Tuning and Understanding your Toyota Viscous Fan Clutch

Landcruiser or Hilux overheating? Your factory fan clutch is probably under-filled and incorrectly set from factory. Fix this first and you may save a lot of time chasing issues.

(4 Runner or Tacoma in the USA)

P3280006 - Copy

The stock Toyota cooling system can sometimes be somewhat marginal. The suspicion for this falls on every component and modification in the system.

  • Radiator (Size / Efficiency)
  • Thermostat (Brand / Effectiveness)
  • Water Pump (Flow, Cavitation)
  • Radiator Cap (Quality, Pressure, Leaks, Recovery)
  • Coolant (Freezing / Boiling points, Specific heat, Anti-corrosion)
  • Hoses (Restriction)
  • Engine Type (Diesel / Turbo / Petrol)
  • Engine Load / Modifications (Diving style, load on vehicle, Mods)
  • Gearbox (Auto Cooling, Slipping)
  • Airflow (Obstructions / Restrictions In / Out, Forced / Natural)
  • Ambient Operating Environment (Temp, Altitude, Terrain)
  • Shrouds (Closeness to Fan, Leaks, gaps between radiators)
  • Fan (Size / Pitch / Airflow)
  • Fan Clutch (Lockup Temp / Stages / % Slip)
  • Temperature Gauge (Damping / Accuracy)
  • Bullbars / Winches / Lights / Antenna’s / Plates / Screens

Ask anyone and they’ll start listing random items from the list above that they have seen before or are suspicious of. It would appear that the issue is simply that the system is marginal in certain areas, and several small changes may be enough to tip it over the limit.

The end goal of a cooling system is to transfer heat to the surrounding air. All the other components are only there to allow this transfer to occur in some improved fashion. There are plenty of air cooled motors in existence that do not have these complexities, and they too may be subject to overheating.

It would appear that Mr Toyota VERY closely engineers his vehicles, with many parts sharing multiple purposes, and many many tradeoffs being made. This is good engineering, but it means that small changes may have many unintended impacts. Despite this, it appears the Landcruiser and Hilux are intended to be frequently modified. There are many attachment points, and the OEM design has many dealer supported aftermarket options that are not from the Toyota factory.

If all the basic checks have been performed on the cooling system – no leaks, nothing obviously blocked, quick warm up, infrequent overheating except under specific circumstances, then it is a fair bet that the overall system is simply marginal. In this case, a dramatic increase in specific areas may yield a significant benefit.

In my case the overheating was limited to situations with a pre-turbo EGT in excess of 550C. This equated to High Load or High Speed driving. Despite expectations, off-road steep terrain (sand excluded) does not yield high EGT’s. Mountain Ranges, Large Trailers, Roof Racks, High Speed or Deep Sand all would yield high EGT’s and therefore problems.

I have measured many temperature points around the engine bay, and spent some time listening to the engagement and disengagement of the fan. All this yielded much confusion rather than understanding.

I replaced most components, some twice. It was during this that I had time to closely examine and understand the Toyota Viscous Fan Clutch. Possibly more than any other component, this is the key item in the cooling system. It is this that creates the airflow, not vehicle forward speed. Without airflow, the radiator is not effective. My experience was very similar  in a Toyota Surf I had owned previously. It is common knowledge that additional Silicon Fluid will often improve these units. What is not common knowledge is:

  • Brand new OEM clutches appear to be under-filled
  • They can be adjusted where they engage
  • There are 4 separate engagement stages
  • Testing cannot be done one the bench. The device requires centrifugal force to operate.

Credit goes to Frank for his guide on how to split and refill the fan clutch. I am just explaining the operation in further detail.

It must be remembered that these types of fluid couplings always have some slip. They may slip by 98% (free spin) or 5% (coupled), but there is always slip. It is difficult to test the slip in any simple manner, and impossible to bench test. Therefore a fan that appears to be engaging and disengaging successfully, may in fact be slipping at 50%, significantly reducing maximum airflow. Worse, the slip will be only happen at high RPM and maximum load.

The key points are that there are 4 operating stages, and that there is not enough fluid to couple the system adequately.

This is why so many people report success with simply adding more fluid. Adding fluid means that when the system is operating with the valve fully open, the rings are full of silicon fluid, and not partly full. The only drive is through the fluid, so insufficient fluid will reduce maximum coupling ability. There was clearly not enough fluid in the unit to fill all the rings to the depth of the final valve.

