Damaged splines of VC housing by TDI engines?

Meanwhile it’s common, that the splines of the VC housing are a weak point and often worn out. Effects of worn splines are not serious in the first place (except for some tictac noises), but at some point the gearing is totally sheared off and the 4WD becomes a 2WD.

Good splines

Damaged splines

Practically we haven’t seen such an example yet, but we will probably see such damage in the next years.

It was suspected that high-torque TDI engines are responsible for fast wear of splines. Until recently, we only visually examined the splines and “felt” the backlash by hand – not a satisfying situation. That’s why we developed a special measuring tool, which measures the backlash in degree of arc. Now we got reliable data for the first time:


From our tested sample of 67 VC’s have got 7 a backlash greater than 1° – what we see as the upper wear limit. The minimum value is around 0,4° btw.


The 7 worn VCs got not special similarities:

  • previous owners had all different engines: Diesel (JX, AAZ, TDI) and Gasoline (DJ) – so the TDI engine can’t be responsible
  • the VC’s come with different years of construction – the assumption that the later ones, which were partly produced in Italy, can be also excluded

The only similarity that can be seen, is that these 7 Viscos were heavily used. Our guess is that housing splines primarily suffer from tension on asphalt due to extremely hardened VC’s.

To see that over 10% of old VCs have these worn splines, was even to us astonishing. Good VCs are becoming quite rare unfortunately.


TDI-engine needs TDI-Visco

You may wonder why we started to offer a special VC-setup for stronger engines (TDI, Subaru…). What looks like a sophisticated marketing strategy was actually born out of necessity:

We started to rebuilt VCs with original Steyr-Puch setup, and testdrivers used to be happy with it. Also we had superior experiences in tough offroad situations (sand, mud..) with our own AAZ-Syncro.

A German Syncro driver went on a Sahara expedition in Summer 2017 with our VC and was rather unhappy. He sent us videos which showed the rear axle digging in sand, while the front axle was standing still. After 30sec the front axle kicked in, but it was much too late.



So he claimed that our VC isn’t working well, and we thought that maybe something is wrong with our VC. After his expedition he sent us the VC back to Austria and we put it on the teststand. Surprise, surprise the VC was working perfectly – and showed practically the identical test chart as before.

VC3076 before / after

After that we asked the German Syncro driver about his engine, and he told us that he is driving a special TDI-Conversion (AFN mTDI) with around 280 to 290Nm and big 16′ wheels. That’s the point when we looked into our documents and started some calculations…

The Steyr-Puch VC was once designed for the 1,6 TD JX with 138Nm. The stronger 2,1 WBX has around 170Nm – but at higher RPM (2800 instead 2500). So the worst thing (worse in case of transferring power) for the VC is high torque at low RPM  – but that’s exactly what a TDI does (RPM 2000).

Originally the torque from the 1,6 TD JX engine goes 60% to the rear and 40% to the front (measured by W.Peschke in his dissertation). All additional torque will just increase the torque on the rear axle – especially when it comes at low RPM.


So we did some calculation and made another VC setup for the German Syncro driver with 70% more torque (210Nm@75RPM). Customer is now happy and did some tests in sand.

VC3131 TDI-Setup

A few months later, Tanja followed Facebook’s call for help from globetrotters “Resfeber”. Their VC showed little effect in the Mongolian sand and the faced several times the risk to get stuck.

The community agreed that their VC is probably “failed open”. We opposed this and assumed that their strong TDI engine causes an inappropriate distribution of driving forces. At the same time we offered our help and sent them a specially adjusted TDI-VC. After we got their old VC to Austria, it turned out that it is even at the upper end of the standard Steyr-Puch tolerance.

The short video clips shows clearly the difference:

We are driving currently the same setup in our own Syncro to get an idea about the increased binding in turns on tarmac. At the original Steyr-Puch VC setup (for JX engine) the minimum torque is around 120 Nm. For a 1Z TDI, we recommend increasing VC’s torque to approx. 180 Nm (+ 50%). We think that’s the upper limitation for driving without a decoupler in urban traffic. But that’s just our personal opinion – someone can see it differently.

But one’s for sure: Since then, we always ask customers about their specific vehicle data (engine / tires / area of application) to give them individual advice about the ideal VC setup.


Selecting the right gear oil

We are frequently asked which gear oil we recommend for the T3 Syncro. The official VW guideline from the ‘80s speaks of 75W90-GL4 for differential & transmission.

 But our recommendation is different:

Differential: 75W90-GL5

For the highly stressed hypoid teeth, in which the crown and pinion touch only along a small surface area (extreme surface pressure), the GL5 standard is the optimum. For this purpose, the base oil is provided with numerous additives.

 Manual transmission: 75W90-GL4+

Although the same hypoid gear pattern is also present in the manual transmission, consideration must be given here to the synchronizer rings of the gear wheels. Because GL5 lubricates so well, it can cause the synchronizer rings in the transmission to slip and partly lose their function – which can cause gears grinding and wear to occur when changing gear.
In addition, the additives of the GL5 oils can attack the non-ferrous metal component (Molybdenum coating) of the synchronizer rings.

But there are also oils with the unofficial name GL4 / GL5 (GL4 +), which manage the balancing act between the hypoid gearing’s ideal lubrication and a residual friction for synchronization.


API ratings

Gearbox oils are classified by the American Petroleum Institute (API) using GL ratings. The higher an oil’s GL-rating, the more pressure can be sustained without any metal-to-metal contact taking place between transmission components.

Designates the type of service characteristic of spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. Gear oils to API GL-4 are today typical representatives in transaxle transversely mounted gearboxes.

Designates the type of service characteristic of hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions. Gear oils according to API GL-5 are today preferably used in differentials.



After a VC fails closed, it then fails open.

We suspected this issue for a long time, but now we found evidence to support our theory.
But let’s tell the story from the beginning.
A customer brought us his old Viscous Coupling (VC) in an unknown condition to our workshop, After the first visual inspection, I suggested to the customer that we could actually save the time on the test-rig, because fragments of the upper X-ring were already hanging out of the lubrication hole of the inner hub (see attached photo). I assured him that his VC had failed open (no drive) and would only reach about 50 nm friction moment.


But when the test-run started, we were all very surprised! The Viscous Coupling was not failed open, but closed (no slip at all)! It had sucked oil inside.

After testing all VCs from my own stock with obvious X-ring damage, I found one more VC in a similar condition.

This Visco clutch shows an advanced stage of failing open. The lowered moment indicates that some silicone oil has already leaked out and the Visco will soon completely be failed open.


Our Theory:

The constant hump pressure (>10 bar) in the oil-sucked Viscous Coupling causes the rubber of the X-ring to creep and slowly be pushed into the gap. As a result, the X-ring can no longer create a seal.

To cut a long story short: After a VC fails closed, it then fails open.