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This , then, brings up the question of what were the criteria the MB engineers used in designing the transmission's automatic shifting "policy" so to speak. And if the transmission design was not driven by less stress on the engine and energy/fuel efficiency , then what was it driven by? Or am I missing something?
I think that you bring up a very good point. I can't really answer your question.
First off let me say that I don't have a performance monitor unit of any type. I do have the OEM engine water temperature gauge, an added mechanical boost pressure gauge, and an added voltmeter.
I tow my 2600# (all up) sailboat quite a bit. Except for that, and our annual pilgrimage to the Florida Keys most of the times my Sprinter is at medium load.
I feel that when I tow my boat it is often necessary to manually downshift on longer uphill climbs. I watch the boost pressure and water temperature. Generally when the engine needs tp supply more power my gauge will spike slightly above 20# and then settle back to maybe 17 or 18#. If my
mechanical gauge boost pressure rises closer to 20# and holds there then I know for certain that I need to downshift. I also will often downshift as a pre-emptive strike when there is an obvious long climb ahead.
I know that many times the torque curves are cited for why you need to intercede, or override the TCM program, and downshift. Around 2800 rpm is often cited as the lower number for T1N's. My observation is that my Sprinter engine also has very good torque and performance in a band below that.
The difference in engine speed between 2800 rpm and 2300 rpm is over 10%. Generally speaking, you can run back and forth across the country many more times without engine troubles at 55 mph as opposed to 75 mph. Less RPM is less wear and tear. Ciprian has over 500,000 ... 770,000 miles on his T1N by using his CSS method.
I watch my boost gauge and my engine temperature. Lately I find that I don't override the transmission program as much as I once did. I do use cruise control fairly religiously. Not even time will tell if what I'm doing is correct or not because my one vehicle being driven by one driver does not a study make.
FWIW. vic