As early as 1917, the American inventor Melvin L. Severy got a patent granted for a viscous fluid coupling. However, at this time the only oils available were viscous oils of mineral origin, which are badly suited for transferring high torques. That’s because on the one hand, viscosity decreases when temperature rises, and on the other hand, higher temperatures cause disintegration.
Only the wonders of modern chemistry made the breakthrough of the idea finally possible. Now silicone oil could be produced synthetically. Therefore a fluid was available which is resistant to highest temperatures and also loses very little of its viscosity when warmed. Even so, using silicone oil is at best a compromise solution, since its pseudoplasticity is actually unwanted.
Thus higher difference revolution speeds (shear rates) cause an apparent decrease in viscosity, followed by a digressive increase in the transferrable drive torque. The opposite behavior, called “dilatant”, would be ideal for the use in a viscous coupling. Sadly there are no known dilatant fluids which react similarly chemically stable to silicone oil when used under the special strains of a viscous coupling.
The basic idea was taken up again by “Harry Ferguson Developments” in the 70s and the modern viscous coupling, using silicone oil as a transfer medium, was born. In the automotive industry, the viscous coupling was first used primarily as torque converter, vibration damper and as viscous cooling fan. In the beginning of the 80s, new applications in 4WD vehicles were found after lots of development work:

The Viscous limited slip diff (VLSD):
Speed-sensitive differential lock

The Viscous-Transmission:
A middle differential and a differential lock between front and rear axle
By the way the first vehicle with a viscous-transmission was not the VW-T3 Syncro but the AMC Eagle, which is said to be the precursor of nowadays’ SUVs. It was produced between the years 1979 and 1987.



The viscous coupling is very similar in its build to a multi-disc clutch (known from motorcycles). The only difference being that the drive torque is transferred through the shearing force of a fluid rather than through mechanical friction.

The free inner volume of a viscous coupling is filled to about 90% with silicone oil.
Actually, classifying silicone oil as an "oil" is rather misleading because the word makes you think of something greasy or lubricating. Due to weak intermolecular forces, the carrying capacity of a silicone-wall-film is rather low though, which is why you could actually call it an anti-lubricant, especially when looking at the material combination steel-steel.
The input variable for the transferrable drive torque in the viscous coupling is exclusively the varying differential speed between the two axles. Concerning the transmission behavior we have to distinguish between two different modes. In principle, a viscous coupling first goes through the "Viscose-mode" and can switch to the so called "Hump-mode" in case of continuous stress.



When the housing and the hollow shaft of the viscous coupling turn at the same speed, the silicone oil is not exposed to viscous resistance. This is because the external plates in the housing are connected to the rear axle and the inner plates with the front axle. In practice this mode is never reached, because there are constantly small differences in rotation speed while driving (tire slipping, turning, slight differences in tire size and so on).
As soon as the external and inner plates rotate at different speeds, cohesion causes inner friction inside the molecules of the silicone oil, which tries to equalize the differential speed again. In this process the silicone oil is exposed to additional shearing forces at the holes and slots between the plates which are rotating against each other.
Silicone oils are pseudoplastic fluids, which means their viscosity decreases with increasing shear stress. This causes a digressive transmission behavior in the drive torque. The transferred torque depends mainly on the momentary viscosity of the silicon oil and the geometry of the set of plates. Then again the momentary viscosity depends on the base viscosity, the temperature and the shear stress.



The inner friction that occurs in Viscose-mode causes the silicone oil to warm up. Since silicone oils have high thermal expansion (about fourty times that of aluminum), the inner pressure rises inside the hermetically sealed viscous coupling. During this process it’s mostly the degree of filling (e.g. 90%) of the viscous coupling which influences the speed at which the pressure increases. In case of a steady differential speed, the contained air therefore gets more and more compressed and creates a solution with the silicone oil, until an effective filling level of 100% is reached. In this state the inner pressure rises abruptly, so that further energy input would destroy the viscous coupling, which is designed for a maximum inner pressure of approx. 100 bar. This causes the viscous coupling to go into Hump-mode. In the past the Hump effect was thought to be a dilatant behavior of the silicone oil. However the Hump effect has nothing to do with sudden changes in viscosity since silicone oil is not dilatant but instead the opposite, namely pseudoplastic.
In reality what is happening is a bit more complex: Due to a destabilizing flow, an inhomogeneous pressure distribution is created inside the viscous coupling. This results in varying gaps between the plates. What is decisive here is the production type of the plates: A rounded leading edge on the top and a sharp burr on the bottom are created by punching. The rounded edge works like a hydrodynamic lubricating wedge on which the plate floats while the sharp burr scrapes the silicone off the neighboring plate’s surface.

If the gaps become too narrow, the silicone film tears, causing mechanical friction between the plates. Hence the transmitted torque suddenly increases, whereby the differential speed quickly declines. On the road this means that a vehicle that got stuck can now either be freed or that the motor dies. The temperature falls and the viscous coupling changes back to Viscose-mode. The Hump-mode is meant for a momentary increase of traction in extreme situations, but it is also a constructive self protection mechanism of the coupling from overheating.

T3 VC flaws

The overwhelming majority of old T3 viscous couplings show the same defect during test:
That of extreme hardening.

Gebrochene Antriebswelle im VA-Getriebe
Those extremely stiff viscous couplings are practically always in Hump-mode while driving. That’s because the front differential usually reaches temperatures of around 60°C, which is why front and rear axle should be regarded as rigidly connected. The resulting tensions in the power train are responsible for excessive rubbing of the front wheels in tight curves on dry pavement. Not only does this lead to increased tire wear, but in the worst case to damages to the transaxle.
However this very common “extreme hardening” of the T3-Visco is no sign of aging but a design flaw. This specific extreme hardening of the T3-Visco only occurs when the viscous coupling has sucked gear oil from the front differential. The problem of “oilsucking” was noticed at Steyr-Daimler-Puch (SDP) in the late 80s, which is why it was also investigated in a diploma thesis.

Conclusion, Diploma Thesis, Thaller

The remarkable conclusion was the following: At wintery outdoor temperatures, a static vacuum forms in the viscous coupling due to the high thermal expansion of silicone oil. This vacuum is enhanced during start-up. Because of the vacuum the viscous coupling sucks in portions of gear oil in the cold-running phase and breaks over time. This problem does not depend on the mileage. In short-distance operation an extreme hardening of the viscous coupling can already happen after 2000 km (approx. 1243 miles). However the results of this investigation came too late in the summer of 1990. Therefore SDP did not make any further effort in solving the problem.

Test setup:

A simple test setup with a manometer shows the virulence of the issue.  

In the first step, an excess pressure of 1 bar (14.5 PSI) was applied to a viscous coupling filled at room temperature of 16°C (61°F). In order to test the absolute leakproofness of the setup, the viscous coupling was immersed in a water bath. There were no signs of leaks.
In the second step, driving in winter was simulated by placing the viscous coupling in a fridge over night, at arctic temperatures of -15°C (5°F).
As a result of this cooling the manometer showed a blatant fall in pressure. The remaining pressure was 0,15 bar (2.18 PSI). Therefore a temperature difference of 31°C (56°F) already causes a static pressure drop of 0,85 bar (12.3 PSI). After reheating, the pressure goes back to 1 bar (14.5 PSI).
The usual filling method at ambient pressure and room temperature thus means a latent danger of an irreparable extreme hardening of the viscous coupling during the winter.

Our approach:

In order to prevent the “oilsucking” caused by the vacuum, the viscous coupling is filled through a special valve.
This makes it possible to insert a slight static excess pressure into the viscous coupling. Moreover we can adjust the static pressure to the ambient temperature via the special valve.

Comparing it with the SDP factory adjustment back then, nowadays we need another filling quantity and an adjusted viscosity. Yet with help of a test bench, working out this modification is a solvable task.



T4 VC flaws

Soft characteristic

Although the T4 Syncro's viscous coupling has no design flaws (unlike the T3 Syncro), it is often at its limit when it comes to rough terrain.

The factory adjustment of the viscous coupling, which transfers roughly 400 Nm (@10 RPM) is enough for icy roads in winter, but overstressed in hard terrain like mud, sand and deep snow.
Especially heavy and tuned vehicles suffer from extended front wheel spin, until the rear axle slowly engages. Usually one is then already stuck.
Standard VC with 400Nm

Therefore we also offer Sport-VCs for tuned and heavy vehicles with ABS , which reacts much faster than the standard viscous coupling. The 3 variants (Standard, Sport, Super-Sport) are described in detail in our FAQ to make the selection for your own bus easier.

Factory and +25% (Sport-VC)
Factroy and +50% (Super Sport-VC)

Increased Hump temperature

Unlike the fast spinning viscous coupling of the T3 Syncro, the T4 Syncro uses the VC’s automatic lock function (Hump-mode) much more frequently while off-road driving. The occurring high temperature peaks in Hump-mode cause premature wear of silicone oil. Which results in an increased Hump temperature. With a new viscous coupling, the Hump temperature is about 40 °C. Due to heavy use the silicone oil can wear out, the Hump temperature can rise to 80 ° C and higher.

An increased Hump temperature may cause the bus to dig in difficult terrain, while the automatic lock function (Hump-mode) kicks in much to late when the rear axle as already holed up. That's why we set the Hump temperature carefully to the lower tolerance limit. In addition, we fill the viscous coupling with inert gas instead of conventional air to increase the temperature resistance of the silicone oil.

Rusty shaft bushing

The most serious flaw relates to the bare, inconspicuous shaft bushing on the steel cover of the viscous coupling. Because of the harsh environmental conditions on the vehicle underbody, the shaft bushing tends to get rusty. Over time the corrosion creeps down to the lips of the shaft seal, which often starts an unnoticed leakage of gear oil from the differential. Due to the rotation and centrifugal forces, it’s furthermore hard to recognize the constant oil loss.

oil mud due damaged shaft seal  

Many rear differential damages have their origin here. The practical experience has shown that the oil level in rear differentials of T4 Syncro’s are almost always too low. In some cases the differentials were nearly dry.

We fixed this flaw by replacing the rust-prone standard bushing with one especially made for us from stainless steel. The surface of the stainless steel bushing is additionally grounded twist free and hardened by a special process to prevent stainless features.

stainless steel bushings

Service life

Steyr-Daimler-Puch had the durability of the T3 Syncro viscous coupling tested in a diploma thesis under specified operating conditions. The investigations showed that the service life of a viscous coupling depends on the differential speed, the power loss and the height of the Hump-torque. On principle we have to distinguish between two separate service life- categories though:


The service life of a viscous coupling in Viscose-mode (fluid friction) depends mostly on the wear resistance of the silicone oil, since the aging of the silicone oil leads to a slow hardening of the viscous coupling. Therefore the increase in viscosity of the oil due to the oxidation of the oxygen causes a rise in transmitted torque over time.

However, this undesired behavior can only be observed with viscous couplings whose gas bubble consists of conventional ambient air.

All our viscous couplings are filled with an inert gas. This will prevent the silicone oil from gradually thickening due to oxidation.


During Hump-mode the plates experience wear-intensive mechanical and thermal stress. From a certain abrasion onwards, the plates lose their ability to initiate Hump. The Hump-moment which is important for self- protection cannot be achieved anymore. This can lead to an overheating of the coupling, causing the inner pressure to rise unstoppably.

In the case of T3’s viscous coupling, the steel cover bulges before the X-rings fail and the VC starts to leak silicone fluid. At the T4 Syncro, the steel cover bulges also at first, but after that the aluminum housing usually ruptures. In both cases, at least the defective plates must be replaced.

Torn steel lid
Torn aluminium housing
Deformed housing
Deformed plates


Hump - Service life

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