Hello Gareth, Within my own field of knowledge, mostly with V8 Efi SD1, try this easy to understand explanation as to what is inside our viscous fan coupling and how it works.
<font color="green"><i># The Rover SD1 Efi Viscous Fan Coupling (Clutch Fan) uses the unique properties of a silicone oil whose viscosity changes both with sheer force and temperature. A grooved wheel (fixed to the water pump) rotates within a grooved housing (fixed to fan blades) and the oil is forced to move within and between the revolving grooves
# The larger the speed differential between oil layers in the wheel and housing the greater the sheer forces which raise the temperature and viscosity of the oil. The increased bond between the two causes the fan to rotate faster increasing airflow through the radiator. As speed difference falls, sheer forces and oil temperature fall, the oil gets thinner, the driving force lessens and fan speed stabilises.
# For Efi cars a forward facing bi-metal coil senses the airflow coming from the radiator. Heat causes the coil to turn a shaft which operates an internal control valve regulating the volume of oil which is able to flow between the grooves. Thus hotter air allows more fluid to enter the grooves resulting in a stronger bond between wheel and housing giving higher fan speed and increased cooling.
# The first of two common fail conditions is when the fan does not speed up due to insufficient bond between the wheel and the housing resulting in overheating especially in slow traffic, urban driving, towing or running air-con. Lightly loaded at normal driving speed there is sufficient air passing through the radiator to cool the engine. There are three reasons for such failure. Faulty bimetal coil, oil control shaft/valve siezed, or loss of oil. A simple check is to press a rolled newspaper against the fan blades when a hot engine is idling. If the fan can be slowed or stopped it is probably faulty.
# A second fail condition when the fan becomes permanently locked is potentially very serious. Because the grooves are thin they can break due to age, internal corrosion, vibration, etc, with debris jamming between wheel and housing resulting in an over speed fan. The air roars loudly and continuously, more so at high engine speeds and such a condition may result in a thrown fan blade, resulting in severe imbalance and further breakage. If this happens broken parts are projected forwards into the radiator. Never drive a car with a locked-up viscous fan.
# There is a third (possible) fail condition whereby the coupling is quite stiff when cold and upon start-up the fan roars at high speed for a short while, say from thirty seconds to two minutes, before it slows to a normal free-wheeling speed. This is due to the internal fluid leeching into the grooves, usually over-night, through poor internal seals and locking up the fan immediately. However the fluid soon reverts to its normal routine and the clutch lets go of the fan until such time as normal heating of the mechanism engages the fan again when hot, as previously described. Perhaps not a true fail condition, but certainly a pre-cursor to total failure when the fan will eventually fail to engage, sometime soon.</i></font id="green">
Taken from my treaty on the subject - here:
http://www.vintagemodelairplane.com/pag ... ous01.html
I'm not familiar with the specifics of your TR7/8 viscous fan coupling arrangement but I hope the explanation goes some way to answering your question?
FYI, I have some further information on how the viscous fan interacts with the other two key temperature control components - thermostat and radiator - importantly combining to stabilise the temperature of our engines.
Download the PDF document from here:
http://www.vintagemodelairplane.com/pag ... ing01.html
Ramon
www.vintagemodelairplane.com