Chevrolet Light Duty Truck 1973 Service Manual page 416

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6 K - 6 E N G IN E C O O L IN G
between fan blades and thermometer sensor
before starting engine.
Position thermometer so that the thermometer
sensor is centered in the space between the fan
blades and radiator. This can be achieved by
inserting the sensor through one of the existing
holes in the fan shroud or fan guard, or by placing
between the radiator and the shroud. On some
models, it may be necessary to drill a 3/16" hole in
the fan shroud to insert J6742-01.
D. Cover radiator grille sufficiently to induce a high
engine temperature. Start engine and turn on air
conditioning if equipped. Maintain a position in
front of the vehicle to observe the thermometer
reading. With a rod, broom handle, or etc., push
on the accelerator linkage to maintain approx­
imately 3000 r.p.m. Use tachometer if available.
E. Observe thermometer reading when clutch engages.
It will take approximately 5 to 10 minutes for the
temperature to become high enough to allow
engagement of the fan clutch. This will be
indicated by an increase or roar in fan air noise
and by a drop in the thermometer reading of
approximately 5-15 degrees F. If the clutch did not
engage between 165-190 degrees F., the unit should
be replaced.
NOTE: Be sure fan clutch was disengaged at
beginning of test.
Do not continue test past a
CAUTION:
thermometer reading o f 190 degrees F to
prevent engine overheating.
If no sharp increase in fan noise or temperature
drop was observed and the fan noise level was
constantly high from start of test to 190 degrees F,
the unit should be replaced.
F. As soon as the clutch engages, remove the radiator
grille cover and turn off the air conditioning to
assist in engine cooling. The engine should be run
at approximately 1500 r.p.m.
G. After several minutes the fan clutch should
disengage, as indicated by a reduction in fan speed
and roar.
Thermostat
A pellet-type thermostat is used in the coolant outlet
passage to control the flow of engine coolant, to provide
fast engine warm-up and to regulate coolant tempera­
tures. A wax pellet or powder element in the thermostat
expands when heated and contracts when cooled (Fig. 8).
The pellet is connected through a piston to a valve.
When the pellet is heated, pressure is exerted against a
rubber diaphragm which forces the valve to open. As the
pellet is cooled, the contraction allows a spring to close
the valve. Thus, the valve remains closed while the
coolant is cold, preventing circulation of coolant through
the radiator, but allowing the coolant to circulate
throughout the engine to warm it quickly and evenly.
Fig. 8—Sectional View of Thermostat
As the engine becomes warm, the pellet expands and the
thermostat valve opens, permitting the coolant to flow
through the radiator where heat is passed through the
radiator walls. This opening and closing of the
thermostat valve permits enough coolant to enter the
radiator to keep the engine within operating temperature
limits.
Coolant
Regardless of whether freezing temperatures are or are
not expected, cooling system protection should be
maintained at least to -20°F. to provide adequate
corrosion protection and proper temperature indicating
light operation. With glycol content less than require-
LIGHT DUTY TRUCK SERVICE MANUAL