Your Guide to Servicing: Air Conditioning

While many people see air con as invaluable, Rob Marshall argues that skimped maintenance could be at the detriment of not only the car’s health but also that of the car occupants.

Once the preserve of luxury vehicles only, many of today’s models, even the most basic of a range, boast air conditioning, whether as a stand-alone feature or as part of a climate control system.

The best-known advantage is the system’s ability to cool the car’s interior in warmer weather but air conditioning also dehumidifies the cabin, allowing misted windows to be cleared rapidly. Therefore, air conditioning has considerable safety advantages, especially during winter months.

Due to its ability to remove moisture, air conditioning is extremely useful to defog interior glazing and to dehumidify the interior

Climate control relies on air conditioning, which is usually activated automatically

HOW DOES AIR CONDITIONING WORK?

Air conditioning’s main function is to transfer heat from within the cabin to the vehicle‘s exterior. The heart of all modern systems is its refrigerant, R134a, which almost every new and used car utilises at the time of writing. Aside from the physical dangers, such as severe frostbite, R134a is also harmful to the atmosphere, which is why a DIY enthusiast should never attempt to discharge an air conditioning system at home.

Never remove any pipe from the air conditioning circuit, without having the gas evacuated first by an approved air conditioning specialist.

All systems rely on the Latent Heat of Evaporation principle to work. Moving away from the science book to beneath the bonnet, the major components remain very similar for most cars. When a working air conditioning system is activated, you should hear a click emanate from within the engine bay, which is the system’s compressor being activated.

This pressurises the refrigerant into the condenser, which looks similar to a car radiator and is positioned at the front of the vehicle, at which point the refrigerant sheds its heat and condenses from a gas into a liquid. Any moisture that could freeze and cause damage is removed from the refrigerant by the receiver/dryer.

The liquid then enters the expansion valve, where the pressurised liquid is metered out, at a low pressure, as it enters the evaporator, located behind the dashboard. As the pressure drops, the liquid boils (R134a does so at approximately -25 degrees Celsius), thus cooling the outer surface of the evaporator rapidly, meaning that any air blown over it will also shed its heat. The gas then passes to the compressor again and the cycle is repeated.

The compressor is driven by the engine via a belt. Its internals are only powered when its integral clutch is activated by the control panel or switch on the fascia.

The condenser looks similar to a car radiator and some types have their separate receiver/dryer unit built alongside the main unit.

All of the above describes the ‘closed’ portion of the system. The ‘open’ part is behind the car’s dashboard, consisting mainly of the blower fan that forces warm air over the evaporator body, prior to the now conditioned air exiting the fascia vents. However, as the incoming air is cooled from behind the dashboard assembly, moisture is likely to be formed and the car’s ventilation system therefore features a means of drainage, which allows the excess water to exit onto the road. Should you stop your car after a long run on a hot day and notice a small pool or trickle of water forming beneath the vehicle, it is likely that it has originated from the ventilation system and is perfectly normal.

AIR CONDITIONING FAULTS

Automotive air-con works in a similar manner to a domestic refrigerator or freezer. However, while a fridge is one of the most reliable of home appliances, air conditioning is, ironically, one of the motor car’s most fragile components. Should air conditioning not be used for long periods, it can either develop leaks or its compressor can seize. The phrase ‘use it or lose it’ is pertinent.

Many people believe that using the air conditioning will increase fuel use and reduce power to unacceptable levels. After all, the compressor is powered by the engine, often via a belt, sapping a certain amount of power from it. While fuel use is increased, most air conditioning compressors, used since the late-1990s, can alter the amount of power they require, according to the temperature of the car interior, by using  variable displacement compressors. Therefore, leaving air conditioning on, to maintain cabin temperature, is likely to use less fuel, compared to the extra aerodynamic drag of an open window.

Yet, unlike the faithful household white good, automotive air-con has to withstand not only a wide range of external temperatures but it also overcomes mechanical vibration and the cooling radiator (the condenser) has to be tolerant of being bombarded by dirt, grit and other detritus as the car moves. Additionally, solid pipes cannot be used solely to house the system’s refrigerant gas under pressure, or else they would fracture. Therefore, flexible rubber pipes are built into the system. Over time, leaks can develop at the joints, which discharge the system gradually and will affect its efficiency. A lack of maintenance also contributes to increased leak rates.

The air conditioning condenser is positioned in front of the engine’s radiator. It is vulnerable to both impact damage and corrosion and is often responsible for leaks.

WHAT MAINTENANCE DOES AIR CONDITIONING NEED?

Apart from being used regularly, checking that an air conditioning system has sufficient gas pressure to work effectively is essential. The refrigerant also contains a special oil to lubricate the compressor. Should the gas pressure drop too low, the compressor will not operate, as a self-protection measure. While it is true that air conditioning systems might suffer from a degree of natural discharge of the refrigerant gas, this is only one side of the story. In the majority of cases, a system that loses sufficient refrigerant to impair its performance seriously over two years is likely to be leaking.

While DIY top-up kits are available, it is inadvisable (from both cost and environmental standpoints) to top-up a knowingly leaky system. A visit to an air conditioning specialist, who will pressure-check the system with non-environmentally toxic nitrogen gas first, is a wiser move. A specialist will also add the required level of oil to lubricate the compressor, when the system is recharged with refrigerant.

The receiver/dryer unit also becomes saturated with moisture over time and it is recommended that this part is changed every two years. This will require the gas to be ‘evacuated’ from the system, prior to the new parts being fitted, before it can be pressure checked and recharged. Unfortunately, a typical cost of between £100 and £150 dissuades most owners from doing so.

On the ‘open’ side of the ventilation system, a clogged or dirty pollen filter can not only reduce the air flow but can also cause unpleasant odours. The damp and warm environment behind the fascia panel, surrounding the evaporator, is prone to harbouring bacteria. Not only is ‘smelly air-con’ syndrome an undesirable result but also the firing of bacteria directly at the occupants is likely to pose a health risk, including ‘sick car syndrome,’ which is a recognised medical condition. However, replacing the cabin filter and using an aftermarket air conditioning cleanser within the car should alleviate the problem.contaminated cabin filters

Old and contaminated cabin filters can release unpleasant odours and bacteria into the cockpit. They should be replaced once every two years at a minimum.

Newer cars can have the status of their air conditioning systems interrogated by computer, to pinpoint any problems.

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