Is it possible that the measurement is wrong because of the soldered resistor an cables?
NO
When I plug in the signal cable to Analogpin 2 on the Arduino i measure about 300 KOHM.
THAT'S IMPOSSIBLE BECAUSE THE ARDUINO CAN ONLY MEASURE VOLTAGE NOT RESISTANCE SO YOUR STATEMENT
MAKES NO SENSE WHATSOEVER
When the signal pin is not connected I measure 1 MOHM what I think is not correct.(Vcc and GND are connected to the Arduino all the time but this cant be the problem or?)
I don't understand this question.
When I assume that the resistance at no load is 10 MOHM my result of 50 is not to far from the theoretic value.
Your result is the same no matter what you assume. You read an analog value with the arduino . I don't know how your are viewing that value. You haven't posted any code so we have no idea what your are doing.
Post your code.
Regardless , the value you measure is the value you measure regardless of any "assumptions" Making some assumption is not going to change the value returned by the code.
So why do i measure 1 MOHM? I dont like to dismantle and desolder all...
You measured it because the ONLY WAY YOU CAN CONVERT THE VALUE MEASURED IS BY KNOWING THE RESISTANCE IN THE CIRCUIT. You are trying to measure a physical parameter. Your code has to convert the value measured in analog counts to the physical parameter units you are trying to measure. (torque)
Many thanks for the tip with the power supply, I hope I understand it right and will try if it works!!
I would be very grateful for any suggestions.
POST YOUR CODE