Chapter 5 Fundamentals of Engine Construction and Operation 83
is shut off until the coolant temperature falls to a certain
point. In addition, the fan is turned on any time the air
conditioner compressor is operating, no matter what the
coolant temperature.
Some longitudinal engines have belt-driven fans.
These fans are usually installed on the end of the coolant
pump shaft so that the same pulley drives the pump and
the fan. Many belt-driven fans have a fluid clutch installed
between the drive pulley and fan assembly, Figure 5-18.
The fluid clutch allows the fan to freewheel, or not be
driven, at higher speeds when it is no longer needed. Some
fluid-clutch fans contain a thermostat that prevents the fan
from operating until the engine warms up. Other fluid-
clutch fans contain a flow-control solenoid. The solenoid
is operated by the vehicle computer based on temperature
sensor inputs.
Radiator Pressure Cap. The radiator pressure cap
is a valve that allows pressure to build up in the cooling
system. See Figure 5-19. It does so by using spring pressure
to hold a seal against the radiator filler neck. The spring is
calibrated to produce the proper system pressure. As the
engine heats up, coolant pressure builds until it overcomes
spring pressure and opens the valve. When the excess pres-
sure is released, the spring closes the valve.
Figure 5-16. Core plugs are installed in various locations in the
front, sides, and rear of the block. The holes sealed by these
plugs are used to remove molding sand after the block is cast.
(Ford)
Typical core
plug locations
Figure 5-17. One or more electric fans are used on all front-
wheel and many rear-wheel drive vehicles. The electric fan
allows for better control of fan operation as well as more avail-
able horsepower from the engine. (Chrysler)
Crossflow
radiator
Air-temperature-sensing
switch
Radiator fan
switch
Electric
fan motor
Fan
shroud
Enlarged
view—note polarity
Automatic
transmission
oil cooler
fittings
Figure 5-18. Some vehicles have belt-driven fans. The fan clutch
allows the fan to freewheel at highway speed. This reduces the
load on the engine.
Fan clutch
Fan
blades
Serpentine belt
Coolant
pump
Drive
pulley
Figure 5-19. Radiator pressure caps are not a frequent source
of driveability or engine problems; however, a faulty cap can
cause diminished overheating protection. Never open a radiator
pressure cap when the cooling system is under pressure.