Borg Warner viscous, Care & Feeding

- 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 ???

James[/QUOTE]

Can't see how you could put more strain on the centre splines, as, to my understanding, they take all the drive already- as evidenced by the fact that the vehicle doesn't go anywhere when they are stripped.

Surely you could use the car for normal driving whilst deciding what to do about your seized VC, by removing the front propshaft and driving in rear wheel drive. Avoid heavy towing, drag racing and burnouts and you should be fine. I drove for a couple of weeks in FWD after destroying a rear diff in my 88RR, RWD is preferable IMO.

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 ???

James[/quote]

Can't see how you could put more strain on the centre splines, as, to my understanding, they take all the drive already- as evidenced by the fact that the vehicle doesn't go anywhere when they are stripped.

Surely you could use the car for normal driving whilst deciding what to do about your seized VC, by removing the front propshaft and driving in rear wheel drive. Avoid heavy towing, drag racing and burnouts and you should be fine. I drove for a couple of weeks in FWD after destroying a rear diff in my 88RR, RWD is preferable IMO.[/QUOTE]

Darn ! - Can't do all the fun things!

James the white haired hairy-faced Hoon in Gosnells

G;day POD,

Took your advice and pulled the front shaft...

- Did a guestimate on the toe-out pulled it towards "IN" by 2 turns on the adjusting rod.

- It feels virtually the same, cornering is now a bit quieter and easier, but as a bitumen-driver there really is no need of AWD over 2WD, IMHO. This gives me time to collect bottle-tops to pay for the VC or an LT230 with the 2WD modification.

Fuel consumption will be interesting....

Just reading this interesting run of posts on the BW viscous coupling centre diff, so a bit late into the game. I'm confused, you see I thought the BW viscous coupling was a unit REPLACING the whole centre diff as fitted to earlier Rangies, Landrovers and Discos, and chain driven from the gearbox with output to both front and rear driveshafts, not gear driven as is usual, but am I right in thinking here some of you guys are talking a BW unit only in place of the normal manual centre difflock function so in fact an actual centre diff with sun gears planetary gears etc all automatically locked together by the action of the VC and its silicon fluid, yes or no?. The industrial units I have worked on during my years as a plant fitter were all sealed units with that "special" silicon fluid inside that thickened up and locked up when the two parts ran at different speeds or one element slipped.
Also the comments from contributors about giving more regular oil changes to the transfer case being a factor in prolonging the life of the VC, is not the coupling a sealed unit containing the silicon medium by which it gets it's drive, so then how does changing the ATF do any more to prolong the life of the coupling other than lubricate the drive chain and bearings of the transfer?
A mate and I have recently fitted one of these gearbox/transfer cases and the VM diesel engine it came with into an old LR County, don't ask why. But I'd be interested to know what exactly we are dealing with and just how much of a headache are they?
Another mate had a Borg Warner VC transfer in his 1984 Classic RR, specially fitted by a previous owner, and it has done in the order of 600,000 klms of all types of driving/towing on and off road, lots of beach stuff, it has been absolutely reliable, I get the impression from this forum that may be not the case generally.

Gazby,

Understand perfectly your confusion, - I thought so too, but with the patience of the thread's contributors and lots of staring at the RAVE diagrams... I reckon I've got it sorted.

I'm not a mechanical fitter (Pre-Press/ Printer) so this explanation is gunna be simplistic in the extreme...

Imagine your standard pre-'88 Classic with its LT230 t/fer case. You drive off the pavement straight into the ruff stuff.... Somehow you end up with one wheel high and dry, and spinning like mad. You're stopped, and the Toyomitsussans are starting to smirk... so you reach down and push the transfer lever over one side to LOCK your centre diff, try and look Kool...and drive on. Next time you'll save face by engaging the lock before you need it !

To save embarrasment, LR came up with the idea of an Automatically Locking Centre Diff.

The method chosen, was to add a gizmo (must be cheap/easy to assemble) in parallel to the centre diff's outputs, Front & Rear prop shafts.

Sort of like a Wife... lets the diff get away with a little slippage... but instantly gets hot and digs her heels in if it goes too far.....and forces the prop shafts to try and turn at the same speed..... which allows her to cool down...and they slip again so she grabs them both again.... with the result that the RR once again Proceeds, and wipes the smiles of the audience's faces. :D

The sole function of the VC is to LIMIT the differance (in speed) between the Front & Rear prop shafts. And to do it all by itself, allowing you to look even Kooler by appearing to do Nothing. Nothing more.

So, technically, you can *remove* the VC and drive around in AWD as an "Open" centre diff.

In practical terms this is impossible, as the drive from the FRONT of the diff is transmitted - literally - through the outer case of the coupling, and the inside is connected to the REAR output of the diff. - Have a really long and hard look at the RAVE diagram...

I'm told that nearly all failures of the VC are caused by the thick fluid becoming a solid rubber mass that locks up the input/outputs. Very few fail by letting go... Also like a Wife !

As to life of a VC, dunno, seems very variable and influenced by front/rear tyre pressures, how the vehicle is driven, wind direction and the entrails of a chicken.....

You have now exhausted the extent of my understanding....

