Fitting Viscous Couplings

How to install your Viscous Coupling

(including parallel instructions from Rainer and Tom Forhan)

No special tools or automotive experience are needed for this job, and because it is so simple, it should not be farmed out to a transmission shop. The best thing I could say, going from memory, is to….

    1. Drive van up on 4 ramps

    2. Put jack stands under van in case ramps fail

    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.

    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.

Rainer Adds This Comment:
‘There’s one very important point missing here, if you’re replacing the viscous coupling with the diff installed: before you remove the oil drain plug by all means make sure you can remove the oil fill plug first (the same holds — of course — for changing gearbox oil)! If, for some reason or other, you can’t remove the oil fill plug, forget about replacing the viscous coupling right under the van.’

    7. Remove the 13ish? 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. 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.

Steve Schwenk ( adds this comment here:

‘At steps 7-9 it might be noted that it is a little tricky removing the casing to get to the VC. It can hang up on the Diff. mount bar, the one in a “U” shape, and I had to remove that bar to get it off and out of the way.’

    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 Loctite 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 viscosity. 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–everybody wisely uses Redline GL-4 synthetic for the rear). You can also use the Redline GL-5 or GL-4 for your front differential.

Rainer adds this comment here:

Lately — at least here in Germany — VW seems to have issued a new order of the day with respect to gearbox oil. They’re now recommending a single new synthetic oil for ALL of their gearboxes (not only Vanagons). It is said to eliminate cold shifting problems, and it is also said to only marginally keep even small metallic particles in suspension.

The new specification is ‘G50’, the weight is SAE 75W-90, and the VW part number is G 005 000 05 for the half litre plastic bottle, and, I think, G 005 000 for the one litre tin can.

Derek’s comment:

If you want to try this new oil and try to order at the VW dealer, it is a reasonable thing to do. Nevertheless, I think most experts in America would still go with a synthetic GL-5 in the front and Redline GL-4 in the back, at least pending evidence that the G50 oil Rainer writes of is indeed somehow superior. There is a possibility that the G50 is both much more expensive, harder to get, and no better. There is also the possibility it is much better. At this time, we simply do not know.

    12. The entire job should take you about 1 hour. But for some reason –I cannot predict in advance why — it will actually take you 5 hours :-). Some members have reported broken bolts holding the diff in place, or broken rubber mounts, so be prepared to hunt these parts down if you decide you need to replace them.

    13. Some people decide to remove the diff entirely to do this job. You can do that, but that means you have to disconnect the front CV joints, which is a pain. Still, those front CV joints probably need their grease to be renewed anyway. And as long as you are re-greasing the inner front CV joints, you might as well do the outers too, and inspect the boots, and then why not the rears and HEY lets not loose focus here…

    14. The best description on how to change your VC was originally posted to the list by Rainer about three years ago, but Steve Schwenks photo essay matched this post in utility and interest. It was on again and off again at in the technical section. I highly recommend a visit to that site to check it out. I will copy Ranier’s post below.

About the pogo stick method

This is actually a method of easily removing the transmission and engine from the vehicle. Essentially, the procedure is to skip using Loctite on the nuts that hold the driveshaft on the front diff, and don’t replace those nuts with new nuts either. Just reuse your old nuts even though the factory says to replace them. When the nuts or bolts eventually fail, the driveshaft will drop down on the highway and get lodged on a rock, crack, or other obstacle and remove your transmission and engine in about two seconds. This also works in the event of a front U-joint failure. If this does not appeal to you, install a strap that holds up the front driveshaft in the event its front attachment point fails. I haven’t put my strap in yet myself, but I think about it from time to time.

At 01:07 AM 7/5/01 +1000, you wrote:


There is an excellent article by Steve on how to replace the VC at in the Tech section called ‘Replacing the VC (photos)’:

Reproduced below is Rainer’s version of the above.

It differs from my version in some important respects so it pays to read both posts. Note that Rainer is one of the world’s leading observers of the Syncro Viscous Coupling, and is of much higher intellect than the average Vanagon or Syncro poster so we are lucky to have him among us. (He gets, mostly unfairly, bashed by some of our other syncro experts from time to time and it hurts to see this.)

