POWER SYSTEM OPERATION (Contd)
SURGE TANK - Filling point for cooling system. On M939A2 vehicles, a float sensor monitors water level
and illuminates a light on instrument panel.
COOLANT PRESSURE CAP - Designed to depressurize cooling system and to access cooling system for
WATER MANIFOLD - Collects coolant from cylinder heads and directs it to the thermostat housing (M939/A1
SURGE TANK-TO-WATER MANIFOLD VENT - Vents air trapped in water manifold (M939/A1 series only).
SURGE TANK-TO-RADIATOR VENT - Vents air in cooling system.
THERMOSTAT - Shuts off coolant flow to radiator until temperature reaches 175°F (79°C) on M939/A1
series vehicles, and 181°F (83°C) on M939A2 series vehicles. Coolant is then directed to the radiator
through the radiator inlet hose.
RADIATOR INLET HOSE - Directs coolant from water manifold to radiator after thermostat has opened.
BYPASS TUBE - Directs coolant back to transmission oil cooler where it is then recirculated through the
engine block until the thermostat opens.
RADIATOR SHROUD - Concentrates air flow through the radiator.
RADIATOR - Directs coolant through a series of fins or baffles so outside air can remove excessive heat
FAN CLUTCH - Regulates use of fan to control engine temperature fan to belt driven pulley when conditions
require additional cooling.
WATER PUMP - Provides force to move coolant through engine.
FAN - Provides force to pull air through radiator.
RADIATOR DRAINVALVE - Permits coolant to be drained from radiator.
TRANSMISSION OIL COOLER HOSE - Directs coolant to transmission oil cooler.
ENGINE OIL COOLER - Reduces heat of engine oil (M939/A1 series only).
TRANSMISSION OIL COOLER - Reduces heat of transmission oil.
ENGINE OIL COOLER TO HEATER HOSE - Directs coolant to personnel water heater when shutoff valve is
open (M939/A1 series only).
PERSONNEL WATER HEATER - Provides heat for cab and personnel.