Frage HELP, HF500 Spindle RPM off when using WinPC-NC

Hello,

After tying successfully making a few wood workpieces on a 820 Black Edition I wanted to try some aluminum. So I needed higher RPM's.
Only to find out that the spindle WONT go over 60% of it's max rpm when controlled by WinPC-NC USB. I've set the code to "S17000 M3"
If I try the controlbox to operate the spindle it CAN go over the 60%.
Also if I try to set the RPM to 4000 the % on the controller does match the RPM, so I the G code states "S4000 M3" the controller box wil indicate 20%, so that seems pretty accurate.
One other really strange thing is that if use the WinPC-NC "signal test" function and go to 10V (Max) the controlbox will indicate as 60% In the area of 8.4V the "Length Sensor" Light turns red.

I've no clue what I'm doing wrong. And I hope there is someone here who could help me find out what I can do to make it work.
I you need additional information about the settings please feel free to comment.