Date: Fri, 18 Dec 1998 12:53:06 +0100
From: “Dr. Rainer Woitok”
To: Derek Drew
Subject: Re: Viscous Coupling Test


Here we go again … 🙂

I’ve attached the ‘How-To’ information at the end. Enjoy! And plenty of luck with changing the silicone-fluid.

Derek’s note:
Rainer was sceptical about my plan to change the fluid. As it turned out, he was right to be sceptical, and I abandoned this idea. I still have the fluid. Anyone want to buy it from me?

Apart from that: Merry Christmas as well as all the best for the next year to you as well as to your family … 🙂

’89 Caravelle GL Syncro 16″

The material below is by Rainer referenced above

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.

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.

As soon as you are encountering problems with your Syncro when cornering, in particular after a long and fast drive, or as soon as — despite the power steering — steering becomes a bit difficult when turning and the Syncro slows down considerably when going round a corner… as soon as one or more of these things are happening, your viscous coupling is most probably due for replacement. When the tires start whining while cornering it might well be too late already …

I once had all of these symptoms and it was immediately clear to me that the viscous coupling was the culprit. But it took some time for me to react, and it took some more time for the new viscous coupling to arrive at my door. Should you ever encounter similar problems I would urge you to immediately get under your van and remove the drive shaft between gearbox and front diff. If you fail to do this and wait too long, severe damage to the gearbox and/or front diff is the probable result. Removing the drive shaft is pretty straight forward, just four bolts and nuts (13 mm) on either end of the drive shaft. Then loosen (just loosen, don’t remove them) the three nuts and bolts (17 mm) which hold the front diff in place, so the front diff can move out of the way a bit for the drive shaft to be removable. Don’t forget to again tighten the 17 mm screws.

When I was shopping for a new viscous coupling (according to all reliable sources I contacted they cannot be repaired) I asked several knowledgeable people how long a viscous coupling is expected to last. Apart from ‘it depends’ (an employee at Steyr-Daimler-Puch in Austria, the firm which was producing the Syncros and in particular the viscous couplings for VW) the answers ranged from ‘some 60,000 km’ to ‘between 170,000 and 200,000 km’. Mine was replaced after 150,000 km but I bought the Syncro with 80,000 km on the tach and don’t know for sure whether or not this was still the first viscous coupling. But I’m assuming it was.

As for the reason why the silicone in the viscous coupling gets too stiff and starts causing trouble the people I asked unanimously answered: too much strain. However, this is not referring to relentless off road driving in groundless mud, but rather to small but permanent differences in rotational speed between front and rear axle while doing normal on road driving. These differences in rotational speed can be caused by such things as unequal tire wear or different tire pressure. Tire diameters should be the same within a 2 to 4 mm tolerance. Or, in other words, if you’re measuring the depths of the grooves in your tires, the differences should not exceed 1 to 2 mm.

Thus people not caring tire pressure and tire wear, people having mounted different tire brands on the front and rear axles, as well as people never routinely using their spare tire so it gets worn roughly the same way as the other tires are most probably ruining their viscous coupling pretty fast.

Removing the drive shaft as an emergency measure: If you have to, and provided you’re slim enough, you can remove the drive shaft on the bare ground (at least if it’s a Syncro 16″), just with a little help of the factory jack. In case your driveshaft was balanced in its current position, it may be a good idea to mark the alignment points of the flanges before removal. Then you’ll have to use a pair of 13mm wrenches (due to space constraints only open wrenches are usable) to remove four bolts and nuts at either end of the drive shaft (sometimes the nuts are only 12mm). If you want to do it properly you’ll also have to loosen (only loosen, don’t remove!) the three nuts and bolts (17 mm) which hold the front diff in place via rubber stops, so the front diff can move out of the way a bit for the drive shaft to be removable. However, when lying on the bare ground just below the van this might be hard to do. If your drive shaft has got this rubber element in it you may try to simply use a screw driver to cautiously separate the two flanges at one end of the drive shaft. Knocking gently at the flanges at the other end will then take the drive shaft down. As to the rubber element: in diesel-engined and FI vans the rubber element was facing opposite sides. Thus you’d better take some notes as to whether the rubber element is at the gearbox end or at the front diff end.

