I bought a 1980 300TD (non turbo) a few weeks ago (my first Benz) and I'm trying to sort some things out. Here's the vin plate, if you'd like to decode it.
The biggest problem is the transmission is shifting way way way too soft, causing flaring/surging between shifts. And sometimes it downshifts when it shouldn't and should not downshift when it should. And I swear that I'm only getting 3 gears, but I'm not sure which gear is missing. Causing some real drive-ability issues.
The guy I bought it from, who only had it for a few months, says it was shifting fine until he took the valve cover off to replace the gasket. Must have not connected the linkage up correctly when reassembly, he says.
So the first thing I do is trace the vacuum lines to see if there's any leaks, and it appears that the Vacuum Control Valve is completely missing! The vacuum diagram decal on the radiator support shows that it should be there, and there is even a spot on the injection pump for it, with less corrosion in that spot. So it is straight up missing.
Instead of a vacuum line going to the transmission, I notice there is a metal rod. I compare this to photos of other 1980 300td engines and that metal rod is not visable, here's an example, and another, and another.
Now I'm starting to wonder if a previous owner swapped in another transmission. So I find the numbers stamped on the left side of the transmission and compare it to this web page. But there's no match! The code stamped on the passanger (left) side of the trans is: 115 270 1701008958. The website implies that I should expect a "722" number somewhere in the sequence.
Here's some more photos of the trans, I'm not sure how helpful they'll be.
Here is a closeup of what I think is the kickdown solenoid. That is an electrical wire going to it, not a vacuum line.
...
The internet hive mind is telling me that this could be an early 722.1xx MB transmission.
Either way, whatever the damn thing is, it has physical linkage instead of vacuum controls. So I'll need to adjust the linkage.
Here's the Hayne's manual entry for adjusting the linkage:
I guess if that doesn't work then I'll have to try adjusting the modulating pressure adjustment spring, as described here: https://www.benzworld.org/threads/diy-w123-transmission-diagnose-and-ad…
UPDATE:
I was able to get it mostly fixed by adjusting the pressure rod such that it's in the middle of its throw when the throttle is off and engine is idling. This fixed the surging/flaring problem, to be honest I don't know why, however now the shift points are higher in RPM, and there is still only three gears. Maybe this is a three speed and not a four speed transmission, or more likely its just not using first gear at all and I don't know why. Probably should just be replaced.