TM10-3930-660-34 CHAPTER 5 COOLING SYSTEM MAINTENANCE Section I.  DESCRIPTION AND DATA 5-1.  GENERAL. This chapter covers maintenance procedures for the engine cooling system components, as well as principles of operation. 5-2.  PRINCIPLES OF OPERATION. a. System Operation.  Coolant liquid is circulated through passages in engine cylinder block and through the engine oil cooler by an engine-driven water pump.  The water pump also serves to force coolant to flow through the radiator, from top to bottom, as long as thermostat is open to allow flow to radiator.  As coolant flows downward in radiator, airflow produced by the engine fan is forced through the radiator.  Heat is removed from the coolant in this way. b. Water Pump.  The water pump is a centrifugal-type unit that is driven by belt off the crankshaft pulley. c. Thermostat.  The thermostat is a heat-operated valve which serves to regulate coolant temperature within a range that provides the most efficient engine operation.  The thermostat is a modulating type, which begins to open at a coolant temperature of 1800 F and is fully open when coolant temperature reaches 190°. 5-1