Transfer Case VCU

[42rangie]

Just a newbie type question. What actually goes wrong with the unit when it
"seizes up?" Does the fluid wear out? Is it a mechanical problem?
I have only used " manually engaging" transfer cases on previous 4X4's and on
HGL(?) fire fighting apparatus.

Thanks All,
Les

[PhilipA]

Just a newbie type question. What actually goes wrong with the unit when it
"seizes up?" Does the fluid wear out? Is it a mechanical problem?

AFAIK there are 2 types of failure.
1 Where the seals fail and the silicon fluid leaks out and causes the VC to have no V. This failure causes the car to be driveable but not have the lock up centre diff .

2 The VCU seizes. I do not know precisely why this happens , but again it may be the fluid leaking out or may be the fluid hardening from overheating, say if the tyres front and rear were a different size. In this case the car should not be driven as it is similar to "windup" where someone drives a car in 4WD on a hard surface and diffs, Cvs, etc etc can be broken.
Regards Philip A

[Hoges]

AFAIK there are 2 types of failure.
1 Where the seals fail and the silicon fluid leaks out and causes the VC to have no V. This failure causes the car to be driveable but not have the lock up centre diff .

2 The VCU seizes. I do not know precisely why this happens , but again it may be the fluid leaking out or may be the fluid hardening from overheating, say if the tyres front and rear were a different size. In this case the car should not be driven as it is similar to "windup" where someone drives a car in 4WD on a hard surface and diffs, Cvs, etc etc can be broken.
Regards Philip A

The VCU is filled with a silicone-based oil which changes its viscosity according to temperature.. when the front wheels are turning at a different rate from the rear ones (slippery conditions for example) the oil heats quickly, becomes a lot more viscous and essentially locks the coulpling. When normal rotation is resumed, it cools quickly and allows a degree of slip which is necessary for normal operation.

For some reason, the oil can can lose its desired properties and retain its more viscous state...to the extent that it becomes so viscous that the unit essentially becomes locked solid (= CDL 'engaged') with consequent strain on the driveline as Philip explains above.

To change the VCU is about a 4 hr DIY one-man job. Shop around for prices--they vary enormously. I ended up importing one from the UK ---delivered in just a few days.. for less than 50% cost of what local LR parts were dreaming of

EDIT:PS they are a sealed unit and require specialist tooling to refurb...assuming you can get hold of the oil. IIRC the volume of oil is less than 100ml so it's working hard!

[superquag]

Philip is correct.

If it fails, you desperately want the first type, where the VC will NOT lock up. - Essentially it is not 'in circuit', so the centre diff does its 'diffing' with no interference or strain imposed on it by a normally operating VC. This includes sealed road driving.
You only discover the failure when you go 4WD'ing, and one of your front wheels is off the ground.... and spins..and spins...and the car goes nowhere.
Your rear wheels are (?) looked after by the Traction Control which will-should- apply brakes to the spinning wheel and thus ensure the other 'grounded' wheel gets drive.
Sadly, this type of failure is rare.

The usual failure is the Constantly locked up mode. Not good.

Simple explanation:
There are two sets of perforated, interleaved plates inside theVC unit. Each attached to one 'drive' input (Rear output of the centre-diff) and the other to the other 'drive' input. (Front output of the centre-diff)

The tiny space between the plates is filled with a thick-ish Magic Fluid which expands and grips the plates when it gets hot. Imagine a thick oil that turns into glue when hot...but reverts to oil when it cools. Turning the Front/Rear at differant rates causes the fluid to shear inside...which makes it Hot & Bothered... and it physically expands and GRIPS the plates, locking them together. Even when Cold & Calm, it still tries to lock the plates together, but not toooo seriously...

Under normal use, with proper tyre pressures, same rolling diameter on ALL wheels etc, the front and rear axles turn at the SAME speed, so there is no 'diffing' to be done by the CD - . VC stays Cool & Calm.

When you lift a wheel off the ground the F to R balance goes out the window, and the centre-diff 'diffs' like mad, as it should, but in doing so the VC gets very Hot & Bothered and locks up, stops the centre diff from diffing wildly...locks it up and you keep moving with drive to the 'loaded' axle.

As you do so, there is no shearing force inside the VC, so the fluid cools off a bit, lets go a bit, and basically reaches a compromise, say 'Warm & Concerned' then becomes Cool & Calm when both axles want to turn at the same rate...

Every time it operates, it shortens its life... the hotter each time etc etc, eventually causes the Magic to leave the Fluid... and it generally stays expanded and locked. - You can hear the front wheels chirping as you make tight turns...and your turning circle in wet weather is more like a ship...

All a VC does in real terms is... saves Sir from the onerous task of reaching down to the Centre-Diff lever and moving it across to the "Locked' position.

Three cures.

Replace the VC.

Replace the BW transfer case with a LT230

Take off the front prop shaft and enjoy a 2WD Range Rover.
This will quickly kill a dying VC, or even a healthy one, so make sure its properly deceased!

I hate VC's

James.

'95 Classic Vogue SE with EAS and 2WD.

[superquag]

EDIT:PS they are a sealed unit and require specialist tooling to refurb...assuming you can get hold of the oil. IIRC the volume of oil is less than 100ml so it's working hard!

Nice explanation.

Ashcroft assure me that opening up the VC is so much of a hazard that they're not interested anymore. - They've had the halves fly across their workshop and smash the wall...
Pressure inside a failed unit can be horrendous, they said. Getting the oil is a big problem normally, but have heard its not in production now. (???)

We need a safe way of extracting the dead silicon and turn it into an 'open' VC. - Then put a locker on the front diff. - Is that possible or practical ?

What makes you think I HATE VC's ?

[PhilipA]

Here is the official version.

Attachment 47626
Regards Philip A

[glenhendry]

Oh oh. I hear my front wheels chirping when tight turns in a smooth concrete carpark! Is this a definite symptom of a trashed VC? This has been a good and interesting thread.

Hoges, how did you determine that your VC was cactus? I really need it to work, as I wheel my RR pretty hard.

I love the idea of an automatically engaging centre diff, but ageing is inevitable, I dont think we can be too hard on the P38 VC can we? I am happy to replace it, but can someone explain why the LT230 is better? What options do we have? Can I slot a Torque sensing TC in from a L322?

[wayneg]

Oh oh. I hear my front wheels chirping when tight turns in a smooth concrete carpark! Is this a definite symptom of a trashed VC?

I would be testing your VC ASAP,,, see..[ame=http://www.youtube.com/watch?v=XddClDAIfXw]Range Rover P38 (P38a) Viscous Coupling (VCU) Test - YouTube[/ame]

You will soon have exploding CVJ`s at best if its seized. Have had this on my last Classic and had to replace both CVJ`s One shattered and the other was on its last legs.

[wayneg]

I am happy to replace it, but can someone explain why the LT230 is better?

The Discovery Lt230 transfer box is quite a common replacement for the RR Classic transfer box. It does away with the Viscous Coupling and instead uses a manually selectable locking central diff. I have not heard of this type being fitted to the P38a. If it were to fit you would need a lever in the cab plus a way to do away with the electrics and inevitable failure warnings.

Replacing the Viscous coupling is quite easy, even I can change one.

http://www.island-4x4.co.uk/transfer...044-p-151.html

[Hoges]

Hi Glen, you might pick it up with chirping wheels on a smooth /polished concrete floor, but a more effective test is to do a full lock turn slowly clockwise on an ashphalt surface..put the driver's window down listen for the 'chatter' /scrubbing/hopping of the driver's side front wheel ... you'll also feel it a bit in the steering.

see also Viscous coupling...vicious price

cheers