The factory engagement points are also quite high. This reduces noise and fuel consumption, but also means maximum engagement doesn’t occur until the air temp is around 95C. Engine coolant temperature will always be higher than air temperature.

This was all tested with a Digital Thermometer and a water bath on the stove.
The water was heated and cooled and the valve set points noted as it moved.


Temperature Set Points (all at 1/2 open)

Stage Original Temp Adjusted Temp
Closed 50 40
Stage 1 55 45
Stage 2 85 75
Stage 3 95 85

Pictures of operation:

Fan Clutch P3280006 - Copy
The 2 halves opened BlueFanClutchApart[1]
The “drive disc” spins freely in the housing except for the silicon fluid. P3230019
The “drive disc” and the “front half” share these closely spaced rings. It is these rings, and the silicon fluid in the gaps between them that couple the system together.The inner ring is taller than the others.

The oil is slowly thrown to the outside of the system by centrifugal force.

P3230004 P3230011
The fluid is rated at 10000 Cst – Centistokes – a measure of viscosity P3230009
The valve that controls where the fluid flows. It operates over 4 stages:0) Closed
A) Some oil to some rings
B) Some oil to all rings
C) Maximum oil to all of rings

This is why it seems to be more than engaged / disengaged.

P3230005 - Copy - Copy
The temperature sensing Bi-Metal spring on the front face that controls the valve. BlueFanClutchthermosideup
The reservoir behind the valve disc in the front half where the fluid is stored. When operating it is held here by centrifugal force, and pumped here by the slipping “drive disc” P3230012
The “vanes” on the edge of the drive disc in the rear half that pump the fluid forward to the outer channel for return to the reservoir. Some slip is required to allow the pumping to occur.The slots in the back of the disc pump the fluid from behind the disc to the edges, and then to the channel at the front.

The rear of the disc is not used for coupling.

The wedge shaped guides and small holes in the front half that collect the fluid from the outer channel and push it back into the reservoir. P3230002
Adding Fluid P3230008
Adjust the valve set point by loosening the 2 screws and rotating the disc.
The outer valve should be 1/2 open at about 45C for Australia. (US quotes 35C). Air temp will always be less than engine water temp.
Getting the fluid level right is a little difficult and involves some guesswork.The minimum amount required is enough to fill the entire outer rim past the depth of the fins in both halves, this fully couples the unit.

The maximum amount is when the reservoir in the front is full and overflows through the central hole. Not so simple though, as full is controlled by centrifugal force, so when operating it fills the “outside” of the reservoir, not the bottom.

Luckily there is a fair tolerance between the two. Overfull will couple the fan all the time.

Mine took 1.5 tubes of fluid in addition to the factory fill to stay a few mm below the level of the valve disc.


32 Responses to “Tuning and Understanding your Toyota Viscous Fan Clutch”

  1. […] Fan Clutch Article I wrote This might interest some of you. Tuning and Understanding your Toyota Viscous Fan Clutch | Paul’s esoteric meanderings It’s an expansion on Franks. Thanx […]

  2. […] yes, you can adjust the temp settings on the fans. Paul has done a good write up on this… Tuning and Understanding your Toyota Viscous Fan Clutch | Paul’s esoteric meanderings __________________ BEU77Y and the BEE55T 105 series GXL Intercooled turbo diesel auto Don’t […]

  3. Chris Fraser says:


    Thanks for the advice! I was about to dish out over $1000 for a new allow radiator until you told me about this. Armed with $10 tube of oil and an hour of my time all was said and done! The truck now runs two full needle widths cooler, which is back to where is was prior adding a turbo!!! My truck is just any old aftermarket turbo mod either, its a highly modified engine running various parts from both the 1HZ and 1HDT engines pumping out 150 rwhp! Keeping this thing cool is essential!

    Thanks again

  4. buddy says:

    where do i get the silicon fluid?

    my local shops dont have it?


  5. […] is great read on the fan-clutch: Tuning and Understanding your Toyota Viscous Fan Clutch Paul’s esoteric meanderings __________________ Chris 07 Salsa SR5, 4wd Cyclone pre-cleaner , Aux Transmission Cooler, […]

  6. eddie says:

    I’ve had a mysterious heating issue with my 87 van. suspect the fan clutch because the thermostat has been removed (by me, to see if the engine would run cooler, and it temporarily did)—– and thus far the van only overheats when the engine has been working very hard for a long time. pressure test came back fine. no leaks. OEM fan clutch replaced 500 miles ago. All is fine until, bam, overheating. Could overhauling the fan clutch solve this issue? Do people replace them because they don’t know how to overhaul, or for convenience? could it be some other component?

