INSTANT EXPERT
A broken aircon system can not just affect comfort, it also has safety implications and can even stop the whole car from working, as Rob Marshall explains.
Once a luxury option, most new cars have had air-conditioning (A/C) standardised for many years. Apart from permitting the interior to be cooler than the outside, it also dries the air, aiding demisting. Using A/C in conjunction with the heater, therefore, has safety advantages in cold and damp weather.
Many cars combine air-conditioning with automatic climate control, which is why these combined systems are called HVAC (Heating, Ventilation and Air-conditioning). As its name implies, HVAC maintains the cabin comfort at the pre-set level, without the driver having to adjust the heating and ventilation settings.
While this feature focusses mainly on the air-conditioning element, it is easy to forget that A/C on cars with climate control works in collaboration with the heating and ventilation system, all of which are controlled by an electronic control unit. Therefore, inactive A/C might not be faulty; the ECU may have calculated that it is not needed. Most A/C systems, for instance, are designed not to operate once the exterior temperature drops below a notional 5°C. Consider also that a fault within the heater box, such as a broken heater flap, a failed control motor, or a blocked heater matrix, should not be confused with faulty air-conditioning.
Operating basics
The air-conditioning system shares its operating principles with a domestic refrigerator. Therefore, its main components are analogous. The coldest part of the system is positioned behind the dashboard – although it is not called the ice box but the evaporator. This unit looks similar to a small car radiator, produced from thin aluminium and its fins provide a large surface area. These characteristics permit heat energy from the cabin to be transferred quickly into it. This is an important factor in understanding how air-conditioning works. It does not ‘make cold’. Instead, it moves heat energy around the vehicle to be shed. In most cars’ cases, this is from the interior to the exterior – or the evaporator to the condenser. On some electric vehicles, the system may also move heat energy from the drive motor to warm the battery and extend its range in cold conditions.
The condenser is positioned in front of the car, ahead of the engine cooling radiator. Like the evaporator, it is produced from thin aluminium, making it vulnerable to damage from stone chips and corrosion. Fridges use rearmounted pipework, which gets warm as it sheds heat. On motorcars, heat energy is removed by air passing through the condenser fins. Fridges also possess
