Transmission problems, syncro, primary front-secondary rear. A case study.


Very interesting!

(*    The included link is not working any more
 **   The missing photos do exist and will be into their position asap. (Today)
18:28 Athens GMT + 02:00 Done)

"The Passat Transmission Problem Solving Process.

Introduction to Problem Solving Techniques.

Almost all manufacturing companies will have a formal problem solving process which engineers will use to find out exactly what a quality issue is and hence how to prevent re-occurence. Such processes can take months to complete due to the amount of testing that is sometimes required and the need to demonstrate the root cause by recreating the problem in the laboratory.
When working on vehicles at home, we do not usually have to hand the vast facilities that vehicle manufacturers have and therefore sometimes repair the symptoms without fully understanding what went wrong. A good case here is the Triumph valve regression I experienced in 2008. Having new valve seats fitted got it back on the road, but it is still unknown what caused it in the first place and hence whether or not the problem will re-occur. This, however, was an acceptable solution to me as the cost of analysis in a metallurgy laboratory is far more than the cost of the work carried out or the value of the bike. It's not even if I'm alone, as garages and professional workshops are sometimes also guilty of such a lack of thoroughness, although their experience makes them less likely to do this than me.

But what if there is re-occurrence? A part is replaced and it fails again. Cut your losses and scrap the vehicle? Sell it as a non-runner? When the Passat started destroying rear differentials it was mainly my curiosity that made me want to investigate, despite the little voice in my ear saying "scrap it, scrap it". Which, of course, turned out to be Gabi and I guess this difference is why she studied accountancy and I studied engineering.

The popular methods for problem solving tend to follow the path of problem definition, cause and solution. In the car industry, one of the favourites is the 8D approach, invented in America. (The story goes that various large companies adopted this approach without the niceties of asking the inventors. They sued, won lorry loads of money, retired, presumably to some Caribbean Island, and now allow anyone to use their methods free of charge). Because the 8D process is my favourite, I will be using it.

D0 Symptom
Passat Transmission failures

Introduction to the Passat Transmission Problems.
A few years ago I bought a second hand 1996 Passat VR6 Syncro 35i (facelift) model with 2.9litre engine and 5 speed manual gearbox. It came with a fully stamped up service book and no reports of major problems.

Problem Sympton.

The Passat has suffered from various transmission failures.
139997km              LH front halfshaft replaced (inner UJ failed).
                                 FL, FR and RR wheel bearings replaced.
ca. 158000km        RR tyre puncture. One rear tyre replaced (winter tyre).
158600km              Rear differential and syncro replaced.
                                 All four wheel bearings replaced.
                                 Front LH hub replaced.
167000km              RR tyre puncture. Both rear tyres replaced (summer tyres).
167200km              RR halfshaft replaced (outer UJ bearing disintegrated).
168000km              Viscous coupling unit failed (casing cracked). Rear differential and viscous coupling replaced.
168100km              Rear differential failed (holed outer casing). Previous differential unit fitted.
168150km              Rear differential failed.

Note that with the second puncture two tyres were replaced because the tyre fitter had this noted as a requirement for VW Syncros. Unlike the first tyre fitter it seems.

D0 Emergency Response
Park vehicle on drive and put up on axle stands.
This would usually be overlooked except for safety related issues. Suppliers tend to write things like "parts sent for testing" here, but realistically, this would be concerned with recalls or production stops - both very expensive and generally requiring top management support.

D1 Create a Team
Okay, an ideal team would have about six members, including a facilitator. Also worth having a manager in there in case some expensive testing is required. Never, ever, allow the manager to join the actual meetings as they all, without fail, think that they alone are blessed with some fantastic engineering insight and immediately know what the problem, and solution, are. You then spend three months following this path until it is shown to be a complete red herring by which time the manager has got bored and stopped attending anyway.

D2 Problem Statement
Torsional wind up in the rear differential.
Torsional wind up is caused when the there is a difference between the rotational speed of the bevel gears in the differential. The input speed is determined by the gearbox and will dictate the output speed to the wheels. However, if the wheels are being forced to travel at a different rotational speed (dictated by the road speed), a conflict arises. Normally the weakest part of the transmission will then fail, which is why half shafts and the viscous coupling have failed as well as the rear differential.

