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 TM 55-1905-220-14-5
3-78. THERMOSTAT AND HOUSING - MAINTENANCE INSTRUCTION.
a. The temperature of the engine coolant is automatically controlled by a thermostat located in the housing
connected to the outlet end of the water manifold and to the heat exchanger (expansion tank).
b. At coolant temperatures below approximately 170 (76.7 ), the thermostat valves remain closed and block the
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flow of coolant to the heat exchanger (expansion tank). During this period, all of the coolant is circulated through the
engine and is directed back to the suction side of the water pump via the by-pass tube. As the coolant temperature rises
above 170 (76.7 ), the thermostat valves start to open, restricting the by-pass system, and permit a portion of the
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coolant to circulate through the heat exchanger (expansion tank). When the coolant temperature reaches approximately
185 (85 ), the thermostat valves are fully open, the by-pass system is partially blocked off, and most of the coolant is
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directed through the heat exchanger (expansion tank).
c. A properly operating thermostat is essential for efficient operation of the engine. If the engine operating
temperature deviates from the normal range of 160to 185 (71to 85 ) remove the thermostat and check it.
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d. The by-pass hoses and tubes of the water and exhaust manifold help to by-pass the thermostat while the engine
is warming up.
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