Hi i have rebuilt a lot of these gearbox,s not just from the m class but just about everyother class of merc as well.
It,s know as 722.600.
They have lots of problems with electrics and planetry sets.
The problem you have could be as simple as the transmission case connector leaking,the oil is fored up the wring harness into the ecu which cause,s it become low on oil,this would not show up on a diagnostic test.
It could also be the pressure regulator spring in the valvebody has broken.
Hope this helps.
Richrd
rbtrans@hotmail.com
Richard, April 2008