This is what has been happening to my differentials. Big hole is where the drain plug has been removed.

Opening the rear differential up (with help from an angle grinder) shows the internal damage. Here, the teeth on the bevel drive have failed as one of the cogs wanted to rotate at a different speed to the other.

D3 Interim Containment Action
Remove prop shaft
Knowing a little more about the problem, it may now be possible to introduce some temporary action to protect the customer.
This was suggested by one of the garages I went to. However I decided not to do this. It should have worked, mind, allowing me to still drive around whilst trying to figure out what to do.

The next step is to determine the root cause, that is what actual mechanism has caused the failure. This is perhaps the trickiest part of the problem solving process and there are, in fact, quite a few methods which would assist the team. The following are perhaps the most relevant:
1.        cause and effect diagrams (also known as Fishbone Diagrams or Ishikawa Diagrams) examines all the factors which may influence a problem
2.        brainstorming is a team method to generate ideas, useful when the investigation stalls
3.        is/is not lists examine differences between what has failed and what could logically have also failed, but has not, and then examining differences between the two sets.

It took John and I four months to get from the Problem Description to the Root Cause. This is because we required a full understanding of how this particular transmission system worked. The information offered on website forums was a mixture of truth and nonsense, VW dealers did not have any in-depth knowledge and were not too interested in assisting, the VW dealer technical information site was a complete dead loss and the workshop manuals were misleading. However, armed with John's knowledge, a few parts diagrams and an angle grinder with which to dismantle one of the failed units, we can now offer the Ultimate Guide to the VW Syncro.

How the VW Syncro system works.

(Blogs alert: the above diagram has an important inaccuracy: the front cv axle shafts are not symmetrical).
The VW all wheel drive system under discussion consists of three main component parts. The front differential, the viscous coupling ("syncro unit") and the rear differential.

The front differential (1) drives the front wheels permanently. The differential is used to allow the two front wheels to rotate at different speeds, which is important when, for instance, the vehicle is turning a corner when the inner wheels will rotate slower than the outer wheels due to relative distance travelled. Sometimes even on a straight road, road surface changes may also require the wheels to move at different speeds.
If you look at a small capacity quad bike, you may wonder why they have such big "balloon-y" tyres. The reason is that they use solid rear axles and hence there will be an amount of tyre scrub when cornering. The tyres used will facilitate this.

There is a third output (2) from the Passat front differential which makes it different from a standard front wheel drive vehicle differential. This is the propeller shaft. The speed at which this rotates is fixed by the input shaft rotational speed and is geared at 16/21 by the bevel drive gear drive. The propeller shaft (3) runs under the vehicle to the viscous coupling.
The viscous coupling (4) is quite a simple mechanical device. It is made up of alternating circular plates. The plates are mounted in a sealed drum, and are located very close to each other. The plates have tabs, or perforations, in them. The drum is filled with a silicone-based oil. When the two sets of plates are rotating in unison, the oil stays cool and remains in a liquid state. When the plates start rotating at two different speeds, the shear effect of the tabs, or perforations, on the fluid will cause it to heat up and the viscosity will increase, which is the relevant property of the silicone-oil. The fluid in this state will essentially glue the plates together and transmit power from one set of plates to the other.
Hence, under normal conditions, the vehicle is front wheel drive only. If the front wheels lose grip and start to spin, torque will then be applied to the rear wheels which (hopefully) will find grip and restore vehicle motion.
Note that the VW have engineered the size of the tab and perforations to allow a 10% torque transfer at normal conditions (meaning the vehicle is not front wheel drive only, but split 90/10 front to back). This is to allow quicker response when the front wheels do slip.

The output from the viscous coupling will then drive, via a bevel drive (5), the rear differential (6) and is geared at 21/16 to restore the same rotational speed in the rear half shafts as the front ones.
Now this is where there is some slight confusion. In my Passat there is a free wheel system (7) which will disconnect drive to the rear wheels. The workshop manuals I have, one in German and one in English and both based on original VW assembly sheets state that:

 "During braking, a free-wheel mechanism in the rear axle differential will automatically engage and interrupts the drive to the rear wheels. This will, therefore, not influence the braking of the rear wheels and braking takes place in the normal manner".

