My new to me 79 116 SD is short shifting from 3-4. It only stays in third for a very short period of time before moving up to 4th gear. I don't have a bowden cable but I have the shift control rod coming off the valve cover and going to the transmission. I lengthened it two turns which moved the 1-2 shift upward but seems to have had little or no effect on the 3-4 shift. There is also a slight flaring on the 3-4 shift.
I haven't found this problem in my searches.
It does have a vacuum line going to the transmission but I have yet to conduct any vacuum tests because I don't know if this kind of problem could be associated with vacuum issues or not.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Looks like I was going the wrong way. Tightened it up two half turns from the original position and it's holding 2nd longer now. I'm still not sure this has had any effect on the 3-4 shift. I'll drive if for a few days and see what happens.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I was going to suggest playing with that adjustment. I don't think vacuum sounds like your issue. Check to see that the kickdown switch isn't engaged.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

After some more thread reading, I think the flaring problem is vacuum related. I've taken up the shift control rod so it is under slight tension with linkage at idle. The shift points are higher but there is still flare on the 3-4 shift with light pressure on the accelerator and a quick shift from 3 to 4. I think this means that I'm getting too much vacuum to the modulator at low accelerator pressure. Since the flare diminishes or disappears with heavy accelerator pressure, I think it's telling me that the vacuum is dropping to the appropriately low level as the rack is opened wide.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13