Viscous Coupling Unit

This site contains affiliate links for which LandyZone may be compensated if you make a purchase.
Grrrrrr

Grrrrrr

Technician, Bodgit & Scarper Ltd
Full Member
Posts
19,297
Location
Buckinghamshire, UK. ('95 DT)
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?
 
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?
Click to expand...

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.
 
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.
Click to expand...
Think he's talking about the VCU not the viscous fan:rolleyes: Too much partying.:)
 
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?
Click to expand...
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.
 
Littelwill said:
What benifits would you get not having it attached?
Click to expand...

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.
 
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.
Click to expand...

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

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

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.
 
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?!
 
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
 
You must log in or register to reply here.

Similar threads

I
Replacing/repairing BCU (Body Control Unit) on 2005 RR HSE
Replies
10
Views
1K
invisible999
I
nevillen
An Honest RR Specialist
2
Replies
20
Views
2K
Datatek
Datatek
S
Freelander seized brakes and transmission fluid
Replies
10
Views
1K
Shabe
S
Skyisthelimit
Disco 1 1991 v8 fuel pump electrical wiring
2
Replies
31
Views
2K
Skyisthelimit
Skyisthelimit
payydg
Defibtech DDU-120 Lifeline Auto Fully Automatic External Defibrillator + Extras
Replies
4
Views
1K
payydg
payydg
Back
Top