Thermostat (Audi 100 C4)

In simple terms, the thermostat determines whether the coolant will circulate or not. That is, it determines whether the coolant should flow through the small or large circuit (see also about this at the beginning of this chapter).

Naturally, the thermostat controls the flow of coolant depending on its temperature. This is done by means of a sleeve filled with special wax and a valve plate fixed to it. When the coolant heats up, the wax melts, expands and opens the valve plate. As long as the coolant temperature rises, the thermostat opens more and more access to cold water from the radiator. If the operating temperature, on the contrary, drops, then the spring in the thermostat again presses the valve plate to the "closed" position ("Zu"), thereby closing the access to cold water until the operating temperature is reached.