This is not true. The free wheel operates in the same way as a motorcycle starter clutch and will allow, for example, shaft A to drive shaft B but not allow shaft B to drive shaft A. So that when shaft A spins faster than shaft B, shaft B also spins. When shaft B spins faster than shaft A, shaft A is not driven, although it can still rotate as long as it spins slower than shaft B. There is no influence on this system of the braking system.
It is also not a requirement for ABS, as some forums state. In fact, not having the free wheel engaged will be an advantage as it will allow the viscous coupling to assist the ABS by transmitting torque to the rear axle and hence matching it to the speed of the front axle.

Now "Volklore" has it for the older T3 syncros that tyres need to be swapped from axle to axle to even out wear. If this is not done, then torsional wind up will occur and the transmission will fail. These VWs do have some detail differences in design, in fact I understand that not all used a viscous coupling as the early models used a de-coupler. Also there was an option of switcheable differential locks for the front and rear axles.
However, the advice to rotate the tyres is relevant. This is because if you have two tyres with different amounts of wear, their effective radii will be different. This would mean that when driving along a straight road, one (the more worn one) will rotate faster than the less worn one.

To understand the relevance further, John helpfully set out two different scenarios:

Front tyres have a greater diameter than rear tyres.
The rear axle is rotating faster than the crown wheel, so the diff case will be free wheeling around the crown wheel shaft.

Rear tyres have a greater diameter than front tyres.
The viscous coupling will be driving the rear axle, trying to speed it up to match the front axle speed.

The second scenario here is what happened to my Passat. Having had punctures, the rear tyres were replaced. With them having a larger diameter, the half shafts will want to rotate slower than the front half shafts. The result is that the input plates (connected to the propeller shaft) of the viscous coupling will be rotating faster than the output plates (connected, via the locked free wheel, to the rear half shafts). The viscous coupling is only designed to cope with a small, temporary difference in plate speeds, so the rotational speed difference between the two plates, likely exaggerated by the Autobahn speeds the car was travelling at for all failures, causes the oil bath in the viscous coupling to heat up. This increases the viscosity, forcing the input and output plates to match speeds. Now we have the classic torsional wind up scenario where input and output speeds in the rear differential are different and the weaving occurs as the wheels try to slip before ultimately the weakest part fails.

As an aside, if you have followed the logic of the design of the rear differential, you may have been wondering what happens in reverse, when the free wheel will be engaged. After all, it would be nice to have four wheel drive if you parked in a muddy field when visiting a traction engine show or were camping. So VW fitted a direct drive which bypasses the free wheel in reverse. It is switched by a vacuum operated plunger, the movement of which is determined by a microswitch at the gear lever.
During the early part of the investigations I found that the vacuum filter was blocked which was possibly interfering with the operation of the plunger.

Here is the (broken) unit removed from the housing, showing damage to the bevel drive gear. Note the pegs that are used for the reverse drive lock as the large toothed ring engages with them. To the right of this is the freewheel mechanism.

Visible here is the rear differential, which allows the driven rear wheels to rotate at different speeds when cornering.


Some views of the reverse plunger, showing (left) the vacuum reservoir on the front left wheel arch and (middle) the mechanism itself. The round thing is the diaphragm which moves a small contol rod which engages the large ringed tooth. In the picture on the right I am working out how the mechanism works. The plunger can only engage the drive, it does not engage any free wheel mechanism. When out of reverse, the free wheel mechanism is engaged due to the return spring just visible in the upper picture.

D4 Root cause
The rear wheels have a larger rolling radius that the front wheels
The root cause is the actual problem. As you have seen, getting from problem description to root cause is the main engineering input.
In this scenario, which is true for many problem solving teams I've been on, I've had to rely on specialist knowledge to look at cause and effect. This method is very useful if you just want to jump to conclusions, so it is important to make sure that the root cause matches exactly the failure mode.
I will have to cheat here as normally I would need to demonstrate the root cause hypothesis is valid by being able to recreate the failure, but as I do not have such a large stockpile of differentials to hand, I'll just need to rely on validating by not having the transmission fail again.
Hence we now have a root cause. In fitting new tyres to the rear axle, the design of the system is such that torsional wind up occurred. This makes the following step quite easy.

