A typical thermostatically controlled system is shown in figure 7-4. This illustration shows the cooling system that is installed on General Motors series 71 diesel engines. These thermostats are designed to open at a specified temperature. This temperature is known as the RATING OF THE THERMOSTAT and is usually stamped on the thermostat. Most thermostats begin to open at their rated temperatures and are fully open at about 20F above their rated temperatures. For example, a thermostat with a rated temper-ature of 165F will start to open at that temperature. At 185F, the thermostat will be fully open.

The conventional type of thermostatic valve is not adjustable. If a higher or lower operating temperature is desired, the valve must be replaced with a valve of a different rating.

Figure 7-16 illustrates the operation of a thermostatic valve. A spring holds the valve shut when the coolant is cold (view A). Thus, the coolant is stopped from circulating through the cooling medium and is directed to the bypass line where it is forced back to the water pump to be recirculated. As the coolant starts to heat up, a special plastic wax-like substance within the temperature-sensing bulb of the thermostat starts to liquefy and expand (view B). This expansion process begins to force the valve open. As the valve starts to open, some of the coolant will begin to enter the cooling medium. When the temperature of the coolant reaches the operating temperature of the engine, the wax-like substance within the bulb becomes totally liquefied and fully expanded. At this point, the valve is completely open and directs most of the coolant through the cooling medium.