Well it is the other cable marked S that is the signal to make it change between 10V and 14V, and it works with only this connected so I know how to make it work in my car.
But it would be good to find out what the other cable is for, maybe it is only a communication cable for fault codes? Or something else that must be used or I will get problems?

And I also wonders if any RS6 cars use Walbro fuelpumps together with J538 ?