Viscous fan hub; is yours working as it should?

A lot of 4WD’s run viscous fan hubs. Over time, they deteriorate and eventually stop working. This means your engine has very little air being sucked through via the main cooling fan and as a result it can overheat.

The viscous fan hub is one of the most commonly overlooked components on a 4WD, and whats worse is it plays one of the most critical roles. It’s a recipe for total disaster; when did you last check yours?

1HDT motor
How often do you pop the bonnet and do a really good inspection?

What does a Viscous fan do?

Some 4WD’s have electric fans, or fans that are positive drive. However, most run a viscous fan, which is a mechanical arrangement that drives your cooling fan faster or slower depending on the engine temperature.

Dmax air flow
Air flow through the radiator is critical for maintaining the right engine temperature

How does a viscous fan work?

The fan hub is a mechanical coupling that operates based on the ambient temperature. A bi-metallic strip on the outside of the hub heats and cools, and expands or contracts, which opens and closes a valve inside the viscous hub.

This allows the fluid to either flow out, or be contained inside the hub. As the fluid flows out, the fan hub grips more and engages the fan more onto the driven side of the pulley.

Under correct operation when the engine is cold, the fan will spin but not very hard. As it warms up, the fan will kick in to keep the engine cool. If it gets really warm, you should hear the fan roar loudly to bring the engine temperature back down.

It’s the perfect mechanical solution to a complicated problem; as your motor warms up the fan engages more, and as it cools down it engages less. 

Land Cruiser Viscous Fan Hub
The Viscous fan hub on our 80 Series Land Cruiser

Why is it so important?

Without a properly functioning viscous hub, your engine can easily overheat, which often causes extremely expensive repairs.

When you are driving at high speeds it is less important, but when you slow down (off road, traffic lights or low speed suburb work) you are at risk of your engine getting too hot without a working viscous fan hub.

How do you know if its working properly?

When your motor is hot, you should be able to hear the fan kick in. If you can’t, pop the bonnet, and turn the vehicle off while watching the fan.

If it comes to a fairly abrupt stop, the fan hub is working properly. If it keeps spinning round and round, your fan hub is knackered and needs to be repaired or replaced.

Another way is to have the engine off after a decent drive, and flick the fan around by hand. If it stops quickly, you are good to go. If it doesn’t, you’ve got a problem. Either way is a pretty good Viscous Fan Test.

Viscous Fan hub not working
Is it working properly?

Alternatively, you might be able to see it on an OBD2 scanner, like an Ultragauge, if you’ve been monitoring it for a while. We picked this up on our Isuzu Dmax, and noticed it was spending a lot of time higher in the temperature range, without the roar you’d like to hear. You can read more about that here – Isuzu Dmax Viscous Hub Replacement.

Don’t rely on the factory temperature gauge

You’ve probably noticed that a lot of factory temperature gauges rise to their normal point (around half way), and you never see them move from there.

Toyota’s in particular, have about a 20 degree band where the engine temperature can go up and down (as it naturally does depending on the circumstances) before you’ll see their gauge move. Once you see the temperature gauge moving, you’ve got issues, and its often too late notice.

The work around is to install an aftermarket temperature gauge, like an engine watchdog, or a coolant temperature gauge. Many modern vehicles already have this information displayed via the computer, and you just need the OBD connections to gain access to it.

Just know that even if your factory temperature gauge is reading within the right limits, it may actually not be!

Our 80 Series cooling issues

I first found out about the viscous hubs importance not long after heading down to Bremer Bay for Christmas one year with our 80 Series Land Cruiser. We travelled from 3AM in Perth and arrived at Bremer Bay in the morning, pulling straight onto the beach to deflate the tyres down.

When I got back in the vehicle (which was left idling), I noticed the temperature gauge had moved up from its normal halfway mark.

Whilst travelling at high speeds enough airflow was making its way through the engine bay, but pull up and leave it idle and the engine would get warm.

The issue actually turned out to be a minor blown head gasket, with coolant slowly being shifted into the overflow bottle at high RPM and not being sucked back in, but the engine was able to get warmer because the viscous fan clutch wasn’t working properly.

Bremer Bay engine getting warm
We had a few overheating issues at Bremer Bay

Why is it overlooked?

Not long ago I was inspecting an 80 Series Land Cruiser for the owner, and had a peak under the engine bay. We’d just stopped after driving on soft sand for about an hour, and I gave the fan a flick.

To my surprise, it kept spinning around and around for some time, before coming to a halt. I mentioned it to the owner, who told me it had just been serviced and checked by a reputable workshop in Perth.

This story is not uncommon; the viscous hub is one of the most overlooked components in a 4WD. Unfortunately for those who don’t know this, it can be the cause of an overheated engine, and they then have to pay to have it fixed.

I’m not exactly sure as to why this gets overlooked by so many people, and even qualified mechanics.

I’d hazard a guess that the work is done on a 4WD primarily when its cold, and you can’t tell very easily then whether it works or not. It literally takes 20 seconds to diagnose, but you need to do it when the engine is hot.

