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Figure 2-2. Outside leakage of engine water jacket.
TM-750-254 Coolind Systems: Tactical Vehicles Manual
Figure 2-5. Radiator caps, plain - and pressure-type.
requires high engine output at comparatively low vehicle
speeds. Under these conditions, the amount of engine
heat increases faster than the natural flow of air through
radiator resulting from movement of vehicle. Therefore,
adequate engine cooling must depend on forced air draft
of the fan.  Fan efficiency is even more important in
stationary engines and in military vehicles having armor,
screens, and other restrictions to air flow. Bent fan blades,
or a loose, bent, misalined, or damaged fan shroud
interferes with proper air flow and reduces cooling.
Periodic Inspection and servicing of the fan and shroud
assembly Is essential to proper engine cooling.
b.  Drive Belts Preventive maintenance of the fan
drive belt is also of first importance, because this belt
usually drives both fan, water pump and often the
generator. Continuous flexing, friction, and heat cause fan
belt cracking, friction, wear, and deterioration.  Loose
adjustment may result in slippage, rapid belt wear, and an
overheated engine. Overtight adjustment also sears the
belt and causes early failure of shafts and bearings. In
fan, water pump, or generator. A neglected fan belt may
break without warning and cause sudden overheating and
operating difficulties.  Therefore, inspection of fan belt
condition and adjustment should never be neglected.
Close examination is necessary to discover small flaws,
Figure 2-4. Bellows-type thermostat.
particularly since belts usually begin to crack through from
inside.  Immediate replacement of doubtful fan belts is
b.  Effects of Failure Automatic control of engine
good insurance against vehicle failure during operation.
operating temperatures provided by the thermostat Is
absolutely necessary (summer and winter) for efficient
Radiator Pressure Cap
engine performance. If valve fails to close properly, the
engine will run too cool; then sludge formation and other
a.  Importance of Proper Operation.  The radiator
harmful effects of overcooling can take place (para 2-18e).
pressure cap has more effect on cooling system operation
If valve fails to open properly, engine temperature will rise
than is generally realized. A properly operating pressure
and overheating difficulties may follow.  Engines should
cap increases the normal margin of safety between
not be operated with thermostat removed, except In case
coolant operating temperature and boiling point from 5F
of emergency.
to 17F on transport vehicles and as high as 50F on some
combat vehicles. This additional margin of safety helps to
c.  Temperature  Gage  Check  on  Thermostat
prevent boiling during operation in hot weather, at high
Operation The temperature gage should be observed
altitudes, and under heavy load.
during engine warm-up and on road tests in order to be
sure thermostat is functioning properly. Whenever gage
continually indicates unsafe low or high temperatures,
thermostat should be removed and tested.
Fan and Drive Belts
Fan and Shroud Military vehicle operation often

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