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 TM 55-1905-219-14-4
3-22. THERMOSTAT - MAINTENANCE INSTRUCTIONS.
a. The temperature of the engine coolant is automatically controlled by a thermostat located in a housing
attached to the water outlet end of each cylinder head. Blocking type thermostats are used with a standard
cooling system, and there are four thermostats.
b. At coolant temperatures below approximately 170 (76.6 ), the thermostat valves remain closed and
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block the flow of coolant to the heat exchanger. During this period, all of the coolant in the standard system 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 (76.6 ), the thermostat valves start to open, restricting the by-
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pass system, and permit a portion of the coolant to circulate through the heat exchanger. When the coolant
temperature reaches approximately 185F (85 ), the thermostat valves are fully open, the by-pass system is
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completely blocked off, and all of the coolant is directed through the heat exchanger.
c. A defective thermostat which remains closed, or only partially open, will restrict the flow of coolant and
cause the engine to over heat. A thermostat which is stuck in a full open position may not permit the engine to
reach its normal operating temperature. The incomplete combustion of fuel due to cold engine operation will
result in excessive carbon deposits on the pistons, rings, and valves.
3-605
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