  7. Bryan says:

    Thank god for this write up!! Just rebuilt my 93 4runner hub based on it.
    Great Job

  8. Tienie Nel (South Africa) says:

    Thank you enjoyed your information as I am having an overheating problem – FJ80 1990 3F Carburator. Tried going the electric fan way with no joy.
    A question: How do you know the fan is engaging correctly at each of the specivied temperature settings?I am going to apply this infoe and will supply feedback.

  9. Hi Tienie, I read an article where the guy put the sensor in a pan over a gas stove and heated the water with a thermometer and actually checked the opening of the valves. That may be the way to go, although I think you did this long time ago!

  10. Mitch says:

    Hey a question I drive a 99 hilux I have a overheating issue have a new head,radiator,fan clutch,radiator cap new coolent, and still overheating! Happins at freeway speeds getting so hot causing cap to let out some pressure. Only then left is water pump and pressure sensor but its phiscaly overheating. Down fall I have a arb bullbar winch numberplate and spotlight friend has same set up on his 97 hilux and dont have the issue running out of hope please someone give me something phisical to check! Thank..

  11. tino says:

    hi just fitted a new clutch fan and it making a lot of fan noise in 1st and 2nd gear is that working normal it is not over heating

  12. shiv prakash says:

    dear sir,
    which fluid be used in viscous fan?
    kindly inform my mail address.


  13. steve says:

    i have a 2003 model v6 hilux which has the original l viscous fan. It doesn t get hot, but in the summer months , the fan is constantly on, making it noisy and loses a lot of power. what is the best thing to do here ?

  14. steve says:

    can i adjust the valve set point on the viscous fan to make the fan engage at a slightly higher ‘air’ temperature ?

  15. William says:

    I have a toyota townace 1996. I have had it for about 8 years. I have never been able to solve the viscous fan problem. I have had about 5 new fans, some of which I added oil to. They work intermittently well for 6 months then go scitziod. Sometimes it will work well, and I can hear it loudly cooling the engine, when the temp gets about halfway. Other times it is as if it decides not to work, and it doesn’t work at all. No fan sound, no cooling. Even at 3/4 hot… just will not work. Then later it will again. I can not see any reason for this. Either it should work or it should not. Not sometimes work perfectly and others not at all. All 5 fans have had this problem. Even brand new there are times when it just refuses to work. But it happens more and more often after 6 months. Just this morning I drove up a steep hill… the fan did not work and the engine got to 3/4 hot. I had to pull over and wait for it to cool. The this afternoon, the same hill, and hotter air temp, it worked perfectly, until the last bit when it decided not to work again. FOR THE LOVE OF GOD, DOES ANYONE HAVE ANY IDEA WHAT IS GOING ON? I think I am going to have to buy something without a viscous fan.

  16. tfns says:

    Firstly thanks for this advice.
    I have got 94 3.0 L diesel surf and it was overheating only when it was towing my aroung 900kg campervan. I was thinking problem was the radiator cause seems like its been reparied. I bought heavy duty aliminium radiator thick one it didnt solve the problem. After reading this article i took the viscous fan off and opened it up there was not much oil. i have cleaned it as much as i could with E10 and i put smoke stopper at local shops around $10 at kmart $5 🙂 it worked very well but i think put too much smoke stopper, its coupled all the time but it doesnt bother me. Seems like burning less diesel now i am not because of that. After this no more overheating. Thanks again.
    Regards all…

  17. Anonymous says:

    […] […]

  18. […] pinvalve controlled by a bi metal strip.http://web.archive.org/web/200903220…/MB_Sachs.html and http://neuralfibre.com/paul/4wd/tuni…ous-fan-clutch Last edited by grober; Today at 11:57 […]

  19. VEnkey says:

    Hi guys pls can any one explaine me how to calculate the Engagement point of the viscous fan clutch base on radiator inlet or thermostart (open & Close temperature)

  20. […] 1. On the loss of fan RPM with age, from the respected Aa1Car.Com library:http://www.aa1car.com/library/cooling_fan_clutch.htm 2. On Tuning and Understanding Your Toyota Viscous Fan Clutch, with another link within the article to overhauling it: http://neuralfibre.com/paul/4wd/tuni…ous-fan-clutch […]