Viscous Fan hub new from Toyota
Brand new genuine Toyota Viscous Fan Hub

Repairing and adjusting a viscous hub

Some viscous hubs can be opened and serviced, as well as adjusted. On the 80 series Land Cruiser, some of the hubs allow you to adjust what temperature it kicks in and out, and you can vary the amount of viscous fan hub oil inside, which also does the same thing.

You can buy the silicon oil from most auto shops (Toyota sell it too). Toyota use 10000 grade CST in the 80 series. If you really want, you can get it from hobby shops, with different grade CST (or viscosity).

I removed my viscous hub and attempted to repair it by replacing the fluid, but it still wouldn’t engage properly, so I ditched it.

Replacing a viscous hub

Viscous hubs can be purchased from a variety of aftermarket manufacturers, for anywhere from $80 – $250. Seeing as how important of a role it plays, I opted to go for a genuine one. I’d read of too many aftermarket ones giving issues.

Toyota in Perth offered me trade price at a whopping $580. Instead, I went to, and purchased a genuine Toyota one for about $350 delivered to my door in around a week.

80 series cooling
Your cooling system isn’t something to mess with

Replacing the viscous hub with a solid block

Just the other day I was looking at a Brunswick 6.5 litre chevy engine (that had a failed water pump), and noticed the viscous hub had been removed and replaced with a solid machined block.

What this does is run the fan at maximum speed all the time. Apparently this is a common modification that Brunswick Diesels do, to combat the many overheating issues they have.

It is an option, but in my opinion a terrible one. For starters, a viscous hub is intended to regulate the engine temperature by changing the fan speed.

If you lock it in all the time, even when the engine is cold its still trying to cool it as much as possible. You want your engine to warm up quickly, and locking the hub reduces this.

From there, you can argue that it saps a fair bit of extra power from the motor (and probably fuel too), is louder and vibrates more than necessary. Perhaps the most important thing though, is that it reduces reliability, especially if the fan runs off the same shaft as your water pump does.

The GU patrol mentioned above had a failed water pump, because one of the bearings had let go and flogged it out. When we reinstalled everything, it was obvious upon watching the fan that it moves up and down, and in and out.

A car cooling fan is never going to be perfectly balanced, and by not having a viscous hub in place you pass all of the imperfection in vibrations onto a couple of little bearings.

It was lucky this failed just out of Perth, as it could have happened at Steep Point, or across at Dirk Hartog Island. Replacing a water pump in one of these is not a minor task!

My advice then, is make sure your viscous hub works properly, and don’t remove it!

Check your viscous hub

Take a moment to walk out to your 4WD, and see if the hub is working properly. It might save you some major repairs!

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  1. Hey brs,

    Yep, you should really hear it roaring when the engine is getting warm. A new hub should sort your issues out!

    All the best

  2. I guess the fan should not be near silent when doing heavy beach work or slow hill climbs. Luckily, I was able to monitor the engine temperature with the auxiliary thermometer I was able to use this to regulate the temperature. I did both tests mentioned above, the hub failed. I have since ordered a new Viscous hub for the FJ Cruiser.

  3. Hey Peter,

    Cheers for your comment, and sharing your experience. Most 4WD’s work really hard towing something that big and heavy, and after 220k, its likely the viscous hub wasn’t working properly. If its automatic you’ll have even more of an issue, as the transmission temperatures go high and that often makes the cooling system work even harder.

    I’d suggest you get an OBD2 reader and keep an eye on the temperatures. If it de-rates then its obviously too hot, but you should be able to get it quite warm without any issues.

    In regards to thermal fans, they simply don’t move enough air, and they’re less reliable, and don’t auto adjust speed mechanically like the viscous hub does. A working viscous hub (which is in most 4WD’s) is far better than a thermal fan, but for some reason many people (and mechanics) forget to check them.

    All the best

  4. Peter Hannah says:

    Just came across your report and advice. We have a 2012 BT50 with 223,000 on the clock. We live in our 24ft Kingdom caravan and the car has been great. Had a couple of major issues, but it works hard so these are forgiven. I first came across the overheating – lack of power issue, towing the van up a steep incline in Tasmania. We struggled to reach the top of the hill, and had to stop and call traffic past on a very narrow country road.

    Recently, travelling back to Devonport across the north coast, we reached a long incline and the car did it again. After stopping and letting it cool down, we finally got to the top of the highway hill. I read other Ranger issues and found this problem. I ordered a good quality viscous drive with 2 year warranty for $135, and had a local mechanic install it. He was great and was able to do it without removing the radiator etc. Hoping to get back to Qld without further cooling or overheating issues. Thanks for explaining this little piece of stupidity. Why wouldn’t manufacturer’s install thermal fans in the first place?

  5. Hi Angelo,

    You’ll get good gear, and bad gear from eBay. I would go with a reputable brand. If you are having an overheating issue and have verified that the viscous hub does not spin freely once the motor is hot, and you turn it off then its likely something else.

