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how to check viscous fan coupling is working properly?


Well-Known Member
Mar 1, 2010
How can I check the viscous fan coupling or the fluid coupling as called in the FSM is functioning properly. From what I understand its only method of failing is seizing, is this correct? The reason I'm asking is because the place that's rebuilding my air con compressor has told my garage that the viscous coupling failing could cause the aircon to fail. They way I see it is that if the VC fails it will over cool the condenser making it more efficient, so I cant see how it will affect the reliability of the compressor. Anyway need to check if the fan VC is working properly.
I think it's more common for them to fail by not being viscous enough and so not turning fast enough than locking up. Get the engine hot driving about not just ticking over on the drive, preferably on a hot day, turn the engine off and straight away try to turn the fan by hand. It should be quite stiff. Is your aircon not working very well? It IS quite common for the first signs of coupling failure to be air con that can't cope with higher ambient temps.
The aircon is currently not working at all as the compressor is getting reconditioned. It seized about a year ago after a ragas. Will check tomorrow morning if the viscous coupling is working.
I don't think a failing VC will damage anything on the aircon, it just gets a bit hot and high pressure without sufficient cooling of the condensor and stops blowing cold. I suppose the pressure could get to a point it was terminal but it doesn't sound very likely. Why was it regased? Was it working at all before that?
It was never working in my possession. It was re-gassed and all the gas leaked out over a couple of days. It the partially re-gassed with dye to check for leaks. It then seized when running to check for leaks while on the M4. When the pump was dismantled the lubrication system was blocked. They mentioned I should check the VC but as I did not speak with them myself I'm not 100% sure why.
Well a failing VC wouldn't block the lubrication of the aircon compressor so I don't know why they'd have mentioned that either :roll:
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Isn't the VC on the fan a consumable item? I.m sure that there is something that you can replace isn't there? Like some wax pellet or something. Why not just see what that entails and do it anyway for your overland trip. I am sure that Mr Rubie could sort that. Tempted myself. I thought that they failed to safe. IE if the wax bit fails they just lock so they run all the time?

Hey what do I know, just remember reading it, I'm sure.

The fan VC is filled with silicon fluid. this can loose its thickening properties over time, or even leak out slowly.
the movement of the fluid within the clutch housing through small holes is controlled by a bi-metallic coil strip on the front of the fan, as this heats up - e.g. due to a hot radiator - it rotates and opens up some valves, allowing the silicon fluid to flow within the fan clutch, this then becomes more viscous due to shearing and locks the fan up until the radiator cools enough to allow the bi-metallic coil strip to cool and close the ports.

if that hasn't bored you enough :confusion-seeingstars:

here is more detail - with PICTURES ! ... fan-clutch
Strewth, I think I shall leave mine alone! Thanks Andy. Your prize is on its way!

Ta Chris! funds on way to you!

i topped up the fluid in my fan clutch last year - bought it from an RC model shop - as the limited slip diffs in RC buggies use the same sort of fluid and it was cheaper than mr T !

may put some more in at some-point.
the fan usually roars when you start off with cold engine, as the fluid in clutch as settled overnight. then goes quite after than initial roar. then should roar when engine is getting too hot.
when i had the 4runner, i adjusted the set point of the bimetallic strip and added more fluid. if i had been driving then stopped at traffic lights for a few mins, the fan could be hearing kick-in roar as i pulled away from the lights. my LC doesnt do this,
at one point i also put too much fluid in the fan clutch on the 4runner, so the fan was engaged all the time - boy did it sound good!
but also robbed engine of power and it drank a lot more diesel!
Mine definitely does that 'roar' on cold start for a bit and then suddenly sort of switches off.

WOW, that is quite a complext bit of kit. Very interesting reading indeed. Thanks
I know what "roar" you are talking about (my dads Navara gets it) and there is definitely a lack of "roaring" on my cruiser when I turn the engine on. Maybe its far quieter then on the Navara? Will listen very carefully next time I turn it on. Really technical post eh... :lol: :lol: :lol:
iv'e noticed my neigbours Navara has the fan roar, whenever they drive up lane past our house, they must be setup differently to a LC
This is very interesting.

I am chasing up on the silicon fluid (08816-10001). I have just spoken to my local dealer who tells me it is not available in this country they can only sell a complete coupling. Oddly he has a price for the fluid but can not order any.

My main supplier supplier of parts lists the fluid, I will email to see if he can supply. Looking like £8 - £9 per tube. How much was the R\C model fluid?

I cant remember where i got it from, but should still have the order conf email on PC at home.
i do remember i did some searching before i found a site that had correct weight of fluid and gave the volume in the bottle,

i seem to remember this was a expensive option ... howall.php

but most places you got a bottle of 50ml for a tenner, were as toyota price was same for 18ml

for example: ... emID=28538
no volume of the bottle given!

here is another discussion on fan clutches with toyota part no's
So does the above mean you can use the silicone oil from the R/C shops in the Viscous coupling? I have also tried getting the oil from the dealer and got the same response as Ian.

Could there be a difference in the quality of the oil even though the viscosity is the same? Or are all silicone oils with the same viscous rating equal in quality and ability to cope with heat. I'm wondering because the heat the oil is exposed to in the fan clutch will no doubt be far greater than anything a R/C car shock absorber would experience.
Be careful on getting the correct viscosity. Silicone fluid viscosities range from that of water to grease.
How much is in the housing? Is a leek obvious?