Just a quick question.
Will low oil level (500ml too low or thereabouts) in the ZF box generate a "gearbox fault" message?
EDIT: It is not a 28 but a 26 box. Sorry but I can't edit the headline.?!

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Possibly. Need to source leak, repair and get the trans topped off to spec ASAP.

 

Well, the loss was caused by myself. I saw the filling procedure and decided to see if my box was full.
About half a liter drained out. I didn't drive the car before I had oil leaks on a cracked power steering supply hose AND a loose hose on the pressure side, and in the process of repairing that, had the car on a ramp on the one side for a couple of days. After the repair, I drove the car a couple of hundred meters. The gearbox fault appeared. I initially thought the message was caused by the car parked at an angle, but it did not disappear. I drove the car 500 meters and returned it to the garage.
I then decided to repeat the oil filling procedure, and carefully selected R and D a couple of times, left it idling in P and measured temperature etc, and I have re-filled about 500ml, and it might even take a bit more.
In the process, it also seems to have removed the "gearbox fault message".
I will check all again today after cooling down, and then see.
i find it absolutely strange that there is not a simpler way to fill these boxes? It smacks a bit of German "overcomplication"
Which reminds me of a Polish Friend of mine. One in his family was an aircraft mechanic. He was forced to work for the Germans in WWII, and one of the jobs he had was to fit wings to German planes. He said that two man would hang on a long spanner tightening the bolts and nuts holding the wings in place. The plane would then perform a test flight, with certain prescribed high G maneuvers, and return to be have wing screws and bolts re-torqued, a lengthy procedure.
As the war went on, that area in Poland was taken over by American forces, and he started to fix the American planes. Fitting a wing here consisted of fitting the wing root in a receptacle in the body and securing it there by knocking a wedge into a slot with a big hammer.
Job done.! No testing, stressing and come backs! (I even think that wedge secured both wings in one go?)

Well, just one more. In Stuttgart in Germany, there is a Mercedes museum. One of the items exhibited there is an (cut open) aero engine from WWII. A striking feature are the cam lobes. They are almost square! That would indicate fast opening, great performance and also lots of wear! A friend who is engine designer, asked the guide what was up with these cams. The guide said "performance" and that the design life of the engine was 30 minutes! (IIRC) My friend looked shocked, then the guide added, "the expected lifetime of the pilot towards the end of the war, was 5 minutes!"

And to answer my own question, will too low oil level trigger a "gearbox fault" message.?
It will. My warning message has now gone again, after filling about one liter of ATF into the box. I suspect the measurement is not oil level as such, but rather some form of pressure, which will fall, if there is no ready supply of oil on the suction side of the pump.
I have just done about 15 km out and 15 back. On the out a couple of hesitant changes and a couple of clonks, and the box did not get into top. A short pause for something to eat, and then return. Far better shift on the way back, and the box went into top gear. So it seems to get used to the higher oil level/pressure now. Still going carefully, but forced shift seem better than nudged ones!