My '81 is still in the tranny shop being rebuilt. (they had to get a couple of parts from the MB dealer) What I found when mine wouldn't move is the filter was clogged. I went through a couple of cycles of cleaning the pickup screen (and changing the filter once), putting all new fluid in which would fix it for a week or so, but in my case the bands/clutches were disintegrating and clogging the filter with tiny bits of friction facing material.

I'm supposed to get mine back from the shop on Friday.

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'81 300CD - 180K, cannot be killed by any conventional means
'99 Ford Escort - good MPG