As you could have read in my previous posts, I was struggling with the dreaded 'Error Piston Movement' problem after the rebuild of my Q4. Also with my previous Q4 i was facing this problem.
I tried to bleed the system with the Alfa Tester MA12A module numerous times, without success.
Als the Course Piston Parameter (piston movement) stayed at 0%, no matter what I tried.
So I decided to try to actuate the solenoid by hand, to see if the piston would move when controlled manually.
To do this, I purchased a PWM controller from Conrad.
The type I used was a Kemo Power Control M171 DC 5A
I connected the PWM controller to the solenoid on the viscomatic unit, and a 12V 2.5 A power supply from my wifi modem

When running the engine, I turned up the potentionmeter of the PWM controller.
After one or two full power settings, I hear a slight 'clunk' at the diff and I got 100% piston movement on the Alfa Tester!!
Disconnected the PWM controller and reconnected the Viscomatic ECU to the solenoid.
Everything now works like a dream.
So it seems the Viscomatic piston got stuck from standing still (car stood for 4 years).
The PWM controller is very handy for debugging the Viscomatic, so I hope this helps the Q4 drivers out there.
Pictures are attached.
Cheers!