  21. Anonymous says:

    […] 80's • View topic – How To Top up your FAN CLUTCH and do the Blue Fan Clutch MOD  Paul’s esoteric meanderings Blog Archive Tuning and Understanding your Toyota Viscous Fan Clut… __________________ Gruss/Erich  7er Reparaturhilfen  Unser Hilfsprojekt […]

  22. I ha e a 2001 3 litre disiel surf which is overheating. I have tagged the fan so its continuioly engaged, removed the thermo wo it`s free flow, changed the, changed the, run. Holt permanent. Well through it but but all with no success. Help

  23. Ray Willsher says:

    Yes I have been buying this for years Toyota have a part number for this fluid

  24. Patrick Farnell says:

    Hey…older Hilux 4X2 ’93 yn85, 2Y engine…original fan clutch. Clutch just now seems to have a lot of resistance…you can still turn the thing, but willingness to spin with engine stopped is very small..I can stop the fan with my hand protected by cloth or towel with engine running, I just get the impression that the thing is probably locked full on at highway speed, I have a lot of fan noise as my revs rise just b4 gear changes..I was going to dismantle it and see about replacing bearings,they seem stiff although having no play as such now reading boards like this, I am discouraged.

    any comments pfarnell@optusnet.com.au

  25. Wendy Richards says:

    Hi I have a Toyota hillux surf it over heats when toeing up hill it automatic have the heater on hot helps also put it in to 2nd which help and bring the temperature down it needs to be at 3500 revs which will help bring the temperature down why does it do this and how do I fix this.

  26. james williams says:

    I have 3.0lte hilux surf my problem is engine oil over heating.

  27. Thanks a lot Paul, excellent write up.

  28. Aw, this was an incredibly nice post. Spending some time and
    actual effort to generate a really good article… but
    what can I say… I put things off a lot and
    don’t seem to get nearly anything done.

  29. chardog says:

    I just bought an 87 4wd van with a failed head gasket for 200. replaced the gasket. put it all back together and drove away. after about 15 miles at highway speed it started to run hot . turned around to head home when it got really hot. waited and filled with water and went home slowly at lo speed. had radiator professionally cleaned. went another 30 miles same thing. re filled it and drove home at high speed with the needle just over half. odd. I noticed that the clutch fan is stamped ” made in usa”. the asin brand (oem) is 75 for new one. getting one but I will try your trick for a back up. thanks

  30. Goomba271 says:

    Here’s an oddball situation for you guys… I purchased a used 1990 Toyota truck ‘mini-RV’ and started driving it home across the US. I know, I know – what was I thinking? I hadn’t gone far when the temperature gauge registered hot. I looked under the hood and for some reason, the fan direction seemed wrong. I build and fly drones as a hobby so I know which direction a prop pitch blows. After a cooling off period I continued down the road and came across a Toyota dealer with a hotel right across the street. It seemed like a good place to pull over and get things checked out. One of the main mechanics came out and when I asked him pointedly about the fan direction, he stated flatly that it was correct and the air was blowing the right way. I put a piece of paper behind the grill, in front of the radiator, and sure enough, the air was blowing towards the front of the vehicle. He insisted that with his 30 years experience, he knew what he was talking about. I left it there overnight and got a room. The next day they had supposedly “diagnosed” the problem… a head gasket for sure but maybe worse. Before their explanation was over, they had me 1/2 convinced it was going to take a whole new engine to get home. I said “Go for it. Tear it down. I want to know what’s going on.” Then I had a change of heart and decided to trust my instincts. I told them to forget it. I was just going to buy an electric after market fan, install it myself, and make sure that at least the air was blowing the right way. As I was getting the keys handed over to me, the service manager came out and said, “Wait a minute. What’s going on here?” I explained it all to him and showed him the paper blowing to the front of the vehicle and he said, “That ain’t right.” Soon enough a young Mexican kid was looking up the part in their system and figured out the vehicle had the wrong fan and that the pitch of the blades was wrong for it. He ordered up the right fan, installed it and the temperature gauge stayed down. I kept the old fan and after looking at hundreds of photos online finally found it… it was for a Lexus V-8 and had likely been switched accidentally when the vehicle was in a shop for belt replacements not long before I got it. I suspect someone somewhere has a Lexus with a similar cooling problem!

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