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Tdi

ENGINE COOLING

COOLANT CIRCULATION

Description

Operation

The Tdi engine uses a pressurized cooling system
and cross flow radiator which is supplied from a
separate header tank. The radiator assembly is in
three sections. The largest section is for engine
coolant and the other two sections which are cast in
aluminium, are the engine oil cooler and the turbo
chargerintercooler.

When the engine is started from cold the thermostat
prevents any coolant circulation through the radiator
by closing off the top hose. During the engine warm
up period, the water pump, pumps coolant towards
the rear of the cylinder block around each of the
cylinders. Coolant as it is heated rises through ports in
the cylinder block and head gasket, into the cylinder
head.Thecoolant flows forwards to the thermostat,
by-pass port and radiator top hose connection.

A belt driven viscous fan and centrifugal water pump
is located in the front of the cylinder block. Hot coolant
is supplied to the heater through hoses. Two small
diameter air purge hoses connect the top of the
radiator and cylinder head water gallery, to the header
tank.

Start from cold (thermostatclosed)
While the thermostat is closed, coolant circulates
around the cylinder block and cylinder head via the
by-pass.
Engine warm (thermostat open)
When the engine reaches normal running temperature
the thermostat closes off the by-pass and opens the
flow to the top of the radiator.

Coolant circulation (engine cold)
1.
2.
3.
4.
5.
6.
7.

Cross flow radiator
Header tank
Viscousfan
Heater hoses
By pass hose and engine thermostat
Air purge hoses
Coolant pump

DESCRIPTION AND OPERATION

1

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COOLING SYSTEM

VISCOUS FAN

Operation

Description

The viscous unit consists of two principal components:
An inner member 1 which is secured to water pump
spindle and is driven by the fan belt.
An outer member 2 which has the fan blades
attached, houses theworking parts and is driven
through the medium of the viscous fluid.

The viscous drive unit for the engine cooling fan,
provides a means of controlling the speed of the fan
relative to the running temperature of the engine. The
viscous unit is a type of fluid coupling, which drives
the fan blades through the medium of a special
’silicone fluid’ injected into the unit during
manufacture.The inner and outer members have interlocking
annular grooves machined in each, with a small
running clearance 3 to allow the silicone fluid to
circulate through the valve plate 4.
The unit also contains a valve 5 which is controlled by
an external bi-metal thermostat 6.
Starting engine from cold
During the time the engine is at rest the silicone fluid
drains down, half filling chambersA and B. Thus when
the engine is first started sufficient fluid is present in
chamber A to provide a positive drive between the
members, as is evident by the initial noise of the fan.
However within a very short period of time, after
starting the engine, the fan speed and noise will
decline indicating that the fluid is being centrifuged
into chamber B (as seen in RR3757M) causing thedrive to slip.

1. Drive in from water pump spindle
2. Drive out to fan blades
3. Bi-metal

2

DESCRIPTION AND OPERATION

Tdi

Hot conditions
When operating in high ambient temperatures or
when stationary in traffic, the bi-metel thermostat will
operate and open the valve port between chambers A
and B.
Opening the valve (as shown in RR3758M) allows the
fluid to circulate between thetwo members, causing
the unit to couple, thus increasing the fan speed and
cooling effect.

Viscous unit slipping (Engine at normal running
temperature)
1.
2.
3.
4.
5.
6.
7.
8.
A
B

Inner member (drive in fast)
Outer member (drive out slow)
Running clearance
Valve plate
Valve (closed)
Bi-metal
Fluid seals
Ball race
Fluid chamber
Fluid chamber

Viscous unit coupled...