Toyota 1HD-T Repair Manual Download

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CO-3

COOLING SYSTEM - Description

The cooling system is composed of the water

jacket (inside the cylinder block and cylinder head),

radiator, water pump, thermostat, cooling fan,
hoses and other components. Coolant which is
heated in the water jacket is pumped to the radiator,

where it is cooled by the cooling fan and the vehicle
windstream. Coolant which has been cooled is then

sent back to the engine by the water pump, where

it cools the engine. The water jacket is a network of

channels in the shell of the cylinder block and
cylinder head through which coolant passes. It is
designed to provide adequate cooling of the cylin-
ders and combustion chambers which become the

hottest during engine operation.

RADIATOR

The radiator performs the function of cooling the

coolant which has passed through the water jacket
and become hot, and it is mounted in the front of
the vehicle. The radiator consists of an upper tank
and lower tank, and a core which connects the two
tanks. The upper tank contains an inlet for coolant
from the water jacket and a filler inlet. It also has a

hose through which excess coolant or steam can

flow. The lower tank has an outlet and drain cock
for the coolant. The core contains many tubes and
cooling fins through which coolant flows from the

upper tank to the lower tank so that coolant has
been heated up as it passes through the water

jacket is cooled heae by the air sucked through the

radiator by the cooling fan, as well as by the wind
generated by the vehicle's travel. Models with an
automatic transmission include an automatic trans-
missions fluid cooler built into the lower tank of the
radiator.

RADIATOR CAP

The radiator cap is a pressure type cap which

seals the radiator, resulting in pressurization of the

radiator as the coolant expands. The pressurization
prevents the coolant from boiling even when the
coolant temperature exceeds 100°C (212°F). A
relief valve (pressurization valve) and a vacuum
valve (negative pressure valve) are built into the
radiator cap. When the pressure generated inside
the cooling system exceeds the limit (coolant

temperature: 110-120°C, 230 - 248°F, pressure;
0.3-1.0 kg/cm

, 4.3-14.2 psi, 29.4-98.1 kPa)

the relief valve is opened by the pressure and lets
steam escape through the overflow pipe. The vac-

uum valve opens to allow atmospheric air to enter

to alleviated the vacumm which develops in the

cooling system after the engine is stopped and the
coolant temperature drops. The valve's opening
allows the pressure in the cooling system to return
to the reservoir tank.

RESERVOIR TANK

The reservoir tank is used to catch coolant which

overflows the cooling system as a result of volu-
metric expansion when the coolant is heated. When
the coolant temperature drops the coolant in the
reservoir tank returns to the radiator, thus keeping
the radiator full at all times and avoiding needless
coolant loss. To find out if the coolant needs to be
replenished, check the reservoir tank level.

WATER PUMP

The water pump is used for forced circulation of

coolant through the cooling system. It is mounted
on the front of the cylinder block and driven by a
drive belt.

THERMOSTAT

The thermostat has a wax type by-pass valve and

is mounted in the cylinder block. The thermostat
includes a type of automatic valve operated by

fluctuations in the coolant temperature. This valve
closes when the coolant temperature drops, pre-
venting the circulation of coolant through the radi-
ator and thus permitting the engine to warm up

rapidly. The valve opens when the coolant temper-
ature has risen, allowing the circulation of coolant.

Wax inside the thermostat expands when heated
and contracts when cooled. Heating the wax thus
generates pressure which overpowers the force of
the spring which keeps the valve closed, thus

opening the valve. When the wax cools, its con-
traction causes the force of the spring to take effect
once more, closing the valve. The thermostat in this
engine operates at a temperature of 76°C (169°F).