D5 Chosen Permanent Corrective Action
D6 Implemented Permanent Corrective Action
Fit tyres with the larger rolling radius on the front axle at tyre change.
I have bundled D5 and D6 together as there is only one corrective action and it will be implemented. Sometimes there are a number of PCAs and, following testing for example, not all are implemented.
So, in future, I will need to control how tyres are fitted. As is common in Germany, the Passat has two sets of wheels, one set with winter tyres and the other with summer tyres. This usefully allows me to compensate for wear due to usage.

This is fitting the winter tyres (the steel wheels on the left - the summer tyres are on alloy wheels. The winter tyres' treads have been measured at several points each and the ones with the deeper treads go on the front.
The cardboard box is the spare set of wheel bolts as the two sets of wheels have different shaped seats. These were not supplied to the previous owner by the supplying garage, incidentally, I bought them after losing three wheel bolts and trying to figure out why. Nice to know that the main dealers are happy to let you drive around with incorrectly secured wheels.

D7 Prevent Recurrance Actions
Use tyres of one manufacturer, type and size on all four wheels.
Good companies always keep records of processes and requirements for products or services with vehicle manufacturers being exceptionally good at this, mainly because they buy so many parts in from suppliers and need to control the quality. So any information learned from a problem solving exercise can be used to update design or process requirements or even result in new quality tests being implemented.

D7 Recommendations of Prevention
More prominent advice should be offered to customers relating to fitting tyres.
Right, I'm going to apologise now for pointing the "j'accuse" finger now, but I am less than impressed with VW and their dealers. Manufacturers of consumer goods have a duty (already tested in law I'm told) to highlight requirements which are not considered as the norm for that type of goods. And by highlight, this means make sure you cannot miss the instruction. So a washing machine manufacturer cannot put a small red button on the back of the washing machine which must be pressed every ten minutes or the motor will blow up and then tell you the instruction was written on page 54 of the instruction manual. Because such an requirement is not what "a reasonable person" would call normal, the manufacturer would be required to stick a prominent label warning you to carry out the procedure.
So, how do VW deal with the issue of how to fit new tyres:
Betriebsanleitung Seite 129
Es empfiehlt sich aber, die Reifen mit der größeren Profiltiefe vorne zu montieren: Sind die Vorderräder stärker abgenutzt als die Hinterräder, haben sie einen kleineren Abrollumfang und drehen deshalb schneller. Dadurch treten Verspannungen im Antriebsstrang auf und die Reifen verschleißen schneller. Ein erhöhter Verschleiß des Allradantriebes ist jedoch nicht zu befürchten.
which, roughly translated, says that tyres with more tread should be fitted to the front because otherwise the faster rotating front wheels will build up tension in the transmission. The last sentence says that increased wear in the transmission should not be an issue. Hence it does seem to miss the point that the differential will explode on the motorway. And hiding it away on page 129? Unbelievable!
So I think VW failed here. If I was an engineer working on this issue for VW, I would also recommend
More prominent advice should be offered to customers relating to fitting tyres.
And in case any VW engineers are reading this and are saying that the issue is common knowledge, remember that the car was seen by a VW garage and two independent garages, one of which sells itself as a VW specialist, and none of them spotted this.

D8 Recognise team and individual contributions
Well, thanks John here for patiently taking me through the design of the transmission system. I owe you a beer.

Despite all the hassle, I have found the whole exercise interesting in many ways. I just wish it had happened to someone else's car. The Passat is now running fine, although there is the occasional clunk in reverse which I tend to attribute to the freewheel bypass engaging. I think one of the lessons here (for me) is that whilst internet forums can be a hugely useful facility when trying to solve problems, not all information is correct and hence it is always worth verifying in someway. Also, I think it unforgiveable of VW that not only do they reduce the significance of tyre fitting, they also allow incorrect information to get out. All it needs is a sticker near the rear wheel well and I would have saved myself a lot of time and money."