13 March 2020 INFO

All current vehicle air-conditioning systems work on a loop system with refrigerant being recirculated around it. The compressor is belt driven from the engine and compresses the refrigerant to a high pressure in gas form.

  Cutaway compressor.

It is then piped to the condenser situated in front or beside the radiator, where fan assisted air cools the gas which condenses to a warm liquid - still under high pressure.

A typical condenser


Some systems use a receiver drier and expansion valve, in these types of system the liquid refrigerant passes through the receiver to remove moisture, impurities and to separate gas bubbles from the flow. It then passes to the expansion valve which restricts the flow creating a cooling effect in the evaporator as the liquid evaporates due to the reduction in pressure. The expansion valve closes at about 2 deg C to stop ice formation inside the heater box restricting the airflow as moisture collects from the rapidly cooled air. The gas is then returned to the compressor via the larger of the two pipes for another cycle.

Typical Receiver/Driers

On other systems the cooling effect is achieved by restricting the flow of liquid in the inlet pipe to the evaporator, the resultant drop in pressure giving the cooling as the liquid vapourises absorbing heat from its surroundings in the evaporator. The gas is then passed through an accumulator to prevent liquid refrigerant returning to the compressor and causing damage. This type of system is regulated by a pressure switch on the return line switching the compressor off if the pressure goes below a pre set point.

Example of an evaporator

Warm air can hold much more moisture than cold air so as the air is rapidly cooled moisture is released and collects in the bottom of the heater box. This drains out through a small pipe to exit under the vehicle. Becase the heater box is often damp, mould growth can become a problem, causing bad smells - for more information on this click here.

Various controls are added to the systems and differ from each manufacturer, to prevent pressures from being to high or low, the evaporator freezing etc, and to operate fans to cool the condenser.

Most problems occur when the refrigerant leaks out and the pressure drops below a pre set level, the system will shut down to prevent the compressor failing. This can be rectified by recharging the system with refrigerant. Loss of cooling efficiency may be noticed as the refrigerant level drops. If the system requires topping up at frequent intervals the leak should be traced and repaired. The most common location of leaks is the condenser due to it’s exposed location and constant heat/pressure changes.

If the system becomes completely empty the a vacuum must be applied before it is recharged as a tiny amount of air or moisture in the system will cause early failure.

A small amount of oil is added to the refrigerant to lubricate the compressor and seals.

This is not intended as a repair manual but merely to give an overview of how it works!!.