Viscous Coupling Unit

Grrrrrr · Apr 10, 2015

There have been a couple of threads recently on VCUs.

They look pretty easy to get on and off (never tried) but apparently they're a sealed unit. Is that really true? I've had few cars now with automatic gearboxes that were "sealed for life" but in reality needed the transmission fluid changing and could be broken open after all.

Are these units REALLY sealed or could you get one apart and replace the fluid in it?

wammers · Apr 10, 2015

Grrrrrr said:
There have been a couple of threads recently on VCUs.

They look pretty easy to get on and off (never tried) but apparently they're a sealed unit. Is that really true? I've had few cars now with automatic gearboxes that were "sealed for life" but in reality needed the transmission fluid changing and could be broken open after all.

Are these units REALLY sealed or could you get one apart and replace the fluid in it?

Viscous fan hubs have a valve in them which opens with heat. This allows the fluid stored in the centre of the hub to flow outwards driven by centrifugal force and flow between two very close fitting slotted plates. The fluid causes the plate fixed to the centre shaft to drag the plate connected to the fan hub around with it driving the fan. It is not normally loss of fluid that causes fail. If the fan fails slack it is usually a failed valve or valve actuator. If it fails locked it is usually a seized bearing between the shaft and outer hub. Fit a new one you know it makes sense.

Datatek · Apr 10, 2015

wammers said:
Viscous fan hubs have a valve in them which opens with heat. This allows the fluid stored in the centre of the hub to flow outwards driven by centrifugal force and flow between two very close fitting slotted plates. The fluid causes the plate fixed to the centre shaft to drag the plate connected to the fan hub around with it driving the fan. It is not normally loss of fluid that causes fail. If the fan fails slack it is usually a failed valve or valve actuator. If it fails locked it is usually a seized bearing between the shaft and outer hub. Fit a new one you know it makes sense.

Think he's talking about the VCU not the viscous fan

Too much partying.

Datatek · Apr 10, 2015

Grrrrrr said:
There have been a couple of threads recently on VCUs.

They look pretty easy to get on and off (never tried) but apparently they're a sealed unit. Is that really true? I've had few cars now with automatic gearboxes that were "sealed for life" but in reality needed the transmission fluid changing and could be broken open after all.

Are these units REALLY sealed or could you get one apart and replace the fluid in it?

The VCU is sealed, look in the Gaylander section for info about re-filling VCU's. When mine goes I plan to experiment as it's not really needed if you don't off road especially with 4 wheel TC.

wammers · Apr 10, 2015

Datatek said:
Think he's talking about the VCU not the viscous fan Too much partying.

OK, Hic!!

Littelwill · Apr 10, 2015

Datatek said:
The VCU is sealed, look in the Gaylander section for info about re-filling VCU's. When mine goes I plan to experiment as it's not really needed if you don't off road especially with 4 wheel TC.

What benifits would you get not having it attached?

Grrrrrr · Apr 10, 2015

Littelwill said:
What benifits would you get not having it attached?

It cannot seize!

My understanding is that it allows the front and rear wheels to rotate at different speeds. If your rear wheels are slipping (for instance) then all the power goes to the back wheels and I think the liquid inside is heated / stressed and goes solid so forces 50% to front and rear.

If you don't have it at all then you're 2 wheel drive unless you put some kind of differential in-between as far as I can see.

Grrrrrr · Apr 10, 2015

Datatek said:
The VCU is sealed, look in the Gaylander section for info about re-filling VCU's. When mine goes I plan to experiment as it's not really needed if you don't off road especially with 4 wheel TC.

Well, that's 20 minutes of my life I won't see again in a hurry. Talk about handbags at dawn.

So, specialists can refill it with the silicone stuff but it isn't for the home mechanic. Even some of the "specialists" don't do it that well so it is a bit hit and miss. All in all, safer just to buy a new one.

