Discovery 2. Manual - part 99

 

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Discovery 2. Manual - part 99

 

 

COOLING SYSTEM - V8

DESCRIPTION AND OPERATION

26-2-9

Viscous fan

Coolant pump pulley drive attachment
Fan blades

Bi-metallic coil
Body

The viscous fan provides a means of controlling the speed of the fan relative to the operating temperature of the 
engine.  The fan rotation draws air through the radiator, reducing engine coolant temperatures when the vehicle is 
stationary or moving slowly.

The viscous fan is attached to the coolant pump drive pulley and secured to the pulley by a  nut. The nut is positively 
attached to a spindle which is supported on bearings in the fan body. The viscous drive comprises a circular drive 
plate attached to the spindle and driven from the coolant pump pulley and the coupling body. The drive plate and the 
body have interlocking annular grooves with a small clearance which provides the drive when silicone fluid enters the 
fluid chamber. A bi-metallic coil is fitted externally on the forward face of the body. The coil is connected to and 
operates a valve in the body. The valve operates on a valve plate with ports that connect the reservoir to the fluid 
chamber. The valve plate also has return ports which, when the valve is closed, scoop fluid from the fluid chamber 
and push it into the reservoir under centrifugal force.

Silicone fluid is retained in a reservoir at the front of the body. When the engine is off and the fan is stationary, the 
silicone fluid level stabilises between the reservoir and the fluid chamber. This will result in the fan operating when the 
engine is started, but the drive will be removed quickly after the fan starts rotating and the fan will 'freewheel'.

At low radiator temperatures, the fan operation is not required and the bi-metallic coil keeps the valve closed, 
separating the silicone fluid from the drive plate. This allows the fan to 'freewheel' reducing the load on the engine, 
improving fuel consumption and reducing noise generated by the rotation of the fan.

When the radiator temperature increases, the bi-metallic coil reacts and moves the valve, allowing the silicone fluid 
to flow into the fluid chamber. The resistance to shear of the silicone fluid creates drag on the drive plate and provides 
drive to the body and the fan blades.

COOLING SYSTEM - V8

26-2-10 DESCRIPTION AND OPERATION

Operation

Coolant flow - Engine warm up
Refer to illustration.
 

 + 

 COOLING SYSTEM - V8, DESCRIPTION AND OPERATION, Cooling system coolant flow.

During warm-up the coolant pump moves fluid through the cylinder block and it emerges from the inlet manifold outlet 
pipe. From the outlet pipe, the warm coolant flow is prevented from flowing through the radiator because the 
thermostat is closed. The coolant is directed into the heater circuit.

Some coolant from the by-pass pipe can pass through small sensing holes in the flow valve. The warm coolant enters 
a tube in the thermostat housing and surrounds 90% of the thermostat sensitive area. Cold coolant returning from the 
radiator bottom hose conducts through 10% of the thermostat sensitive area. In cold ambient temperatures the engine 
temperature can be raised by up to 10

°

C (50

°

F) to compensate for the heat loss of the 10% exposure to the cold 

coolant returning from the radiator bottom hose.

At engine idle speed, the by-pass valve is closed only allowing the small flow through the sensing holes. As the engine 
speed increases above idle, the greater flow and pressure from the pump overcomes the light spring and opens the 
by-pass flow valve. The flow valve opens to meet the engines cooling needs at higher engine speeds and prevents 
excess pressure in the system. With the thermostat closed, maximum flow is directed through the heater circuit.

The heater matrix acts as a heat exchanger reducing coolant temperature as it passes through the matrix. Coolant 
emerges from the matrix and flows into the coolant pump feed pipe and recirculated around the heater circuit. In this 
condition the cooling system is operating at maximum heater performance.

Coolant flow - Engine hot
As the coolant temperature increases the thermostat opens. This allows some coolant from the outlet housing to flow 
through the top hose and into the radiator to be cooled. The hot coolant flows from the left tank in the radiator, along 
the tubes to the right tank. The air flowing through the fins between the tubes cools the coolant as it passes through 
the radiator.

A controlled flow of the lower temperature coolant is drawn by the pump and blended with hot coolant from the by-
pass and the heater return pipes in the pump feed pipe. The pump then passes this coolant into the cylinder block to 
cool the cylinders.

COOLING SYSTEM - V8

DESCRIPTION AND OPERATION 26-2-11

Viscous fan operation

A = Cold, B = Hot

Drive plate
Fan body
Clearance
Valve plate
Valve
Bi-metallic coil

Fluid seals
Ball race
Fluid chamber

10 Reservoir
11 Return port

When the engine is off and the fan is not rotating, the silicone fluid stabilises within the fluid chamber and the reservoir. 
The fluid levels equalise due to the return port in the valve plate being open between the fluid chamber and the 
reservoir. In this condition, when the engine is started, silicone fluid is present in the fluid chamber and causes drag 
to occur between the drive plate and the body. This causes the fan to operate initially when the engine is started.

As the fan speed increases, centrifugal force and a scoop formed on the fluid chamber side of the valve plate, pushes 
the silicone fluid through the return port in the valve plate into the reservoir. As the fluid chamber empties, the drag 
between the drive plate and body is reduced, causing the drive plate to slip. This reduces the rotational speed of the 
fan and allows it to 'freewheel'.

When the coolant temperature is low, the heat emitted from the radiator does not affect the bi-metallic coil. The valve 
remains closed, preventing fluid escaping from the reservoir into the fluid chamber. In this condition the fan will 
'freewheel' at a slow speed.

As the coolant temperature increases, the heat emitted from the radiator causes the bi-metallic coil to tighten. This 
movement of the coil moves the valve to which it is attached. The rotation of the valve exposes ports in the valve plate 
which allow silicone fluid to spill into the fluid chamber. As the fluid flows into the clearance between the annular 
grooves in the drive plate and body, drag is created between the two components. The drag is due to the viscosity 
and shear qualities of the silicone fluid and cause the drive plate to rotate the body and fan blades.

As the coolant temperature decreases, the bi-metallic coil expands, rotating the valve and closing off the ports in the 
valve plate. When the valve is closed, centrifugal force pushes silicone fluid through the return port, emptying the fluid 
chamber. As the fluid chamber empties, the drag between the drive plate and the body is reduced and the body slips 
on the drive plate, slowing the rotational speed of the fan.

 

 

 

 

 

 

 

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