I chased the same problem (no transmission activity in any position) on a 300E but eventually failed.I got into the valve body and found a broken K1 and broken B1 spring. I even removed the valvebody completely and inspected the internal linkages but found nothing. The broken K1 and B1 spring did not fix the problem. The pump was pumping but I dont have a gauge but my theory was a pressure relief valve in the valve body had stuck open but I found nothing wrong with the valve body. I eventually resolved to telling the guy it was deeper in the transmission than I want to dig and he should find a good used one. I recommend removing the pan and valvebody to see if you find anything however I did not.

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1981 300SD 512k OM603