WD Catalog 2018

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Chapter 3 - Heater Components and Controls
3-2 - -
Figure 3-1
The thermostat is shown in
both closed and open posi-
tions. When closed coolant
does not circulate through
the radiator.
COOLANT OUTLET
TO RADIATOR
CASTING
THERMOSTAT
PISTON
COOLANT
FLOW
COOLANT
FLOW
THERMOSTAT VALVE CLOSED –
COOLANT CIRCULATES IN ENGINE
THERMOSTAT VALVE OPEN–
COOLANT CIRCULATES THROUGH
ENGINE AND RADIAOTR
ENGINE BLOCK
OVERFLOW PIPE
TO COOLANT
RESERVOIR
UPPER AND
LOWER SEALING
SEATS
CAP PRESSURE
VALVE
CAP VACUUM
VALVE
RADIATOR
UPPER TANK
RADIATOR
CORE
1. Thermostat - controls the direction of flow of the coolant
from the water pump, through the engine and radiator. As
the engine warms up the thermostat opens, allowing the
coolant to flow to the radiator. The coolant gives up excess
engine heat to the outside air as it moves through the radia-
tor. If the engine is cold, the thermostat stays shut and the
coolant by-passes the radiator and circulates in the engine,
as well as the heater core.
2. Radiator – radiator cap seals the cooling system at the inlet
on top of the radiator. Caps are pressure rated to match
cooling system design. Each pound of pressure on the cooling
system raises the boiling point of coolant three degrees Fahr-
enheit, so the pressure cap extends the cooling system oper-
ating range above the normal boiling point. Coolant can flow
through the heater core regardless of thermostat position.
Altitude lowers the boiling point of coolant by two degrees Fahrenheit for every
1000 foot increase of elevation above sea level. As you can see, the pressure cap
tends to compensate for a lower boiling point at higher altitude. When high
system pressure forces the radiator cap to open, coolant escapes to the coolant
reservoir. A low pressure or slight vacuum inside the system sucks coolant back
to the radiator from the reservoir.
Figure 3-2
The pressure radiator cap
seals the cooling system.
The cap pressure valve
opens when operating pres-
sures are high.