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Chris,
I have fallen foul to the VC/rear diff problem and, when checking back, realised that the occurrences were after punctures where one (winter tyres) or two (summer tyres) were replaced. I have been working with a differential engineer and cut open the rear differential in an effort to finding out what is going on.
The conclusion for larger (i.e. new) rear tyres is how you describe. There will be a permanent speed differential in the input and output plates of the VC which will cause the silicon oil to heat up, which means the input tries to drive the output faster than the speed dictated by the rear wheels. The resultant torsional wind up manifests itself by breaking the weakest point in the transmission, in my case the bevel gears and VC itself.With larger front wheels I can’t see how the engaged freewheel would fail. The system is similar to motorcycle starter clutches (but with eleven pins, not three), and motorcycle starter clutches spend almost all their working life in freewheel mode. Can you explain the reasoning as I was going to just fit the new tyres to the front?
There are also open questions I was hoping you may know:
1. I find it hard to understand that VW engineers did not foresee this problem, given the testing that is carried out. I have seen nothing in handbooks or workshop manuals explaining the tyre issue. Is this problem therefore a by-product of oil deterioration in the VC which alters the characteristics? Normally oils get more viscous (runny) with heat, which should mean a lower inclination of the input plates to drive the output. In other words, should I fit a brand new VC?
2. The manual states that the freewheel engages automatically when the brakes are applied. I see no automatic function. Any idea why the manual makes this comment, given that they are derived from VW worksheets?Oh, and hello to everyone.
