Repairing airconditioning systems
An in-depth guide to what goes wrong with aircons and how to put them right.
While the humble refrigerator is one of the most reliable of household appliances, vehicle air-conditioning requires regular servicing and periodic regassing. Even though domestic fridges and car aircons share similar operating principles, a typical white good is kept in a cossetted environment, whereas automotive air-conditioning units are subject to extremes of temperature and vibration. A refrigerator’s soldered joints would fracture were they used in a car, which explains the use of flexible pipes and rubber seals in the automotive application, through which the pressurised gas escapes over time.
How air-conditioning operates
While air-conditioning seems to be a standalone element, it actually works in conjunction with a car’s heater, which explains why some aircon components are housed within the heater box. The whole unit tends to be described as the heating, ventilation and air-conditioning system (HVAC).
The heater part of this is fairly straightforward, although some diesel cars may have an electric, or fuel-fed auxiliary system, which complicates the traditional layout slightly. Warmth is fed into the cabin by blowing air through a water-to-air heat exchanger known as the matrix, found within the heater box, behind the fascia. The degree of warmth can be varied by either altering the flow of hot engine coolant through the matrix with a restricting valve, or by mixing hot and cold airflows together using mixer flaps. Other than failed heater fans, burned-out resistors that vary the fan speed, or the matrix developing a leak (covered in the June 2018 issue of CM), the heater unit is usually maintenancefree and fairly reliable. On certain newer models, though, the heater flaps can warp, or break, and their stepper motors can seize.
Vehicles equipped with airconditioning incorporate a ‘cold radiator’ inside the heater box, called an evaporator. Crucially, an air-conditioning system does not create cold air, it removes heat by transferring it from the interior to the exterior. Warm air is blown through the evaporator and, as its surface is colder, the incoming air is chilled, prior to being expelled through the vents. As warm air tends to contain more moisture, the cooling process causes water to condense onto the evaporator’s surface, ultimately forming frost. When the engine is switched off, the frost melts and the water is discharged via a drain hole onto the road. This is why a small pool of water can collect beneath a stationary car, especially after completing a long journey on a hot day.
The evaporator can be likened to the ice box of a domestic refrigerator. Inside the evaporator, a liquid refrigerant boils at a low pressure and absorbs heat in the process, chilling the evaporator body and any air that passes through its fins. The basic physics – referred to as 'latent heat of evaporation' – is the same as dabbing spirit on your skin: as the liquid evaporates, the area feels colder, because evaporation pulls away your body heat.
The low-pressure vapour is drawn into the compressor, literally the heart of the system, the drive of which can be controlled by an electromagnetic clutch, powered by the auxiliary belt system (some applications might use a clutchless variable displacement compressor, which applies a variable voltage to an internal solenoid by a separate ECU). The compressor pumps the gas from the evaporator to the condenser under high pressure, which increases the refrigerant’s temperature significantly.
After being pumped into the condenser, the hot refrigerant releases the heat that it gained from the evaporator and the compressor, resulting in it changing state again as it condenses from a high-pressure vapour into a highpressure liquid (known as the 'latent heat of condensation'). This condenser serves the same purpose as the pipes on the back of a refrigerator, which tend to become warm. In an automotive application, the condenser becomes quite hot and is mounted in front of the engine’s radiator, cooled either by the ram air effect that results from the car being driven forward at speed, or else by an electric cooling fan. On some cars, the cooling fans run continuously whenever the air-conditioning is activated, to keep the condenser as cool as possible.
Mounted to the condenser is a receiver/ dryer, which may be incorporated within the condenser body on some cars. Apart from storing the refrigerant until it is needed by the evaporator, the receiver/dryer removes any moisture contamination, which could otherwise freeze. Solid ice can cause mechanical damage, as well as preventing the system from working efficiently. Still under high pressure, the refrigerant passes through the engine compartment into the expansion valve (or fixed tube orifice), which is either mounted to the evaporator, or very close to it. This regulates the flow, causing the liquid to expand and the pressure drops significantly as it enters the evaporator, prior to being sucked back into the compressor. This cycle repeats itself.
