I am parting out an '84 300TD. The trans works great so I'm wondering if it will fit in a '77 300D? I suppose it is a long shot but is it possible?

the '77 would be missing the components needed to properly control the vacuum modulator on the transmission

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1981 NA 300D 310k miles

The 1984 300TD is a turbodiesel and the 1977 300D is a N/A diesel. Automatic transmission are specific for the engine. It will fit physically and even if you transfer the bowden cable and vacuüm controls mentioned above, it will not necessarely work.

I live in WA, what is the mileage on this car and history of ownership... should you wish to sell the transmission i might be interested.

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1985 300D, 250k
1980 300SD, 180K

for what is worth theres a turbodiesel trans (from an 80SD) on my 81 N/A. so it may still be worth exploring, if you can get all the other pieces

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1981 NA 300D 310k miles