G643 (E) Service Manual Chapter 3. Engine Mechanical System 54
Thermostat
The thermostat is the wax pellet type. A jiggle valve
(which improves air bleeding during water supply) is
provided on the flange part. When the thermostat is
closed, the circulation of coolant is stopped, thereby
making warm-up faster.
Operation
When the temperature of the coolant is low, the
valve is closed by the spring, with the result that the
coolant circulates within the engine, without passing
through the radiator.
When the temperature of the coolant rises and
reaches a certain specified temperature, the valve
opens and the coolant also circulates through the
radiator.
When the temperature increases further and
reaches a certain specified temperature, the valve
opens fully, allowing even more coolant to circulate
through the radiator.
Thus, in this way the degree of valve opening is
varied according to the temperature of coolant, and
the temperature of coolant is adjusted by varying the
amount of coolant caused to circulate through the
radiator.
Thermostat Test
To test the thermostat opening temperature, use the
following procedure:
W
ARN
I
N
G
The pan, water and thermostat will be very hot
and can cause burns. Do not touch the pan,
water or thermostat. Handle the components
with an insulated device for protection.
1. Remove the thermostat from the engine.
2. Hang the thermostat in a pan of water. Put a
thermometer in the water. Put the thermostat
completely under water. Do not let the thermostat
make contact with the pan.
3. Put heat to the pan of water. Make the water in
the pan move around. This keeps all of the water
at the same temperature.
4. The thermostat must start to open when the
temperature is 82°C (180°F). The thermostat must
be fully open at 96°C (205°F).
Cooling System Heat Problems
To check if there is a good reason for heat problems
do the checks that follow:
1. The indications of a heat problem are as follows:
a. High coolant temperature indicator light is on
or needle of coolant temperature gauge is in
red range.
b. Coolant boils out (comes out because of too
much heat) of the cooling system during
operations.
c. Coolant boils out on the floor when the engine
is stopped.
d. Coolant must be added at the end of each
shift but Steps b and c are not present.
2. If indication in Step 1a is only present. It is
possible the problem is only a damaged gauge,
light or sender. Make a replacement of the
defective part.
3. If indication in Step 1b is present, do the
procedure that follows:
a. Run the engine at medium idle (1200 rpm) for
three minutes after high idle operation. This
cools off the hottest parts of the engine before
it is stopped.
b. Install a coolant recovery system on the truck,
if not already equipped.
4. If indications in Step 1b, 1c or 1d are present, but
Step 1a is not and the high temperature indicator
light does work, the problem can be a damaged
radiator cap seal or there can be a leak in the
cooling system. Complete the procedure that
follows:
a. Do the Pressure Cap Test, Cooling System
Leak Check, Thermostat Test and Belt
Adjustment in the Testing And Adjusting.
b. Clean the radiator with hot water (steam
clean) at low pressure and use detergent or air
according to the different types of debris that
caused the radiator to be dirty (plugged).
c. Check the engine high idle setting.
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