TM 55-2815-574-24
0003
COOLING SYSTEM - Continued
Water Pump
The centrifugal-type water pump circulates the engine coolant through the cylinder block, cylinder heads, or heat
exchanger and the oil cooler. The pump is mounted on the engine front cover and is driven by the 66 tooth front
camshaft gear (water pump drive). The water pump gear has 42 teeth and meshes with the water pump drive
gear. A bronze impeller is secured to one end of a stainless steel shaft by a locknut. The water pump gear is
pressed on the opposite end of the shaft. Two ball bearings are used to carry the shaft. The larger bearing is used
at the drive gear end of the shaft to accommodate the thrust load. An oil seal is located in front of the smaller
bearing and a spring-loaded face type water seal is used behind the impeller. The pump ball bearings are
lubricated with oil splashed by the camshaft gear and the water pump gear.
Figure 8. Water Pump.
Thermostat
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 when a standard cooling system is
employed. Two thermostats are employed by the 8V engines. At coolant temperatures below 160F to 180F
(71C to 82C), depending upon the thermostat used, the valves remain closed and 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 bypass tube. As the coolant temperature rises
above 160F to 180F (71C to 82C), the thermostat valves start to open, restricting the bypass system, and
permit a portion of the coolant to circulate through the heat exchanger. When the coolant temperature reaches
approximately 185F to 197F (85C to 92C) the thermostat valves are fully open, the bypass system is
completely blocked off and all of the coolant is directed through the heat exchanger. A defective thermostat, which
remains closed or only partially open, will restrict the flow of coolant and cause the engine to overheat. 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. Properly operating thermostats are essential for efficient operation of
the engine. If the engine operating temperature deviates from the normal range of 160F to 197F (71C to 92C),
the thermostats should be removed and checked.