The aircon system (more accurately termed HVAC) comprises heating, ventilation and cooling, but most people regard it as the air cooling or refrigeration system. On a hot day it is fairly obvious when the system is not cooling – no cool air from the vents. But in temperate or cold weather it is not so obvious, yet it is still important to have it working since it provides dehumidification and stops the cabin becoming stuffy – especially on pre-1993 XJSs which have no fresh air control.
When the system is working correctly the compressor should cycle on and off roughly every 30 seconds depending on the outside temperature. You can observe the clutch on the end of the compressor alternately spinning and stopping and you will hear a distinct click as the clutch engages, and on early cars the engine will slow down a bit (the compressor takes about 4bhp!). In addition, the pipe out of the compressor (“high side”) should be very hot (careful!) and the input pipe (“low side”) should be very cold – perhaps with condensation or frost appearing on it. The picture below shows where to find the pipes – 6 cylinder engine at the top, V12 below.
Sometimes the compressor can be cycling ok but the pipes are not very hot or cold. This means the gas quantity has fallen below that required to refrigerate but is enough to keep the compressor running. [The system has a pressure switch which switches off the compressor permanently if the gas runs out].3:21 pm