Okay I searched "300SD transmission ratios". Waded through 3 pages of results here, on google I mostly got foreign alphabets.1. What are the ratios of the 4 gears in the '80 300SD auto transmission? 2. The owner says it starts in second unless the full throttle switch is depressed, is that right?3. He says 4th gear might be overdrive, is that right? 4. Is that a torque converter or a fluid clutch behind the flexplate? Is the converter ratio 1.4:1? 5. Is it a lockup converter? 6. Is the transmission valve body separate like a Ford, or is it cast into the transmission case like a Chevy? I can't drive the car, the bolts broke the edge off the flange of the back driveshaft coupling.

There are only 4 gears. No overdrive. 4th is as high as it goes.
Yes, it is a torque converter.
I'm pretty sure the valve body can be removed.

That is all I can answer. If you do need a working transmission, I have one from my 80 300SD that I parted a few months ago sitting in my garage I can sell to you for cheap. It shifted great and I had no problems what-so-ever with it.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

what Graplr said. Also- the W116 300SD starts in 1st, but with the aid of an electromechanical device in the passenger footwell that senses when one take's one's foot off the brake and downshifts to first for the start.

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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 (??)