There's an interesting side issue here, regarding how 4wd rolling roads actually work. I'm assuming the roller pairs are independent, as the applied torque will be different front and rear (although speed must be the same)..., and hence why they can tell that the centre VC is knackered..
But how are the rollers loaded? does the operator have to load them up progressively in relation to each other, whilst maintaining the same speed on each?

Surely they can't rely on the centre VC to equalise the speeds on the roller pairs?? But if that's the case, then what does it matter if the VC is faulty?...