Figure 2-4. Bellows-type thermostat.

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

2-9.

Radiator Pressure Cap

engine performance. If valve fails to close properly, the

(Fig 2-5).

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.

2-8.

Fan and Drive Belts

Fan and Shroud Military vehicle operation often

2-6