I put an '85 W123 300D transmission into my '82 W126 300SD 5 months ago and it only required minor modification to make it work. (conversion from mechanical speedo to electronic). It has worked perfectly, even without all the fancy vacuum stuff on an '85. I've put about 5,500 miles on it so far. 81 more every day!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)