A while ago the viscous coupling on my SD1 went to Rover heaven. This happened in a queue with the car suddenly overheating because the fan blades weren't driven properly anymore by the coupling.

Because they are sealed units they can't be repaired so I looked around and obtained a good 2nd hand one. Curious by birth I couldn't resist opening the old unit to find out how it worked.

The units operation consists of two main parts

• The viscous coupling.
  In the housing there is a wheel with fine grooves which runs in the also finely grooved housing to which the fan blades are attached. Between the housing and the wheel is silicone oil. Now a property of this oil is that it's viscosity is not so much depending on temperature (as with normal oil) but on velocity. The greater the oil velocity the greater the viscosity. The greater the difference in speed between the oil layer on the housing and the oil layer at the wheel the thicker the oil becomes and so the wheel (which is attached to the water pump shaft) takes the housing with it and the fan turns. As the fan speeds up the speed difference decreases and the silicone oil gets thinner....thus less driving force.....this system prevents the fan from over revving and limits it turning speed at about 2500 rpm

• The bimetal.(only for EFI)
  At the front of the housing there is a bimetal spring.When it heats up (by the air blowing warmed up by the radiator) it turns a small axe. Inside the coupling this axe moves a thin metal plate which controls the amount of silicone oil which is able to flow between the grooves. Thus hotter air results in more silicone fluid to enter the grooves resulting in a stronger connection between wheel and housing and giving a larger fan speed.

• The bimetal turns a small arm in the housing
• The arm moves the oil plate which regulates the amount of oil to the wheel
• the wheel in the cut housing

There are two ways this beautiful thing can let you down

• By locking itself up
  The grooves are very thin and probably can break due to vibrations of the fan blades, engine,etc. this could lead to a lock up between wheel and housing resulting in an over revving fan. It can be clearly heard because suddenly the fan makes a very loud noise, especially when the engine is revving hard. There is a chance that a fan blade breaks off, this can cause such an unbalance that more blades also try to find their way to freedom. But even more damage can be done. Because the blades push air towards the engine, when a blade comes off the reaction forces move the blade forward against the radiator!. Now if your unlucky this can result in a damaged (even leaky) radiator.
• By not picking up the revs
  When the connection between the housing and the wheel isn't good enough the fan blades will turn slower than desired. This can result in overheating of the engine. especially when driving in a queue, town driving or towing. Reasons for this happening can be some loss of silicone oil along the drive shaft, malfunctioning of the bimetal spring or a seized oil plate You can check this by carefully (!!!can be dangerous!!!!) trying to press a thick cloth against the fan blades when the engine is at operating temperature and idling. If you can stop the fan...the unit is faulty and should be replaced.

New viscous couplings are quite expensive...about £ 160 or fl500,= so trying to find a good second hand one might be worthwile. However it is impossible to see what state a 2nd hand one is in. It might fail as well within a year or give good service for years and years. It is also possible to use viscous fan from other engines.In the picture below you can see a viscous fan on a EFI engine which came from a BMW. You have to redrill the holes for the fan though to make it fit. If you do, make sure the fan is absolutely in the middle otherwise bad vibrations will give a lot of trouble.