Mazda Training manual — part 252

6 – COOLING SYSTEM

93

Piston Engine Fundamentals

TC010-05-01S

If the pressure in the cooling system exceeds the specified limit, the
pressure valve opens to avoid bursting the radiator or hoses. Steam
and coolant can then escape through the reservoir hose (attached to
the filler neck) into the radiator reservoir.

When the engine is shut off, steam in the system condenses back into
liquid, creating a vacuum in the system. In this case, the vacuum valve
on the pressure cap opens, allowing coolant from the reservoir back
into the radiator through the radiator reservoir hose. Without a vacuum
valve, the radiator tanks and hoses could collapse.

The pressure cap protects the cooling system from springing leaks due
to excess pressure or vacuum. For the cap to work correctly, the entire
cooling system must be air-tight.

Testers are used to check the cooling system for proper sealing. The
tester is attached to the radiator filler neck and pumped up to see if the
system will maintain pressure. The cap is also tested separately.

Removing the radiator cap while the engine is running, or when
the engine and radiator are hot is dangerous. Coolant and steam
may escape and cause serious injury. Turn off the engine and wait
until it is cool before removing the cap. Even then, be very careful.

FIGURE 63. The
pressure cap raises
the operat- ing
temperature of the
engine.

Pressure cap

Radiator
reservoir hose

Pressure valve

Vacuum valve

Filler neck

To radiator reservoir

6 – COOLING SYSTEM

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FIGURE 64. A
wax pellet
expands and
contracts to
open and close
the thermostat
valve.

Valve

Coolant flow

Spring

Wax

THERMOSTAT

The thermostat restricts the flow of coolant through the system until the
engine reaches its operating temperature. This helps the engine warm up
quickly, which improves fuel economy and emissions. A quick warm-up also
keeps combustion chamber gases from blowing by the pistons and entering
the crankcase, contaminating the oil.


The thermostat contains a heat-sensitive wax pellet, as shown in Figure 64.
When the engine is cold, the wax remains solid, and the spring holds the
valve closed.

When the coolant heats up, the wax turns to liquid and expands. The
expansion pushes the body of the valve down, which opens the flow of
coolant to the radiator.

Closed

Open

6 – COOLING SYSTEM

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To provide an outlet for air in the cooling system, many thermostats
include a jiggle pin, either in the thermostat itself or in the thermostat
housing. Figure 65 shows how a jiggle pin works.

When there is air in the cooling system, the weighted end of the jiggle
pin drops down, allowing the air to escape. When the engine is
operating, pressure from the water pump pushes the jiggle pin against
its seat. The closed jiggle pin prevents coolant from flowing to the
radiator until the thermostat opens.

FAN DRIVES

The cooling system fan draws air through the radiator core to cool the
engine coolant. Most fans have four or more blades to increase their
cooling capacity. A fan shroud may surround the fan to concentrate the
flow of air.


On some engines, an electric motor drives the fan. In this design, a
thermoswitch

(engine coolant temperature sensor) monitors the coolant

temperature. When the coolant reaches a preset temperature, the
thermoswitch activates an electrical relay, which turns on the fan motor.
When the coolant temperature drops, the thermoswitch turns off the fan
motor.

On other engines, the coolant fan is driven by a pulley and belt. This
design is called a mechanical drive.

FIGURE 65. The
jiggle pin opens
when air is in
the cooling
system.

Jiggle pin

Thermostat
housing

Open

Closed

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FIGURE 66. A
viscous drive fan
clutch conserves
engine power by
match- ing fan
speed to air
temperature.

Clutch plate

Bi-metal
thermostat

Viscous Drive Clutch

Most mechanical drive fans use a clutch drive, which allows the fan to
turn at lower speeds when the temperature is lower. If the fan were
constantly turned at the speed of the engine, the fan would become very
noisy at high speeds, and it would sap engine power.

One of the most common types of fan clutches is the viscous type. A
viscous drive is a fluid coupling. Viscous refers to the viscosity, or
thickness, of the fluid — usually a silicon oil — that is used to control the
clutch. A typical viscous drive clutch is shown in Figure 66.

The amount of coupling is controlled by a bi-metal thermostat, which is a
spring made of two types of metal. The spring expands at higher
temperatures and contracts at lower temperatures. The thermostat is
connected to a valve that controls the amount of fluid available to couple
the clutch.

The thermostat responds to the temperature of the air passing through
the radiator. If the air temperature is cold, the flow of fluid in the clutch is
restricted. Little or no coupling occurs, and the fan turns very slowly or
not at all. At higher temperatures, the fluid operating on the clutch
increases, causing a tighter coupling and faster fan speed.