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). Diagram of the Syncro Viscous Coupling 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. 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.

The Effect of Heat on the VC

As the chart below depicts, at 110 degrees centigrade, the VC silicon fluid viscosity increases dramatically, thus engaging the VC.  Since temperature is a cause and not just an effect of VC engagement, it  seems probable that the "engagement temperature" of a "cooked" VC (a VC where the fluid has been ruined by prolonged overheating) is lower than for a non-cooked VC, and that with cooked VC fluid, the VC engages at the higher end of normal VC operating temperatures, with the result being that the VC is always engaged at the higher normal operating temperatures even if the wheels are turning within the 6% threshold for engagement.  This puts incredible strain on the entire drive train.

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Test for Proper Function of the VC by "Dr. Rainer Woitok"

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How to Replicate the VW Test

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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! ___________________________________________________________________ Warning! You could conceivably get killed or kill someone performing this test.  If you die doing this, don't get mad at me! ___________________________________________________________________ A test to see if the VC is engaging is to take your syncro out into some fresh snow or mud and induce wheel spin.  If you get none up front, but the rears are spinning, you have a problem...which could be blown VC seals or perhaps a bad front differential.  This test, however, does not help you diagnose a VC that is in the process of self-destructing, or is putting undue strain on the rest of the drive train, because it is engaging when it should not.

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Replacing the Viscous Coupling

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   Photo essay of repalcement of VC

Tools and torques needed:

Front diff mounting brackets: 17mm socket and ratchet, perhaps with  extension. 
Use 17mm wrench to counter at the other side, 45 Nm (33 ft lb.)

VC housing: 13mm socket with extension, 20 Nm(15 ft lb.)

CV-joints: either 6mm  hexagonal  or 8mm  multipoint socket with extension  and  ratchet, 35 Nm (26 ft lb.)

Drive shaft: Two open 13mm wrenches (sometimes only 12mm for  the nuts), 35 Nm (26    ft lb.)

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Removal Procedures:

1.  Hose off and wash under van around VC casing. 2. Raise front of van using jackstands. 3. Record and mark the alignment of the driveshaft to the front differential so that you can put the same bolt through the same holes of each unit upon reassembly. This will reduce the chances of your ending up with an out of balance driveshaft on reassembly. 3. Unbolt the four forward bolts holding the driveshaft on with either a 1/2 inch or 13mm open end wrench and some liquid wrench. If the 13mm wrench doesn't work that great, try the 1/2 inch open end wrench. 4. Loosen the bolts holding the front differential so that differential may be shifted around.   Loosen the two bolts at the top rear of the diff that support it from on top, on an upside down "U" bar. Don't remove them yet. Take out the three mounting bolts here, one in front (front is front), two at the rear.   Put a jack under the diff, and removed all bolts and the mounts at this point. 5. Shift the front diff forward so that the driveshaft will fall away from the front diff. Shift that driveshaft out of the way. 6. Remove the oil from the front diff through the oil drain hole. Throw that oil away by bringing it to your nearest auto repair shop for disposal.   Shift the front diff forward so that the driveshaft will fall away from the front diff. Shift that driveshaft out of the way.   Before you remove the oil drain plug, make sure you can remove the oil fill plug first. 7. Remove the 13? 13?mm bolts holding the back half of the front differential onto the vehicle and then pop the rear third of the differential off backwards.  Use your floor jack to reposition the diff so that it slopes down as much as possible toward the rear. That way you will have complete access to all the bolts without shifting things around. The bolts are 13MM.  Also have the remove the 14MM banjo bolt and two small copper washers near the top that connects the air vent hose for the diff. Then jack the back of the diff up so that it is more or less horizontal again. Your jack must be on the main diff housing, not the VC housing. Good luck on just "popping it off", mine was glued quite nicely, at least the first time. This is why I suggst leaving the "U" bar on the diff-  use two pieces of nominal 2x2 about a foot long, levering against the subframe and the two sides of the "U" bar to break the seal. Once the rear housing is free, remove the "U" bar.   Do not loosen the big bolt at the rearmost point in the front differential 8. Have something on the ground to catch the residual oil that will spill out. 9. Pull the VC out and replace, being careful to reinstall the little metal washer that is wedged in there. No special tools or measurements of any kind are needed. 10. Bolt everything back together, but bolt the front differential down last after shifting it around to properly seat it in relation to the rear transmission. Make sure everything front to back are arranged in a perfect strait line front to back. This is very important, as there are (unconfirmed) arguments that not doing so can lead to excess stress on your VC. When bolting the driveshaft back on, either replace the 4 driveshaft nuts with factory new ones the way VW says to do it (proper way), or just use Red Loctite the way about half the people on the list do it (universal list method) or reuse the original nuts with no loctitie the way the other half does it (pogo stick method; see below). 11. Refill the front differential with GL-5 Transmission oil using the factory specified viscocity. Mobil 1 makes a good GL-5 for the front diff. (Make sure not to use GL-5 in the rear transmission, however, as that takes only GL-4--eveybody wisely uses Redline GL-4 synthetic for the rear.) You can also use the Redline GL-5 or GL-4 for your front differential. (Thanks to Derek Drew for contributing toward these instructions)