    All the best

  6. just to ask ,if there are many differnces in clutch fans for toyota hilux rn85 model clutch fans as there is a plethora on ebay and prices do vary alot , if the cheaper one does the job and offers 3 years warranty should i take it or is it too good to be true ? just scepitcal and would like to resolve overheating issue in ute, p.s i did try to stop the warm engine fan from spinning and it does stop at instant command. anybody out there would be awesome to chat… thank You Angelo..

  7. Hey Johann,

    Cheers for the explanation. I only briefly had mine apart, and didn’t spend much time looking at it. From memory though there was no adjustment at all. Looking on Youtube now and there’s some pretty good explanations of the bimetalic strip opening and closing the internal valve and adjusting the fluid. I’ve made some minor changes to the article to reflect this.

    I guess the real take away is that the fan should kick in really well when the motor is running warm, and not so much when it isn’t, but I appreciate the extra detail.

    All the best

  8. Hi There Aaron. I agree on most of the stuff you mentioned. But you were nearly there when you answered the adjust part.
    The fan does not kick in when the oil are getting hot, might be some little truth in that.
    The heating coil infront heats up from hot radiator air hitting. That start the coil to operate.
    The hotter the air, the more it moves and opens up two port holes inside the coupling, where a disc that runs permanently runs the same speed as the waterpump locked up for as long these ports are opened by the coil in the front.
    To let this process to happen slower or faster you can adjust the plate to let this locking up quicker or slower.
    This just my view as I was playing around with one to understand the working of it.
    Not all of them work like the Asian coupling but on the same principle. They all work with some kind of coil heating up.
    But if not sure if I try to talk nonsense.
    Google is your friend ?

  9. Hi Samantha,

    Something sounds suss there. When you say ‘staying on’, what do you actually mean? They will never stop all together, they just slow down and speed up depending on temperature. Their slowest speed is still fairly quick.

    The hubs cut in at a certain temperature, and out at another. Some are adjustable, but many are not.

    All the best

  10. Samantha white says:

    Hi there,
    Has anyone ever had the issue of the vicous fan NOT cutting out? My mechanic has tried 3 different ones and all of them are doing the same thing not cutting out constantly going. I drive a 2011 Holden Colorado the original viscous fan stopped working and we have tried 3 after market ones and all doing the same. I’m happy to spend the money and get a genuine one but I’m worried it will stay on to so wondering if anyone has any advice as to why its doing it.
    Cheers Sam

  11. Hey Craig,

    Simple things hey! Not sure why it gets forgotten so much, but its easy to fix at least

    All the best

  12. Very good article had the same problem when driving from Coral Bay to Perth. Tried everything turned out to be the viscous hub. I had a hot car the interior floor pan ect for some time before It went really bad.

  13. Hey Tim,

    Sounds like a nice build! Don’t go down the path of an electric fan. You’ll have a hard time getting it to fit and it will struggle to move nearly enough air.

    I’ve seen plenty of people try this and very few succeed. The factory fan and hub works perfectly, when in good condition.

    Test it, try the top up with oil or replace it with a quality unit. Much easier and more reliable and you know it’s fit for purpose.

    All the best

  14. Hi Aaron,
    I have a Coaster with a fresh 1HDT in it. Water temp is perfect when driving, oil temp is 90° to 100°. When I pull up or hit traffic, water temp quickly goes to high 90’s or more. I am yet to test my viscous hub but was initially considering an electric pusher fan. What are your thoughts on the Electric versus Viscous debate ?

  15. Hey James,

    Good to hear you’ve come across a solution. I’ve not seen engine oil coolers used much in the 4WD scene. Well done

    All the best

  16. Hey les
    I have a 6.5l brunswick in my 75 series ute been in for 15 years now i lived up north so overheating was a big issue i tried everything with the coolant system. In the end i finally found that the oil pan in the 6.5l was not big enough so with less oil as you should know oil is a coolant as well the oil was pretty much overheating. They did install a small oil cooler when the motor was installed but in a bad spot and not big enough. So i installed another oil cooler in the back of the bull bar and run the pipes in series with the other oil cooler. So now with 2 oil coolers and more oil capacity (about 1 litre more) the motor has never even come close to overheating.
    I think the oil cooler was $100 bucks online and about 1m of good hose it was the easiest fix and cheapest too.

  17. Hey Les,

    Good to hear you sorted your overheating issues. It can be a right pain in the backside chasing issues like that.

    Just watch the bearing on the water pump – having a solid coupling like that puts a lot of extra strain on it


  18. I have a Brunswick 6.5 L it is always overheating I have tried Hayden with a German span it still gets very hot change the water pump change the radiator attached checked to see if you got a leaky head gasket once I put a solid fan on it it never gets hot any more it Sitra and 85 I can four-wheel-drive or down the sand and it doesn’t budge and I have a video gauge I swap that for another one just in case it was faulty I done thermostat it’ll came down to the business hub