Helmuth, sorry about the delay. I am on the road this week and not much time for access to XO. (I could buy you a beer if you happen to be in Vancouver tomorrow? :-)

The torq converter has a clutch in it that bridges the turbine and if locked it becomes a fixed link between the engine and the gearbox thus eliminating the slippage in the TC. It locks under certain conditions and is regulated/controlled by the transmision. I am not an expert on this and I do not know what failure mode the diesel torq converter has as it might be different from the V8. (in my V8 the torque converter had a failed bearing)

I don't think there is a level sensor for the transmission fluid. Not sure if the diesel transmission has a dip stick or is like the V8 auto and needs to be checked under the vehicle.... Maybe Phil can pitch in?

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.