Thread: Borg Warner viscous, Care & Feeding

... 'Cos the lifter bit looks like a pair of spectacles !

Don't we just hope that someone tows an AWD with VC, owned by an irascible lawyer with mechanical savvy !!

Ian, from what you've just said about transmitting "X" amount of torque, THEN slipping (and getting hot and Not Happy...) this would also mean - here comes my twisted logic - that running lower tyre pressures on the front is essential as it keeps the rolling radius smaller than the rears, lessening the load on the front half of the centre diff (?) - so that the VC does'nt need to slip.

Say the front wheels are bogged solid, rears resting on smoooth concrete, then applying oodles of power will mean the VC slips and gets hot and bothered, and the majority of the drive goes to spin the rears....and if you do it often enough and long enough to cook the silicon.... it changes state to 'solid', the VC locks up, and your RR then becomes a "Permanently engaged Part-time 4WD" without the driver realizing it... Till maybe he comments on this forum and folk like Mike RR diagnose it for him !!!

So what is the transfer case 'diff doing during this happy time ? Can't it operate as a normal diff ? that is, supply differing amounts of torque to front/rear shafts as the conditions change...

Still confused, even with the RAVE manual....

If one of the prop shafts was removed from my Classic, would it still drive ? (with a seized VC still in place)

Confused James in Gosnells

The EH, 95 Vogue SE, currently an expensive Garage Decoration... (Already have a "Lawn Sculpture" a.k.a. hovercraft hull.....)

Most times the VC is operating as an "open" limited slip diff. Each shaft has a series of plates in a fluid. The plates have holes in them. Oil is forced between the plates through the holes. Minor variations in rotation between front and rear...such as turning etc have no effect.

However, when the front wheels are moving at significantly different revs from rear wheels the different speeds of the internal plates causes friction within the fluid which causes the fluid to heat up. The resulting expansion in a closed space results in an increase in viscosity and the whole lot starts to spin as a single mass (like an AT) thereby acting as a gradually locked diff. When back to normal driving, the fluid is not being forced through the plates so it cools and the plates separate and it reverts to being an open diff.

The problem in a VC unit is that depending on the quality of the silicone oil, it can degenerate and become highly viscous, essentially forming a solid mass which causes the diff to lock... Other times the oil can deteriorate and it ceases to change viscosity with temp. so the plates will not lock up and so you don't get the LSD effect

The increased sophistication of traction control on all 4 wheels in later P38s should have reduced the likelihood of premature failure...not that it seems to have happened in my case...!

Originally Posted by Ian Ashcroft

Hi Mike yes the couplings are physically identical and can for be interchanged, there may be a slightly different slip limit but for all practical purposes they are the same.
As a matter of interest these couplings transmit a fixed torque up to their design limit (then they slip and overheat), this is why you cannot spectacle tow a RR or a freelander ( freelander visc coupling is same principle but more plates as it is a direct drive not a visc controlled diff). Many people think th couplings slip up to a limit then start to lock up, not so, for exampile you can drive a RR without the front prop but if you drive it agressively the coupling will slip, overheat, and go solid.

Regards Ian Ashcroft

Thanks, this explains how a customer with a P38 managed to drive for a couple of days without his front driveshaft. The only telling point was the rude squawking noises the viscous made when he tried to drive up my steep drive. Not like a viscous fan drive at all.

Originally Posted by Ian Ashcroft

Hi Mike yes the couplings are physically identical and can for be interchanged, there may be a slightly different slip limit but for all practical purposes they are the same.

Cheers for that ... Gives me confidence towards reco'ing a B/W replacement box. I've got a LT230 but don't really want to use it, as I enjoy the RRc and the way it operates just as it is.

Originally Posted by Ian Ashcroft

As a matter of interest these couplings transmit a fixed torque up to their design limit (then they slip and overheat) ..... if you drive it agressively the coupling will slip, overheat, and go solid.

Noted ....


Cheers
Mike

I was musing over the function of the various components of these boxes last night and came up with an idea, not sure whether anyone has tried this and I want to be clear that I put it forward for interest's sake only, not as something I would advocate, in fact I hesitate to post this at all because someone is going to misinterpret this as some kind of tutorial, BUT if you can just take it as a thought on the way these boxes function, and the relationship between the VC and the centre differential....
Say you had a box with a seized viscous coupling. If you were to leave the seized VC in situ and take out the rear output shaft housing, this shaft is splined onto the rear prop shaft hub, splined into the centre of the centre differential unit (this is the spline that stripped on my car last week), AND splined onto the centre of the viscous coupling.
IF you were so inclined, my theory goes, you could put this shaft in the vice, take out your trusty hacksaw and CUT OFF the bit that goes into the viscous coupling. If you were to put the (now abbreviated) shaft back in the transfer case, you would now have your rear propshaft driven directly from the inner hub of the centre diff, and your front propshaft driven from the outer hub of the centre diff via the seized VC. This would result in constant 4WD with a permanently open centre diff.

Again, I want to stress that this thought just occurred to me as a matter of interest regarding the way these transfer cases work. I'm not sure what the implications would be on bearings etc and would DEFINITELY not advocate trying this at home...

Hi IMO I think the center diff will now behave as a normal diff would with a broken half shaft, as there is nothing to restrain the other sun gear,
.no drive transmitted, but you are correct the front would have drive. A seized coupling will not stop you but a stripped center spline will.

Regards Ian Ashcroft

... Now I'm totally confused.!

Maybe I'll have to see one in bits in order to understand exactly how everything moves.

I could understand it if the Vicious Coupling was effectively in 'parallel' to the centre diff outputs, that is, it slips a little whilst transmitting most of the power fed to it.... - to a point. But locks up when the slippage is tooo much and it gets hot and bothered, so it progressively - smoothly - locks solid and effectively by-passes the centre diff which was quite happy slipping most of the power to one prop shaft...

So, in my case of a seized VC, what happens - will the Beast still drive without the REAR propshaft ? Or should I take off the FRONT shaft instead?

Until I either get a s/hand VC or make Ian a happy chappy, it would be nice to take it driving on "Light Duties" - and to let the sun shine on that patch of grass!

The LT 230 conversion is still tempting, if only because it only fails under brutal treatment...at least its either open or locked centre-diff. But the co$t of getting a box, time etc to fit it..... sooo much easier to undo a handful of bolts and put in a new VC.
Besides, its now a matter of Principle to not let the VC beat me !

I believe there is a thingy that can be substituted inside to turn it into a 'Part-time 4WD', meaning you can drive the highways in fuel-efficient 2WD mode and the bush in open or locked centre 4WD.

Anyway, I've taken the Paj out of storage, so I've got reliable wheels for the time being...

Befuddled James in Gosnells.

".....no drive transmitted, but you are correct the front would have drive. A seized coupling will not stop you but a stripped center spline will." - Ian Ashcroft.


- So, if I take off the REAR shaft, I can tootle around town... Would this put mortal strain on other components,,,, such as the centre spline ???

- Murphy's Law says "Yes, the most expensive bit will now break"

James