fittings for the low and high side ports are different sizes, to prevent you from making this mistake, but it bears repeating. I've seen people with limited knowledge of the system do dangerous things in the past.
With the car running and the A/C system turned on full blast, take a reading on the pressure gauge. A properly charged system should read between 25 and 40 psi on the low side. If the pressure is low, add more refrigerant into the system. Be aware that the pressure gauge reading will automatically elevate as you are adding more coolant, so periodically close the valve on the can or release the trigger to check if the pressure is rising in the system. If the pressure doesn't increase after adding one complete can, you most likely have a major leak in your system and should seek the help of a professional A/C system mechanic. Also keep an eye on the high side reading. Typically, you don't want this to exceed 200psi.
A set of air conditioning manifold gauges should only cost around $50 or so. You'll need the gauges to check the different pressures on both the high and the low side of the air conditioning circuit. Most auto parts stores these days also sell cans of refrigerant with a pressure gauge built into the dispenser. You can use this gauge in lieu of the manifold gauges, but your readings will be less precise without also knowing the high side pressure.
With the system properly filled and measured with your gauge, head to the passenger compartment and check the temperature of the air exiting the vents. On a system that is operating really well, the temperature will be in the upper-30s Fahrenheit. Also keep in mind that if your system is cooling air in the 30 degrees Fahrenheit range, the compressor will tend to turn itself on and off, causing the temperature to go up and down slightly. This is not a defect of the system; the compressor turns itself off as the evaporator nears the freezing temperature of water. This prevents the evaporator from becoming frozen and clogged with icy buildup.
Accessing the high side pressure port is easier to access by removing the engine cover. See our article on Removing the Engine Cover for more information.
Shown here is the low side refrigerant port on the Mk4 Jetta (green arrow). Unscrew the plastic cap and remove it.
Attach the low side pressure fitting for the gauge assembly. Make sure that the valve is in the closed position when attaching. This attachment can be different based on the manufacturer. Read all instructions with the gauge set you use.
Shown here is the high side refrigerant port near the left rear of the intake manifold (green arrow). Like before, unscrew and remove the plastic cap.
Attach the high side pressure fitting for the gauge assembly. As before, make sure that the valve is in the closed position when attaching. Don't worry about mixing up the low and high side fittings as they are different sizes.
Hang the gauge set on the underside of the hood as shown here and follow the manufacturer's instructions for adding refrigerant via the yellow or fill line shown here.