My 1980 N/A 300D does not have the bowden cable but instead all shifting is performed via vacuum adjustments. On my 1975 the shifting isall linkage controlled on the passenger side of the motor. There is a cross over rod that connects it to the throttle.
Both my 81 & 84 SDs have the bowden cable and

On the 1980 it is a balancing act between the vacuum bleed valve adjustment on the back of the injection pump and the modulator T-key adjustment on the tranny itself. I have what I think is too firm of a 1/2 shift but this is what I settled for in order for the balance of the shifts to be acceptable by my standards.

Do you have the black vacuum line running from the bleed valve on the IP to the tranny? This is common to both my 80 and newer models. I have not had much chance to really get into the ffine tuning of the newer tranny with the cable so I am afraid I am not much help there. I do have mucho experience with the 1980 and can help out in making adjustments to this model if in fact that is what you have.
Hope this helps...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique