Status report: T4 Syncro Viscous Coupling

As recently requests for overhauling the T4 viscous coupling are piling up, we want to give you a short overview, what we are currently working on and what the main challenges are.

Basically, overhauling the T4 viscous coupling to VW standard is already possible.

If you use the bus as intended by VW, as an on-road vehicle with off-road capabilities, you will again be happy for many years with your Syncro. In fact this also shows the data of our long-term testdrives.

However, if you intend to use your T4 Syncro as an expedition mobile and challenge your bus often off-road, then two problems will occur:

  • The original adjustment of the VC is too soft – especially for the powerful TDI engines. There is too little torque being transferred to the rear axle. Even a fresh overhauled VC (according to VW standard) can give the impression of a defect.


  • The silicone fluid ages under hard load, and the VC starts to get softer again. This effect results from the high temperature peaks in hump mode, which occurs quite often (unlike the T3 Syncro) at offroad-conditions. Even the Co2 filling can only limit the damage.


Currently, a dozen of our T4 VCs are running in test vehicles. Three of them, which were used in different ways in hard terrain, should exemplarily demonstrate the problems that occur. The recorded curves show before / after measurements of the tested VCs:

 VC4005: T4 with gear reduction +9,6°C Hump temperature after 10000 km

VC4006: Expeditionsmobil (Lübeck to Kirgistan) 25tkm on Snow/ Gravel / Sand

+7°C Hump temperature after 25000 km

VC4010:  Offroad-Bus, always at Limit

+7,5°C after 5000 km

We are currently working on the problem of premature aging of the silicone fluid. We are testing other inert gases, anti-wear additives and silicone oils with solid additives. Meaningful data can only be generated during extensive long-term tests, the testing is still ongoing and will take some more time.

Off course, we could already offer overhauling the T4 viscous coupling for mainly on the road T4 Syncro drivers, however, we decided not to. Our approach is to get the best and lasting technology for all Syncro drivers and furthermore driving habits can change eventually.