i did check all the fuses under the hood, including those on the K40 relay. I have not yet looked at those under the rear seat.

Engine cranks and cranks because I'm holding the key in the start position. Cranking stops as soon as I release the key. The odd thing is that it's not even sputtering or firing at all. Not even a little bit. I'm on day 3 now of cranking a bunch every morning and again every evening, with the car on the charger overnight.

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'98 E300 turbodiesel

Reason I asked is because in a normal start scenario you just turn the key and let go and the car will start itself , my Jeep with the Mercedes om642 engine does the same .... so if no signal from sensors (usually crank sensor)within a second or two then they stop cranking.
My Jeep had my swirl motor delete resistor short out on the inlet manifold and this blew a fuse (no 16) and the ASD (auto shut down relay )resulted in a no start and the engine would keep cranking by itself until I removed the key , I obviously had a warning light and four engine codes for inlet / turbo /boost etc as the blown fuse powers those circuits.
You having no codes I presumed that there was no ecu communication which would then lead to non stop cranking on a key flick start.