Recent developments to the HVAC system include adding sophisticated electrical sensors and controls, which allow it to maintain any setting without further driver intervention. While air-conditioning systems tend to have a number of electrical switches that will stop the compressor from working if the evaporator temperature is abnormal, or the gas pressure is too low, climate control adds an additional layer of complication by having multiple sensors positioned around the car’s interior to assess temperature, humidity, sunlight intensity and even pollution levels. Apart from ensuring a consistent atmosphere, the information collected has efficiency benefits.
What goes wrong?
As with all repairs, spending time on an accurate diagnosis is far more costeffective than replacing expensive parts using guesswork, in the hope that it solves the problem. This is also true if you are engaging professionals to undertake the work for you. While non-operating aircon can be due to numerous factors, the main faults are leaks and/or component failure.
A non-working compressor might be due to a lack of refrigerant pressure (which can have a host of causes), a faulty low-pressure switch, or a failed sensor in the system, especially if climate control is fitted. This means that the climate control ECU is best interrogated for live data readings.
If faulty, compressor clutches can fail to engage, or could seize, especially if the air-conditioning is not activated for a long period of time. They can rattle loudly, which is often blamed mistakenly on the engine’s dual mass flywheel. The compressor’s internals are lubricated by a special oil, the specification and quantity of which varies between vehicles. This oil is carried by the refrigerant gas, so low gas pressure can damage the compressor through lack of lubrication. Any other fault that creates excessive pressure can overload the compressor and cause its clutch to slip and fail. This includes being recharged at pressures beyond the system’s design limits, by too much oil being added or through a faulty or under cooled condenser that could be caused by a failed electric fan.
“Fitting a new compressor is not like fitting many other car parts, where you simply bolt on another one,” says Simon Parker, director of Cool Car, which has a growing network of air-conditioning franchises around the UK. “A process must be followed, which many garages ignore, then wonder why the new compressor stops working soon afterwards. Compressors do not just fail; the cause must be identified or it will happen again. You should change the expansion valve and receiver drier too, but few repairers bother and wonder subsequently why a new compressor breaks down after just a few weeks. Additionally, unless you remove all the oil from the system, prior to fitting a new compressor, and flush it with suitable solution, then you have no idea how much oil to add. An overfilled system causes ‘liquid lock’ in the compressor, promoting failure. On inspection, we can find that the system has not been cleaned, drained or flushed, and that the compressor has been damaged by swarf.”
Simon says that if you entrust a garage to replace a compressor, ensure that the correct procedure has been followed, because many garages are not full-time air-conditioning specialists.
As condensers are located in a vulnerable position right in front of the car, they risk being punctured by road debris. The refrigerant escapes and moisture-laden atmospheric air enters the system, saturating the receiver/dryer. In this type of leak, the compressor oil isn’t always expelled with the refrigerant, unless the bottom of the condenser is damaged. The aluminium pipes that connect to the condenser can also corrode, incurring extra expense if they fracture.
Due to its location within the heater box, the evaporator tends to be fairly reliable, with the main causes of failure being contamination, or damaged connectors. A gurgling noise from behind the dashboard with the aircon activated usually indicates low gas pressure. The expansion valve tends to be less tricky to replace, although it may be soldered directly to the evaporator pipes on some vehicles. A fault with either of these components will result in poor aircon performance, but restricted access makes it difficult to assess their condition. This could also affect your health, because bacteria thrives on the moist evaporator surface and can build-up to such an extent that the spores are blown directly at the passengers, triggering a variety of health conditions, from asthma to ‘sick car syndrome’. Stripping down the heater box to clean the evaporator is not a practical option, but there are a number of aerosol cleaners that claim to kill bacteria, which need to be used annually.