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Throubleshoot Viscous Coupling - Printable Version

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Throubleshoot Viscous Coupling - deanace - 05-24-2011 12:44 PM

Introduction

The Viscous Coupling (VC) is the drive train component that transmits power from the center drive shaft to the front differential, and on to the front wheels. Inside the coupling there is a silicon liguid/goo that turns nearly into a solid when exposed to shear force caused when the VC input and output shafts rotate at different speeds. The input shaft is connected to the rear wheels. The output shaft is connected to the front wheels. When the front and rear wheels turn at different RPMs (at a rate greater than 6%), the shear force raises the temperature and viscosity of the silicon liquid inside the VC. The silicon becomes solid like and engages plates inside the VC with the result that power is transmitted from the center drive shaft (the engine) to the front differential (the front wheels).When the front and rear wheels begin to rotate at relatively the same RPMs again, the liquid "deactivates," becomes less viscous and more liquid like, and as a result the front wheels/diff. disengage, and the van is powered again only or primarily by rear wheel drive.

VC Fail - Causes & Effects

A VC can fail in two basic ways: it can stop engaging altogether or it can remain constantly engaged (sometimes only at higher operating temps). VC fluid loss caused by leaking seals seems to be the most likely cause of a total failure of the VC to engage at all. When this happens, you never have 4 WD. The more insidious and costly failure occurs when the VC remains engaged when it should not be engaged. This can lead to destruction of the entire drive train, including the expensive transaxle.

There appear to be two primary causes for the VC to engage when it should not: (1) having tires that are not all the same size and wear (all 4 must be the same), and (2) age...VCs appear to have a nautural life span, at least where subjected to routine high operating temps. (One shop also claims that having a drive train that is not properly aligned also causes premature engagement, but this theory seems questionable.)

When tires of different wear or size are used, it causes the VC to engage prematurely or even constantly. It makes the VC "think" your wheels are slipping and that you need 4 WD. Premature or constant engagement of the VC, particularly at highway speeds, overheats the VC, thereby "cooking" the viscous fluid. Over time, this causes the properties of the fluid to change so that it engages the VC prematurely or even permanently, thereby stressing the other components of the drivetrain.

A typical scenario leading to a cooked VC would involve a syncro with tire sizes that vary slightly in treaddepth. The syncro is regularly driven at highway speeds for hours at a time over a period of time. Eventually, the driver notices binding in the drive train whenever she pulls off the highway into a gas station for gas. It may be subtle at first. Eventually, though, as the fluid gets cooked and ruined, the wheels seem to stiffen or bind much easier than before, and ultimately at the slightest turn of the wheel. When it gets bad, the drive train may lock up completely in the parking lot at very slow speed upon a relatively slight turn of the steering wheel. This total engagement of the drive train puts tremendous strain on the components of the drive train when the van is moving at speed under power. With continued use, the transaxle soon fails, the drive shaft and CV joints are also strained.

There is some dispute as to whether a properly functioning VC will cause binding in very tight turns, like when turning while backing out of a driveway, or doing a sharp turn in a parking lot. The VC engages when the front and rear wheels turn at different speeds, greater than 6% in relative RPMs. When the front and rear wheels turn at a greater difference in RPMs during sharp turns (above 6%), it would seem that the VC would engage and cause some binding. However, a brand new VC will not do this, even when warm.

Chirping/binding in tight turns at least provides good cause to check your tire tread depth and to keep an eye on whether the symptoms become progressively more pronounced. However, symptoms indicating that the VC is engaging sooner than it used to or should be, such as obvious binding at low (parking lot) speeds, easily induced binding (i.e., upon less turn of the steering wheel) and heat related binding (after long high speed summer trips), should not be ignored and should be investigated immediately in order to avoid huge repair bills. Brand new OEM VCs can be had for about $1100. From the dealer, they cost over $2,000. If you cannot afford or find one right away, remove your center drive shaft until you do to avoid causing damage to the drive train. Have a qualified mechanic do this, or follow the procedures in Bentley. There are a few not apparent procedures that should be followed.

Test for Proper Function of the VC

VW's original (German) repair manual doesn't say much about how to test the viscous coupling. They only recommend placing the rear wheels in a break testing stand. If you then switch to the G-gear (creeping gear), the front wheels should move the van out of the test stand as soon as the engine is revving slightly above idle. If the front wheels fail to do so the viscous coupling is to be replaced, VW says. VW adds another tiny sentence to this, saying that only when the engine is revving at idle and with the G-gear switched in, the viscous coupling is able to absorb all the torque to the front wheels and keep them from moving.


