Mazda Training manual - part 252

 
6 – COOLING SYSTEM 

 

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Piston Engine Fundamentals

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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 

 

94

Piston Engine Fundamentals

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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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Piston Engine Fundamentals

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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

 
6 – COOLING SYSTEM 

 

96

Piston Engine Fundamentals

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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.