Replacing the viscous coupling: To replace the viscous coupling you’ll have to pull the front differential first. I don’t think it’s feasible to replace the viscous coupling right under the van with the front differential in place.

Derek’s Note:
Californians on the list proved otherwise so if you want to leave the diff in there go ahead. Sam Walters did it on the ground, but he indicated it was a pain.

In particular, reassembly would thus be much more difficult and would cause plenty of cursing. And you’d have to drain the oil first. By and large there is no witchcraft involved in pulling the front diff, but you should do it with the van on a lifting platform or above a grease-pit. And it takes two people as well as a floor-jack. First of all loosen but don’t yet remove the three 17mm nuts already mentioned which hold the front diff in place. Then remove the speedo cable as well as the six screws in each of the inner CV-joints. For this screws you’ll need either a 6mm hexagonal or an 8mm multipoint socket. If you haven’t done it yet you should now remove the drive shaft as described above. If you have a front diff lock (never officially exported to the US) unplug it electrically as well as pneumatically (tag the pipes so you can’t confuse them later). Now remove the screw holding the vent pipe of the differential and then remove the three 17mm nuts and bolts holding the front diff. Get the floor-jack in place and remove the front and rear mounting brackets. Then together with a second person cautiously move the diff forward along the skid plate and out. Look out for the diff lock if there is one.

Place the front diff on your workbench in such a way that the flange for the drive shaft is facing upwards. You can now easily remove the ten 13mm bolts with which the housing of the viscous coupling is attached to the front diff without risking any oil leaking. Lift the housing which is containing the input shaft. You will now see the viscous coupling itself and a little spacing collar sitting on top of it. Take this spacing collar, grease the top rim of it and again place it on the input shaft in the housing and slightly press it with its greased rim against the bearing so it will stay there. This will greatly ease reassembling the diff later. Now remove the old viscous coupling and put in the new one. Then apply silicone sealing compound to the contact flange of the housing and, inserting the input shaft into the viscous coupling, put the housing back in place. Finally, gently tighten the ten 13mm bolts crosswise, applying 20 Nm (15 ft lb.), and your front diff is ready to be reinstalled.

Perhaps you should use the opportunity while the diff is still sitting on your workbench to check what has been caught by the magnet at the oil drain plug.

To reinstall the front diff in the van simply reverse the steps laid out above, at the very last fastening the screws of the front and rear mounting brackets as well as the three 17mm bolts through the rubber stops (apply 45 Nm (33 ft lb.) to these).

Tools and torques needed:

Installation of the front diff:
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 and ratchet, 20 Nm (15 ft lb.)

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.)

Special thanks go to Wolfgang Carolsfeld in Canada who not only proofread this text, helped me with some technical English terms I failed to find in my dictionary, and did the math involved in converting the torques from Nm to ft lb, but who also successfully applied the underlying German version of this text to his Syncro to make sure it works 🙂

Tom Forhan’s Addendum Appears Below:

We’ll folks, I’ve done it three times now and I finally got it right: -once with a used VC with a housing leak leaked, once with the used VC no leak but it bound, and once with a new VC! Thanks Derek!

I’ve annotated Derek’s original instructions ( with a lot of detail that I learned the hard way. Original instructions are in quotes, my comments follow.

    4. Loosen the bolts holding the front differential so that differential may be shifted around. Before doing this, 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. The take out the three mounting bolts here, one in front (front is front), two at the rear. All have big nylon/rubber mounts- I 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.

    7. Remove the 13ish? 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. Do not loosen the big bolt at the rearmost point in the front differential

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. They bolts are 13mm. I used a ratchet, two 3″ and one 6″ extensions in various combinations, and a universal joint for the three bottom bolts. A screw driver type handle was very handy for spinning out/in once the bolts were loosened. You 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 suggest leaving the “U” bar on the diff- I used two pieces of nominal 2×2 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.

    8. Have something on the ground to catch the residual oil that will spill out. Amen.

    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.

First, put the car in lst gear. This will maximize the clearance between the VC housing and the shifter rod, which is one of the many things that limit your ability to move the housing around. As you gently pull back the housing – not too far, or too fast, at the start- look inside the crack between the two housings to see if the VC is going to stay with the diff (good) or slide back with the VC housing (bad). If you cannot get the VC to stay on the diff, the clearance issues are much more difficult, you may want to jimmy the VC around and try to get it back onto the diff shaft once you have a few inches of separation.