Hi There is a mis understanding about how the VC works in a RR gearbox and this mis-understanding is causing some people to destroy their VC.
The VC is (partially) filled with viscous silicon fluid, this viscous fluid causes a drag on the alternating plates in the VC. Like any visc fluid its viscosity gets less as it is heated, thick grease becomes thinner as you heat it.
The VC is set up to transmit torque up to the design limit if the load is further increased the VC just slips and generates even more heat, then ultimately fails. The VC does not begin to lock up after it starts to slip and generate heat it is 'locked up' from the start and slips more if it is overloaded , the point where it fails.
You can try this by applying a load to the VC, with a fixed torque time 90deg rotation, slowly increase the load and the slip will increase, apply an overload and the VC will 'break away' almost free.
Best thing to do if your wheels are spinning is ease the throttle you will not make the VC lock up by spinning it.
Hope this helps someone save their VC !!

Regards Ian Ashcroft

Superquag and Ian, thanks both for your explanations of principle of operation of the Viscous coupling as fitted to the Range Rover transfer case, but from reading your replies I get two different pictures, (I think, but then I may be just thick in the noggin').
Superquag, you explained it to work as I thought it would i.e. the more slip, (differential movement) between the members the more tendency to apply drive to the slipping member, I recall from my Physics there are fluids that thicken with agitation and fluids that thin out, that is what I have seen with the industrial applications I have worked with in the past. This is why I thought the VC could tolerate the slow differentials generated say, when doing a sharp turn, maybe a three pointer or some other turn on blacktop that makes the front diff pinion shaft rotate at a different speed to the rear one, after all this is the prime reason for having that centre diff in the first place, something would break if you tried to turn sharply on the bitumen or at least bind up, as I'm sure many of us have found when trying to do a "U" turn on the road with the vehicle still in 4WD (centre transfer diff locked), is it "transmission wind-up"?.
Ian, on the other hand you are saying that the VC is partially locked from the start and can apply torque to a "pre-determined level" but after that it will slip and then generate so much heat internally it will cook the Silicon fluid. I had a mate in NSW years ago with an old Jeep Cherokee Chieftan, I think they may have had a viscous centre diff system on an auto tranny, either that or a limited slip centre diff, called "Powertrak" or some such name, anyhow every time the vehicle was doing a sharp turn, u turn left r right the 'orrible noises and banging that came out from the transfer was enough to drain the wallet of a millionaire, maybe that was the result of something like you explained Ian.
Guys, is there any similarities between the operation of this VC and the VC type fan drive as fitted to my Disco 300Tdi, I know they work well for a time then mysteriously, not work, for some reason, and I don't understand them either.
Can either of you suggest a site where I could see a diagram, exploded veiw or layout of the RR transmission showing the VC, as they say "a picture is worth a thousand words", all in the hope of understanding more.
Regards Gazby

I have linked below to the description of a VC.
[ame="http://en.wikipedia.org/wiki/Viscous_coupling_unit"]Viscous coupling unit - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/File:Text_document_with_red_question_mark.svg" class="image"><img alt="Text document with red question mark.svg" src="http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/40px-Text_document_with_red_question_mark.svg.png"@@AME PARAM@@commons/thumb/a/a4/Text_document_with_red_question_mark.svg/40px-Text_document_with_red_question_mark.svg.png[/ame]

It shows a cutaway and components and I have no reason to doubt it.
The key issue is that the fluid in the VC is Thixotropic or Dilitant, in that it thickens with heat.

This text and explanation seems to support the contention that a VC does stiffen with heat .
Regards Philip A

Quote:

---

Originally Posted by Ian Ashcroft

Hi There is a mis understanding about how the VC works in a RR gearbox and this mis-understanding is causing some people to destroy their VC.
The VC is (partially) filled with viscous silicon fluid, this viscous fluid causes a drag on the alternating plates in the VC. Like any visc fluid its viscosity gets less as it is heated, thick grease becomes thinner as you heat it.
The VC is set up to transmit torque up to the design limit if the load is further increased the VC just slips and generates even more heat, then ultimately fails. The VC does not begin to lock up after it starts to slip and generate heat it is 'locked up' from the start and slips more if it is overloaded , the point where it fails.
You can try this by applying a load to the VC, with a fixed torque time 90deg rotation, slowly increase the load and the slip will increase, apply an overload and the VC will 'break away' almost free.
Best thing to do if your wheels are spinning is ease the throttle you will not make the VC lock up by spinning it.
Hope this helps someone save their VC !!

Regards Ian Ashcroft

---

Ian, I always thought (like Phillip A) that VCs worked because the silicone fluid was a shear thickening, non newtonian fluid???

My anecdotal evidence supports this. The best example was watching Trav (aquarangie) on here attempting to climb a steep, dry, high traction hill offroad in a VC-equipped RRC (said VC was likely old and tired).

All attempts were at low speed (just above idle) with rear ARB locker engaged. Vehicle had just left camp so all fluids would have been cold.

1st attempt - vehicle stopped part way up hill with one front wheel spinning.
...reverses down for 2nd attempt...
2nd attempt - exactly the same as first, vehicle stopped at same location
...reverses back down...
3rd attempt (same speed as 1st 2) - vehicle continues on all the way up the hill, with drive now going to front and rear.

Quote:

---

Originally Posted by PhilipA

The key issue is that the fluid in the VC is Thixotropic or Dilitant, in that it thickens with heat.

---

To be more correct, it thickens with shear.



EDIT:
here is a youtube video of the VC in a VW synchro:
[ame="http://www.youtube.com/watch?v=zMqOebMnZKo"]YouTube - testing viscous coupling[/ame]
It clearly works like an open diff initially, and only starts to transmit drive with a sufficient level of shear in the fluid.

This also seems a good description.
The Viscous Coupling

The Dow Corning site lists the VC fluid but there is no product description.
The above also confirms that it is different FR tyre sizes that do the most damage.
So Phew, I have always had same size tyres.
Regards Philip A