Standard on many vehicles, air conditioning provides chilled air on hot summer days and a lot more. By dehumidifying the atmosphere inside the vehicle, air conditioning can also increase your comfort and safety with fast window de-misting on wet days and an altogether fresher travelling environment all year round. Being able to keep noise levels to a minimum by keeping windows closed will put much less stress on a driver during a long journey. The windows form part of the overall shell of strength and if they are closed then occupants are better protected in the event of an accident. Open windows create drag on the vehicle, increasing fuel consumption, and reduce security.

Air conditioning in a vehicle plays much the same role as the air conditioning in your home or office. The system will, to a greater or lesser degree, control the temperature, humidity and, together with the pollen filter help keep the air in the passenger compartment free of many airborne pollutants. When the air circulation switch is active pollen, soot, dust and spores from the outside are filtered from the air. During normal operation the fresh, outside air is drawn into the vehicle to be mixed with the air already in the car. As it flows over the cold evaporator surface it makes contact with the cold metal and moisture condenses on the surface. In turn, this water collects pollutants in the air and removes it together with the condensate through the drain leading to the under side of the vehicle.

The gas contained within the system is mixed with oil that protects the compressor from wear and also helps to keep the rubber seals gas tight. Refrigerant that travels around the system is also used to cool the compressor. In order to keep the seals working the system should be run for a few minutes once a month regardless of the time of year. You may find that yours switches itself on and off automatically or our handbook may advise you to periodically run the a/c for a few minutes. This should be carried out when the outside air temperature is above around 4ºC. Different manufacturers specify different temperatures so you should check your vehicle handbook.

All vehicle air conditioning systems will leak some refrigerant and oil. The flexible hoses can degrade or there can be leaks on the seals. When the refrigerant leaks out the pressure within the system will drop and sensors will eventually override your controls and switch the compressor off in order to protect it from irreparable damage – friction and overheating. The problem here is that if the compressor is not used regularly, as described above, the seals will dry out and become damaged. If your air conditioning system stops cooling it should be checked and repaired immediately to save long term, expensive damage. If the pressure is allowed to drop too low then air and moisture can enter the system and cause even further, expensive damage.

If you find that the cooling is not as effective as it was do not leave it until it stops altogether. It may be that the system just needs re-gassing.

What’s that smell?

As well as avoiding long term damage to your air conditioning system a service could help to keep you and your passengers safe from Streptococcal infections and exposure to Cladosporium, Aspergillus, e-coli and various other pathogens. While your ventilation system is in operation you don’t notice these fungi and bacteria that have colonised the evaporator and condensate tray in the system. Leave your car parked up for an hour or so on a warm day with no air movement through the ventilation system. As soon as you switch your blower on the spores and various bacteria breeding in the moist warm air at the evaporator exit through the vents. That smell is bacteria and spores from the fungi. It has been reported that 70 percent of new cars fitted with air conditioning systems contain Cladosporium and Aspergillus to some degree. Scientists are now claiming that un-serviced air conditioning systems could account for “sick car syndrome” leaving motorists susceptible to headaches, flu like symptoms and Asthma as a result of being exposed to micro organisms. Fungi can also cause blockage of the drain tubing and can result in misting of one or both sides of the windscreen, damp carpets in the front foot wells, and unpleasant odours.

As with any other part of a vehicle, maintenance is important in order to keep the a/c system running at its best. Surprisingly, very few dealer maintenance programs for vehicles include any allowance for the air conditioning system.

Our mobile service consists of checks on the major components of the system, operating pressures check, system leak check, condenser air flow check, refrigerant gas charging, oil charge and optional application of evaporator sanitiser*.

If you set the blower on high and turn your a/c off about ten minutes before the end of your journey then you can dry the evaporator and therefore not leave an environment so conducive to fungal and bacterial growth.

Vehicle A/C System

And How It Works

Basic receiver drier system


The Compressor transports the refrigerant at the required pressure through the air conditioning system. The refrigerant is a low pressure gas as it enters the Compressor from the Evaporator. The Compressor increases the refrigerant pressure and also its temperature so it becomes a high pressure gas which, in turn, helps the refrigerant condense more rapidly in the next component, which is the Condenser. The Compressor is mounted on the engine and is driven by a belt from the engine's crankshaft pulley. The most common reasons for failure are lack of oil, pulley bearing wear, electromagnetic coil burn-out and lack of regular servicing.


The Condenser works in the opposite way to the Evaporator. The refrigerant gives up its heat generated by the Compressor by passing cold air across its fins and tubes by ram air or by an extra fan. The end result of this change is that the refrigerant is now a high pressure liquid and is forced out to the Receiver Drier through the liquid line. Because the Condenser is usually located at the front of the vehicle it can become damaged by debris or corroded by materials from the road. It can also fail because of weak spots in its construction and also lack of regular servicing.


Depending on the type of air conditioning system fitted, this item can be called a Receiver Drier or an Accumulator. (The Accumulator is fitted on the low pressure gas line of an air conditioning system between the Compressor and the Evaporator and is used in conjunction with an orifice tube). The Receiver Drier is fitted on the high pressure liquid line of an air conditioning system between the Condenser and Expansion Device. The Receiver Drier has two parts to it, the receiver and, of course, the drier. The receiver section holds the right amount of refrigerant required by the system to ensure correct operation and to supply a steady flow of liquid refrigerant to the Expansion Device. The drier section is responsible for removing moisture from the air conditioning system by means of a bag of desiccant which absorbs small quantities of moisture. This is a very important part of the air conditioning system and should be changed at least every two years or when the system is repaired. The most common reasons for failure is corrosion and desiccant deterioration which leads to severe system failure.


The Expansion Device comes in many forms. It can be a brass internally or externally equalised valve, a block type valve or an orifice tube (the latter being part of an Accumulator type air conditioning system). Expansion Device's have an inlet and an outlet which separates the high side of the system from the low side. A small restriction in the valve allows only a small amount of refrigerant to pass through it into the Evaporator, the amount of refrigerant passing through the valve depends on the Evaporator temperature. The most common reasons for failure are contamination, moisture and lack of regular servicing.


As soon as the liquid pressure drops, the refrigerant begins to boil (R134A refrigerant boils at approximately -26 degrees centigrade). As it continues to boil the Evaporator absorbs the heat passing over its tubes and fins and as a result the air is cooled. Remember that heat is being removed from the warm air and cold air is not being created. The compressor, on its suction side, removes the low pressure vapour from the Evaporator and the cycle starts all over again. The most common reasons for failure are corrosion, weak spots in construction and lack of regular servicing.