No vibration on Dreamcast Concept 4 racing wheel

Hi there,
The main question is about the vibration detection on the dreamcast.
I tried the interact concept 4 racing wheel. I have two units, one is fully working including the vibration feature, the second one has no vibration detected (in any game, the best example is F355 because it shows both controllers on the same screen, so you can be sure it's not a game or console problem).

I disassembled the two racing wheels, then I swap some parts to find the faulty part.
Also, I tried to plug an external rumble pack in the vmu port (on the faulty racing wheel), the vibration option appears and the rumble pack works. That means the connection and logic with the console is ok.
But the internal vibration is not working. By the way, if I plug a 5V source on the internal vibrator, it turns.

Here is a picture of the "faulty part"
http://novatux.com/share/renzo/fichiers/download/PHOTO_20170618_171409.jpg

I checked many wires, and I didn't find any clue yet.
But I would like to understand the logic, how is detected the vibration feature on the dreamcast ?
I could bypass the logic of the vmu port to use the internal vibration ?
Thanks for your help