I was going to put this point to David directly but felt the whole group may benefit from an explanation of this vital component to our cooling systems.
I am still having intermittent cooling problems since my engine was rebuilt, as described on the home page, and tend to go weeks between problems.
My idea as to how this item works goes like this. The fan and the engine are attached to either side of this coupling which is filled with a fluid that hardens as it gets hotter. Within this coupling are two sets of blades, one set for each side of the coupling.
When you start the engine, the fluid is cold and watery in consistency, so although the fan turns there isn't a very strong bond between the fan and the engine. This allows the fan to be stopped with little effort ( not with your hand ) and the engine to reach optimum temperature. When your engine reaches this temperature the fluid in the coupling is hotter and has a thicker consistency. The thicker consistency causes the bond between the engine and the fan speed to improve causing the fan to run faster at higher speeds and harder to stop ( not with your hand ).
Is this correct or have I just wrote a load of drivel. I have read here before that this can be tested in two ways :
1/ When cold you should be able to tie a length of cotton / thread to the fan and start the engine without the thread breaking. When you do this when the engine is up to temperature the thread will break.
2/ Drive the van along the motorway at greater than 3000rpm / 65mph and observe the fan. If it stops the coupling is faulty.
Are these tests correct ?
I am here in my shadey room awaiting the light of wisdom to shine on me........OTT I know. - (#1554) Mark, 4 Dec 01

I know ford had the same fan fitted on the old for Granada and I had one that given me a lot
of trouble , what I did was to get a fan that have the same hole as the one Granada had the
size was also about the same and I bolt it on and from then on I never had any more heating
trouble .
What I think is if any one knows of a car that have almost the same permanent fan that would
fit the four stud that hold the Viscous fan I would think such fan would do the trick it would
work more better then electric one and sure it would cool the water down and would fix the
trouble some of us have .
It would be great of all of us look and find out about that sure one would soon find the right
fan the can inform the others I am one who would like to do it to my bus
bigmoe - (#1555) bigmoe, 4 Dec 01

Yup, your tests are all OK. Apparently, you can buy bottles of the silicone fluid to refill the coupling with from your dealer.
A fixed fan, if you could find one to fit(remembering that it has to stand-off the engine by four inches or so to sit within the shroud) would cool the engine well, but at the expense of much wasted power. A viscous fan is reckoned to sap 10% power, a fixed fan moreso. - (#1556) david miller, 5 Dec 01

Well done Mark. A very good explanation of the operation of the viscous coupling/fan arrangement which will give us all a better understanding of it's operation. There is, also, a bi-metallic coil fitted to the coupling and this allows more, or less, silicone fluid to move through the coupling. A bit like a fluid flywheel but, dependant on temperature for it's operation. There has been much discussion, and very helpful contributions, on this board, about the various cooling problems of these vehicles, all of which have added, in my case especially, to understanding the system and, although the viscous fan operation has been highlighted before, I feel that it's importance could be overlooked, especially during the colder weather when an indifferent operation will not have much effect. I feel that owners are not aware, until it is too late, when the viscous fan coupling starts to fail. Unless the temperature gauge is constantly watched, the first indication that the fan has stopped turning is the system boiling over and, in some cases, dire consequences for the head & gasket etc. I should like to fit some sort of warning device such as a light senstive cell and beam, being interupted by the fan blades and sending signal to a warning lamp or audible device, which would indicate whether or not the fan blades were still turning. Has anyone on the BOK done this or, especially the electronic experts, has anyone any thoughts on the feasability of such a device ? It should be quite inexpensive, and straightforward, by using components from, say, the Maplin catalogue and would be a worthwhile addition to safeguard the cooling system at motorway speeds when there is little or no ram cooling effect. I am nervous of the integrity of my 10 year old viscous coupling and, before next summers's warmer weather I should like to take David's course of action, ie, convert to an electric fan, or fit some sort of safeguard to ensure that the coupling is still doing it's job - (#1557) John Davis, 5 Dec 01

I thought of that (remembering that it has to stand-off the engine by four inches or so to sit within the shroud) and I recall that when I did fit a fixed fan on the ford I got a cover which extend the shroud by about 2/3 inches I think one may find a extra cover to do the trick I am looking at some covers now and trying to find a way of geting the fan and cover all to fit right once I do it i would let you all know how I got on .
the only thing that worry me would that fan make the bus run worm not hot I think it should be right tembture to run corect would a fix fan make run a bit cooler then it should or it should be ok ,what do you think .
any how I well try it I have a lot to gain and if it work it should save me the trouble of hade going on the bus . - (#1558) bigmoe, 5 Dec 01