Hi all!

So my beloved 85' w123 wagon transmission has finally decided it doesn't want to live anymore after 500000km. I have been working to find a solution for the problem, which of course requires me to do a ton of research on the different transmissions, problems and fixes, and I think I have myself figured out now, but I would still like to confirm what I've found with this ever so helpful community.

So anyways, when my transmission first went I immediately went in search of a parts w123 to salvage the automatic transmission from that. Coming from a background of working on Volkswagens, where every part within like 20 years of each other works fine across all platforms, I found a 79 300D non-turbo, and bought it like an idiot.

I pulled the trans, and even took it to a euro transmission shop to get it re-sealed and inspected. So come time for me to put it in my wagon, I get under my car and immediately notice the transmission cooler lines are in a different spot. Lord what have I done.

After doing some research (after the fact which was probably my stupidest "mechanic" mistake of my entire life) I found that I have a 4spd 722.1 trans, ready to go into my car that is meant for a 4spd 4Gtronic.

So now I am weighing my options, and I am unsure of what to do. My transmission clunks badly and has burnt black fluid, which is why I am sure the transmission is bad. The other problem is that the transmission shifts into 4th gear pretty much as soon as it can which makes it incredibly slow. This concerns me because I feel it is quite likely one of the various complicated, hard to fix vacuum controlled transmission shift systems giving me grief also. (such as the VCV).

So what should I do?

Should I keep looking for a good 4Gtronic transmission, or retrofit the 722.1 into my wagon? How much work would it be to make that happen, is the vacuum control system different and would I need to swap over (and likely troubleshoot and fix) the system from the donor car, or would the likely non-working system from the 4Gtronic wagon work with the older transmission? are the gear ratios different? Are the torque converters different? Should I get the torque converter rebuilt before re-installing the resealed transmission? is this even possible at all?

The other option (which I would ideally love to do) would be to find a 4spd manual from a 240d and just do a full manual swap. I will have the pilot bearing issue to deal with, but that might not be so bad compared to switching auto transmissions. Other problem is these 4spd manuals are even harder to find than the 4Gtronic. My last option is just buying the 6spd swap kit from MBZ 4x4, which costs almost as much as I paid for the car

https://mbz4x4.com/manual-6-speed-transmission-shifter-kit-for-mercedes-om616-om617-diesel-in-w123-w460/

So yea...

some insight and internet opinions would really be awesome right about now, as I have gotten myself into a bit of a pickle.

If it means anything the engine is coming out because I am going to change every gasket on this engine to stop these pesky oil leaks (including head gasket) which will make any fabricating I need to do much easier

Thanks PeachParts!