Also, the exact torque required to turn the wheel is very much debatable but as long as it turns it probably isn't seized ... yet!

chrisrr · Apr 10, 2015

Also, the exact torque required to turn the wheel is very much debatable but as long as it turns it probably isn't seized ... yet![/quote]

I couldn't make sense of that neither
It's siezed when turning on full lock the front wheels 'chatter' or on tarmac squeal.
Replacing is pretty straight forward - on the soft dash classic . I'm guessing it will be same.

Datatek · Apr 11, 2015

Grrrrrr said:
It cannot seize!

My understanding is that it allows the front and rear wheels to rotate at different speeds. If your rear wheels are slipping (for instance) then all the power goes to the back wheels and I think the liquid inside is heated / stressed and goes solid so forces 50% to front and rear.

If you don't have it at all then you're 2 wheel drive unless you put some kind of differential in-between as far as I can see.

My own view is that it loads the diffs unnecessarily even when working correctly, contributing to early failure.

Bromsgrove Defender · Apr 11, 2015

Datatek said:
My own view is that it loads the diffs unnecessarily even when working correctly, contributing to early failure.

Hmm not to mention unsympathetic driving style

Datatek · Apr 11, 2015

Bromsgrove Defender said:
Hmm not to mention unsympathetic driving style

Bromsgrove Defender · Apr 11, 2015

Datatek said:

If the VCU operating normally would stress the Diffs. Then surely an unsympathetic driving style would exacerbate the problem and or ensure the early demise of the VCU.

wammers · Apr 11, 2015

Bromsgrove Defender said:
If the VCU operating normally would stress the Diffs. Then surely an unsympathetic driving style would exacerbate the problem and or ensure the early demise of the VCU.

If your unsympathetic driving style was driving around in a tight circle at speed for hours on end yes maybe it would fail sooner.

Bromsgrove Defender · Apr 11, 2015

wammers said:
If your unsympathetic driving style was driving around in a tight circle at speed for hours on end yes maybe it would fail sooner.

Agreed it's the build of heat that destroys the fluid but is it the heat itself or the heating/ cooling cycle that does the damage?

wammers · Apr 11, 2015

Bromsgrove Defender said:
Agreed it's the build of heat that destroys the fluid but is it the heat itself or the heating/ cooling cycle that does the damage?

All the above possibly. Anything of a mechanical nature will fail at some point, specially with the close tolerances and stress involved in the VCU. If you have large rotational differences front to rear it will be loaded more. If all your tyres are of similar wear and therefore rolling radius there should be little rotational difference front to rear and the VCU should cope easily, only on lock would the VCU be loaded greatly. The VCU unit should be treated as a consumable item and not one that is fitted to the car when new, then removed when the car is scrapped and fitted to another. It don't work that way. Like any other stressed item it has a life span.

Grrrrrr · Apr 11, 2015

May not need unsympathetic driving. If you drive on slippery surfaces a lot then the wheels will spin (and with TC the brakes will brake!) and you'll be wearing the VCU (and brakes) out faster!

I'm pretty sure mine is fine. I've no idea if it has been replaced in the past but seeing as the car has done over 120k miles I'm guessing it must have been. I do like to play in the mud a bit though so I'm obviously not a totally sympathetic driver. Maybe I lack empathy with the old bus?!

GARY MCGREGOR · Dec 28, 2017

anyone recommend best place to purchase genuine recon or new vcu

johnnyb70 · Dec 28, 2017

I'd be surprised if the vcu has been changed in mine at 150000! I think many are on the edge of seizing. But I think they are reasonably robust. If your wheels are roughly similar and you drive in a normal way then they aren't failing all the time like the early freelanders. I think they had a unsympathetic gearing so the vcu was always on the edge of working. I don't know anywhere that advertises they recon them anymore. I bet new are tricky to find as well

jamesmartin · Dec 28, 2017

GARY MCGREGOR said:
anyone recommend best place to purchase genuine recon or new vcu

google rtc6044

Viscous Coupling Unit

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