Rover's Vikingship SD1 Viscous fan Rover's Vikingship

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 three viscous couplings

There are three types of viscous couplings around for the SD1.

  • A viscous coupling with the drive shaft going right through the unit. This one has no bimetal operation.You can find it in all models, including 2300/2600 from '76 until '82 and carburetted models from '82 onwards with no air conditioning
  • The models from '82 onwards with air conditioning have a similar type (heavy duty)
  • The EFI models have a viscous coupling whereby the shaft doesn't go right through the coupling but has a nut which is screwed onto the driveshaft. At the front there is a bimetal spring (This is the coupling as shown in the pictures)

viscofan back side 12,7 kB viscofan front 15,6 kB
The back side of the fan (towards the engine) the nut must be loosened clockwise !! The front side of the fan, facing the radiator. The bimetal spring can clearly be seen.
viscofan 18,4 kB wheel grooves 17,8 kB
Picture above, the coupling opened up. From left to right.
  • 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
The grooves in the wheel and the housing can be seen here

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.

An electric fan?

Maybe you'll ask, why don't you install an electric fan. It is possible and using an electric fan has its own advantages and disadvantages.
  • An electric fan is cheaper than a viscous fan,especially when using one from a scrap yard
  • The fan doesn't absorb any power when it's not in use
  • You can switch the fan on/off by yourself if you by-pass the electric thermostat


  • The viscous fan is always blowing some air through the engine bay. An electric fan only when the water temperature is high. So the viscous fan provides a cooler engine bay which is better for the electric components
  • With an electric fan you introduce more wiring, thermostat, electric motors which can fail

BMW viscous fan on a 3.5 V8 EFI 47,1 kB
BMW viscous fan found on a Swiss EFI Vandenplas now being restored in Holland

mainpage © A3aan may 2001