To me this last and rather ill formulated (in the German manual) sentence is the key to testing the viscous coupling. For in most cases we are not dealing with viscous couplings doing less than their share, but rather with hard going viscous couplings which don't have a problem at all in moving the van out of the test stand with the engine just idling. [i.e., prematurely engaging VCs -ed.]
Thus the really important thing here is not the van successfully leaving the test stand. On the contrary, the important thing here is the van not moving and staying put in the test stand with the G-gear switched in and the engine just idling. If your Syncro doesn't pass this test your viscous coupling is probably worn out and ready for a replacement. Or put the other way round: as long as your van's viscous coupling is working properly you will not notice your van has got one. [Except, perhaps, when making tight turns in a parking lot. Some binding and chirping is ok when you make very tight turns. It is the temperature-relarted, stiffer and more easily triggered binding that is symptomatic of a problem needing immediate attention.

How to Replicate the VW Test

Using a heavy floor jack with wheels, put a block of wood on the jack and raise the rear of the van using the skid plate. Be careful, you can bend it. The wood block, if long enough, distributes the weight across enough of the skid plate to minimize that risk. Lift both back wheels off the ground 6" or so on a smooth level parking lot, with the jack's wheels parallel to the Syncro's. The rear of the van will be moving on the jack wheels, so you need to make sure there are no obstructions that could catch the jack wheels and cause the van to fall off the jack.

Now, put a 1x1 piece of wood in front of each front tire, 2x4 may work too. You need to block the front wheels like this to be able to test whether the VC is capable of absorbing the spinning of the rear wheels without locking up and causing the front wheels to engage and climb over the wood. If you can get the rear wheels to turn/spin in the air with the clutch fully disengaged, and without the van climbing over the wood blocking the front wheels (can't be too high...1-2"), then the VC is definitely good, or the fluid is not cooked. The van should climb over the blocks as soon as you increase the engine RPMs.

It may take several tries to get the van to do this. The VC is very sensitive, and it will want to engage as you let the clutch out. Some advise using the hand brake to help slow the spin of the rear wheels ... or to start them spinning slowly at first. It is really neat when you get it to work. Suddenly, the VC is working before your eyes in a very graphic way!


Re: Throubleshoot Viscous Coupling - deanace - 05-24-2011 10:39 PM

[youtube]4E_dz_DkRm0[/youtube]


Re: Throubleshoot Viscous Coupling - ALIKA - 08-21-2011 03:33 AM

If you plan on rebuilding it,278grams is what it takes to get it factory set.

Some videos in German,but it's pretty simple to do,I mean,anything is simple to do on our vans,right? :lol:
I wouldn't work on a T5 4motion for a gold bar :x

Edit Smile

[youtube]IkKAFpIMVlA[/youtube]
[youtube]PFn9-jnO5nc[/youtube]


Dean,how the hell do you put the videos from youtube here?I tried whole link,whole link in youtube brackets parenthesis,partial code in parenthesis,I can't figure it out :oops:

Thanks


Re: Throubleshoot Viscous Coupling - deanace - 08-21-2011 05:09 PM

Fixed your post already but here is where you can see our tutorial on how to do the embedding
Have you checked out our youtube channel yet ?


Re: Throubleshoot Viscous Coupling - ALIKA - 08-21-2011 08:59 PM

Hi Dean!!

no,I haven't yet,I did see some videos here but didn't notice there was a channel:let me go back to my homework then :mrgreen:

Thank you for the embedding explanation :-)

Have a nice week end!


Re: Throubleshoot Viscous Coupling - Busfan - 03-30-2012 12:33 PM

All Videos are originally there:
<!-- w --><a class="postlink" href="http://www.2wd-goes-syncro.de">www.2wd-goes-syncro.de</a><!-- w -->

Elaboreted by Thomas Rausch

3. Copyright
The author intended not to use any copyrighted material for the publication or, if not possible, to indicate the copyright of the respective object. The copyright for any material created by the author is reserved. Any duplication or use of such diagrams, sounds or texts in other electronic or printed publications is not permitted without the author’s agreement. :roll:


Re: Throubleshoot Viscous Coupling - ian - 03-30-2012 01:06 PM

It's good you brought up the copyright issue as there are a few items posted here that don't even credit the original source. Not that that fulfills the copyright limitations but at the very least one should do that so it doesn't look as if you are trying to pass the info off as your own work.


Re: Throubleshoot Viscous Coupling - syncromad - 03-30-2012 01:11 PM

Yes but is that person really the copyright owner?


Re: Throubleshoot Viscous Coupling - Busfan - 03-30-2012 01:51 PM

That is the copyrightowner, acording to the website. But I think, if you doubt, you coud mail there :?
I posted that, because there where videos missing here.
Also to come to my soon requiered 10 postings Smile


Re: Throubleshoot Viscous Coupling - ian - 03-30-2012 01:54 PM

Who knows and I personally wouldn't worry about the copyright issue too much. I can't see someone suing you for posting their article on another Syncro forum. But it does understandably pee people off if you copy their stuff without acknowledging the source, so I think it's a good idea to at least do that.