Play around with the jack to move the height and angle of the diff as well as the VC housing as you try to pull it back. You will have to get the VC housing back pretty far to clear everything, and then I found I could turn its tail 90 degrees or so to the drivers side, and then I was able to slide the VC off, and replace it. The little metal washer Derek is referring to is the item labelled “spacer” in the Bentley, it goes on the shaft in the VC housing, not on the shaft in the diff housing, though it will fit ;-).

Reattaching the two faces, of course use a sealant – I used Loctite blue- and make sure to torque everything together, 20 Newton Meters.

    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.

With the driveshaft and the “U” bar reinstalled, jack the assembly around to get it into position to install the three mounts and bolts – loosely, but make sure the top and bottom nylon half circles in the two rear mounts are aligned. To get the right to left alignment, I measured to make sure the distance between the VC housing centerline casting mark and the right and left drive shaft skid plate mounting points was the same.

Took it for a test drive this am – everything works, turns like a dream, no vibration, no rear pinion howl ( Karl M and I replaced the trans with a used unit I had laying around right before I did the VC).

Replacing the VC is not difficult for one person to do in about two hours…the third time.

Hope this helps someone out there-

Tom F.

Did mine in July. Slide show is good. only tool is a 17 mm Allen socket. Drew says a two hour job that takes five. That’s about right. Two points not covered cost me about two hours.
1. Go ahead and remove the cross piece
2. The VC should stay forward. Mine came back with the cover three times, but finally stayed forward. Make sure you can get the fill plug out before you start. Not a hard job, just requires a little patience.

John Parsons
’87 VW Syncro/Country Homes Pop Top
’68 VW Single Cab Pickup

Ben’s contribution on his installation:

To: syncro
From: Ben McCafferty
Date: Sat, 09 Feb 2002 00:21:09 -0800
Subject: [Syncro] VC Viscous Coupling installation notes

For the archives, I wanted to add a few thoughts to the great picture essay on about replacing a VC.

Since I had the tranny out, I had the skid bars out of the way already. For my money, taking the time to remove them before this job is time well spent. It’s 6 bolts, and makes a huge difference.

The pictures reference using a jack stand, which is, in my opinion, a necessity (unless your name is Brutus and you bench press 350 with one hand). I found a jack more useful, though, because it did the work of lifting the front diff for me.

There are 10 13mm bolts, and one 14mm bolt for the breather hose.

Once the bolts are out, I laid under the van on my back, feet to the front bumper. I could then brace my feet against the frame, and use the mounting bracket to pull the cone loose. If it’s stuck, a couple of shots from a dead-blow hammer (non-marring) should kick it loose.

The next thing I found that worked really well was to rotate the cone 180 degrees. To do this you have to first remove the mounting bracket (2 more 17mm bolts, 3 minutes). Once the cone is upside down, the slot in its bottom side no longer catches on the frame as you slide it to the rear. Also, I found it essential to remove the front diff mounting bolt, and slide the front diff as far forward as possible. I used the jack under the front diff to move it up as high as possible as well.

Going back together was a snap–slide the cone back in place, flip it over, put back the bracket…..wait, did I put in the VC yet? 🙂 I ended up using Permatex #2, non-hardening sealant.

One other note that I think is worth mentioning–there’s lots of dirt/grease/grit stuck to the undercarriage above the front diff, and it’s really easy to knock it loose and into the open diff while replacing the VC, etc. I made sure to wipe out all dirt that might have fallen into the cone before reassembly. Also, some of it may sit on top of the new VC, out of sight, so be sure to get that too.

Total job took me about 1.5 hours, 1 hour the second time (did two yesterday).

Regards, bmc 🙂

When I did mine, it was similar. I found that removing the bracket made it easier. But when I pulled the nose cone back the VC came back with it, I tried everything that I could think of with no luck until I pushed the cone back on, then yanked it back. That is when I learned that the VC should stay forward with the diff.

John Parsons

*Edited by Clive Smith for Club80-90