A new Viscous Hub for our Dmax

There’s a lot of very helpful information online, if you can filter through it effectively. One of the snippets I picked up a few months back was that our era Isuzu Dmax often started to have performance issues with the viscous hub at around 120,000km, and I put that in the memory bank for later on, when we were to get there.

Our Dmax in a river
Picking up common issues from other owners is hugely helpful

What is a viscous hub?

We actually wrote a post on this a while back, because its one of the most overlooked, but critical parts of a 4WD that even good mechanics often seem to miss. It’s a mechanical coupling that turns the big radiator cooling fan on your vehicle, pulling air through as needed to maintain suitable and correct coolant temperatures. As your vehicle gets hotter, it engages more, and the fan spins faster, drawing more air to cool your radiator down.

Obviously, if it does not do what its mean to, your engine is going to run hotter than it should, and you could have some nasty issues down the line. You can read more on our Viscous Hub post.

Viscous Fan hub new from Toyota
The viscous hub on our 80 Series Land Cruiser

How did I know ours was on the way out?

Our Dmax has had an Ultragauge fitted since virtually day dot, and I’ve always paid close attention to the coolant and transmission temperatures. I picked up early on that the coolant temperature would climb to 97 degrees when the vehicle was being worked hard, and almost instantaneously the viscous fan hub would really kick in, and you could hear it roar. This would take place at any speed, any RPM and you’d easily hear it in the cab.

On our 3 month trip through the Northern Territory, on warm days towing our camper it was common for the temperature to hit 97 degrees, the fan to come on and it drop down to around 90, and then it would slowly climb again to 97, and repeat. Occasionally it did get warmer than 97 (which is perfectly fine), but you’d always hear it road.

On our Lap of Australia, we got to see how flat WA really is, and the hills through Adelaide, Victoria and even Queensland put our Dmax to work at a much harder rate than ever before, mainly when towing our 2.3 tonne Hybrid Camper.

Climbing a big hill
Some of the hills on the East coast are brutal

If it was cold outside (like the Victorian High Country often is) then the temperatures were still decent, but I began to notice on several occasions that the viscous fan hub was not roaring at 97 degrees anymore.

The more I thought about it, the more I noticed that it was only in really warm ambient temperatures, when the was substantial heat soak. We could hit 97 degrees on a cold morning towing the camper up a hill, and you could still hear the fan come on and roar, but if it was 35 degrees outside and you’d been towing for an hour, it seemed to lose its effectiveness substantially. 

The vehicle also seemed to be spending a fair bit of time in the mid to high 90’s, which I didn’t think it used to do.

Ordering another hub

There’s a lot of places you can get viscous hubs from, and I’d also read of people running aftermarket ones that didn’t improve the situation, and they ended up going genuine. Whilst I’m not completely sold on always using genuine, there are some cases where it’s a good idea, and I was prepared to pay for a new hub.

I grabbed one at Gippsland Isuzu Ute, for about $450, which I prepaid for, and it took a few days to come in (which was perfect as we hadn’t arrived there yet).

Replacing the viscous fan hub

I had a quick look at replacing the hub myself, and it became fairly obvious that I’d probably have to remove the shroud for the radiator and muck around, and I’m not a fan of working on vehicles, so I got Moss Vale Isuzu to replace it for me. They charged an hours labour to do the job, and that’s probably fair.

Dmax radiator support replacement
The viscous hub is under the fan shroud

What difference does it make?

I immediately noticed a huge difference in temperatures. Where you’d frequently see 100 degrees on the coolant temperature, it this became the new absolute maximum, with it very rarely going over 98 degrees, even when being worked really hard. Low 90’s, and even high 80’s became the new normal even when towing, on flat ground.

The big difference between coolant and automatic transmission temperatures appeared again, with the cooling system often sitting on 86 degrees when not working too hard, and that was rare before.

You can hear the fan roar again, and it comes on when the vehicle is sitting at 92 degrees or so, proportionally. I also wonder if this has helped fuel economy a little, in a weird kind of way, but who knows.

Isuzu Dmax at Perisher
Our Isuzu Dmax runs much cooler now

Check, and change your viscous hub

If you’re having overheating issues with a Dmax or MUX, or its just running warmer than it used to, get the viscous hub replaced. We’d already done the coolant hoses, thermostat and coolant, so it was a no brainer to finish the rest of the cooling system maintenance and know that we’re gold for another 120,000km.

In all honesty, 120k out of a viscous hub is a pretty average lifespan; you’d expect at least 200,000 in my opinion, and potentially longer, but when they stop working, you’re not in a good way.

You can top them up with fluid, but I know from previous experience you’re usually wasting your time.

Have you had to replace the viscous hub on your Dmax or MUX? How many kilometres had it done? Did you go genuine or aftermarket? Let us know below!

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