The tranny is the same, for the most part, but the SD (w126) uses an electrical pick up for the speedometer, while the 300D (w123) uses a speedometer cable.

I would recommend that the tranny come from the same year w123 as the w126, so that you do not get bit be some small modification for the model year.

No idea on what can cause loss of gears 3 and 4. What condition is the fluid in?

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D