TM-9-2320-272-23-1
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; FIELD MAINTENANCE MANUAL; VOLUME 1 OF 5
SEPTEMBER 2012
TM-9-2320-272-23-1 - Page 223 of 1313
COOLING SYSTEM OPERATION
This system provides cooling of the engine and transmission. It differs slightly between the M939/A1 series and
M939A2 series vehicles because different engines are used. Major components of the cooling system are:
A. WATER MANIFOLD
- Collects coolant from cylinder heads and directs it to thermostat housing
(M939/A1 series only).
B. SURGE TANK-TO-WATER MANIFOLD VENT
- Vents air trapped in water manifold
(M939/A1 series only).
C. SURGE TANK-TO-RADIATOR VENT
- Vents air in cooling system.
D. 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.
E. RADIATOR INLET HOSE
- Directs coolant from water manifold to radiator after thermostat has opened.
F. BYPASS TUBE
- Directs coolant back to transmission oil cooler, where it is then recirculated through the
engine block until the thermostat opens.
G. RADIATOR SHROUD
- Concentrates air flow through the radiator.
H. RADIATOR
- Directs coolant through a series of fins or baffles so outside air can remove excessive heat
from coolant.
I. FAN CLUTCH
- Regulates use of fan to control engine temperature fan to belt-driven pulley when
conditions require additional cooling.
J. WATER PUMP
- Provides force to move coolant through engine.
K. FAN
- Provides force to pull air through radiator.
L. RADIATOR DRAIN VALVE
- Permits coolant to be drained from radiator.
M. TRANSMISSION OIL COOLER HOSE
- Directs coolant to transmission oil cooler.
N. ENGINE OIL COOLER
- Reduces heat of engine oil (M939/A1 series only).
O. TRANSMISSION OIL COOLER
- Reduces heat of transmission oil.
P. ENGINE OIL COOLER-TO-HEATER HOSE
- Directs coolant to personnel water heater when shutoff
valve is open (M939/A1 series only).
Q. PERSONNEL WATER HEATER
- Provides heat for cab and personnel (M939/A1 only).
R. SURGE TANK
- Filling point for cooling system. On M939A2 vehicles, a float sensor monitors water level
and illuminates a light on instrument panel.
S. COOLANT PRESSURE CAP
- Designed to depressurize cooling system and to access cooling system
for filling.
TM 9-2320-272-23-1
0003
0003-18
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