Air conditioning has worked pretty much the same way for its entire existence: it cools and removes humidity from the air. There are three main parts to the system — the compressor, condenser, and evaporator — that achieve this, plus a few other parts to keep the system running smoothly.

Compressor: The compressor is a pump driven by the engine or by an electric motor. When the refrigerant is drawn into the compressor, it is in a low-pressure gaseous form. Once the gas is inside the pump, the compressor puts the gas under pressure and forces it out to the condenser. Compressors cannot compress liquids, only gasses.

Condenser: The condenser is basically a radiator. The refrigerant enters the condenser as a pressurized gas from the compressor. The process of pressurizing the gas and moving it to the condenser creates heat, but air flowing around the twisting tubes of the condenser cools the refrigerant down until it forms a liquid again. The liquid refrigerant is now a high-pressure liquid and nearly ready to cool the car.

Evaporator: This is where the magic happens. While all the other parts of the system are located in the engine compartment, this one is in the cabin, usually above the footwell on the passenger side. It also looks like a radiator, with its coil of tubes and fins, but its job is to absorb heat rather than dissipate it.

Refrigerant enters the evaporator coil as a cold, low-pressure liquid, ideally at 32 degrees Fahrenheit (0 degrees Celsius), which is why you don’t want any water in the system. The refrigerant doesn’t freeze at this temperature, but it does have a very low boiling point. The heat in the cabin of the car is enough to make the R-134a in the evaporator boil and become a gas again, just like water turning back to steam. In its gaseous form, refrigerant can absorb a lot of heat.

The evaporator also takes humidity out of the air in the car, which helps you feel cool. Water in the air condenses on the evaporator coil, along with dirt and pollen and anything else floating around in the cabin. When you stop the car and see water dripping underneath, it’s probably the water from the AC evaporator and nothing to worry about.

R-134A is a hydro-fluorocarbons refrigerant that is essentially used as a residential and automotive refrigerant. It was developed to replace ‘chlorofluorocarbon (CFC)’ refrigerants that were detrimental to the ozone layer.

Once a fault within the air conditioning system has been detected the following procedures are used to determine the source of a defective component;

• Run system to observe operation of compressor and interior vent temperature.
• Attach pressure gauges to identify gas level.
• Evacuate system and pressure test to identify any leaks.
• Visually examine system for signs of leaks.
• Refill system with gas, lubricating oil and leak detection dye.
• Observe internal pressures to identify faulty internal A/C components.
• Check operation of ventilation fan, cabin filter, condenser fans and drive belts.