The system works as follows: When the engine is cold, the thermostat is closed, the water pump pumps water through the internal channels, through the bypass hose (and the heater circuit, if it is on) through the thermostat capsule and back to the water pump inlet.
The circulating water cools the engine cylinders, combustion chamber surfaces and valve seats. However, when the coolant reaches a certain temperature, the thermostat begins to open. The coolant now begins to circulate through the upper hose to the top of the radiator. As it passes through the radiator, the liquid is cooled by the air flow as the car moves, boosted by an electric cooling fan if necessary. Finally, the coolant returns to the water pump through the lower hose and the open thermostat.
The electric cooling fan is controlled by a thermal switch located at the base of the radiator. The fluid temperature is monitored by a sensor in the cylinder head.
Note: The electric cooling fan starts to operate when the coolant temperature in the radiator reaches a certain level, even if the engine is not running. Therefore, be careful when working near the fan blades.
(This article was copied from an online resource: AUDImanual)