Hi, when changing seal on the diff i noticed the nut beeing loose,, it was pinched down but could undo with socket in hand. Diff have never been messed with last 26years and if the torque was original 90nm what happend here, there was no movement in out on the pinion and no sound from it, what to do know?

What's your question? Just torque it back to spec and call it a day. If you're concerned with it loosening up either use a thread locker or a nylon lock nut.

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1990 190E 3.0L