Job finished - just a few hiccups, but nothing serious
First snag was after unscrewing the overload protection switch - the rest of the barrel didn't come out. Just to make sure, I hand screwed the switch back in (hoping to re-engage it) and then pulled it out again- no luck. I put a 90 degree pick tool in there (at an angle), lifted the tool up perpendicular to the barrel and was able to grab on and pull the barrel straight out. More ATF came out than I was expecting! Had to clean that off the driveway before the Mrs sees it
This is a 722.5 transmission on a 1994 SL320. I really like the M104 after digging in and fixing up a 1993 300E 2.8 (guides here were massively helpful). I checked CARFAX and had a VMI pulled... the transmission was replaced in 2018 by Mercedes-Benz of Flagstaff (VMI says "remanufactured transmission") and I can confirm the serial doesn't match the data card. That said, the overload protection switch had a date stamping of "1207" (December 2007). I'm guessing they just put a used one in (versus actually rebuilding).
Here's what the old switch looks like pulled off. It was corroded bigtime which explains why it was so difficult to pull off
I followed the guide everything went as expected
Reinstall was easy. Hand screwed it in until I couldn't turn anymore - then pulled out the big daddy torque wrench and the new 36mm 6 point socket. Torqued it to 70 newton meters
The electrical connector was interesting. You can't slide the boot off (or at least I didn't see a way to slide the new boot on) without removing the plastic fittings and the electrical caps themselves. I cut the existing caps, pulled the plastic bits off, and put the new kit from Kurth Classics on. You just have to think out the order of operations - putting the brass fitting (and washer) on first, then the rubber boot, then the circular plastic piece.
After that - you get to do some soldering under your car in a super cramped space

That was definitely tough not only due to being 40 and having a "dad bod", but because I suck at soldering. My only real experience is replacing the saved game batteries for old Nintendo games (NES/SNES).
Being under a car in a cramped space is hard enough- but the caps used are hard to hold while you're also trying to hold the soldering iron and the rosin core. Would be a piece of cake with three hands! You can't be messy either- otherwise the squared plastic fitting won't fit. The two plastic fittings sandwich the cap into place (circular fitting on top; squared fitting on bottom and held in place by 2 small screws which go through the top of the circular plastic fitting).
It was at this point that I realized I don't know which pin (on the switch) goes to which wire. I had to look at old photos... when you look at the protection switch dead on, it says "Moto Meter" stamped at the top. My pictures were showing brown going to the right pin, black going to the left pin. When I removed the old parts, I didn't bother to realize the plastic bit has the polarity on it (+ and -)... but usually black is + and brown is ground / neutral so I wired it that way.
Definitely not the happiest with my soldering job - but I think it'll work. If it doesn't that's another great opportunity to try all this again
After wrapping it up, I hand tightened the brass fitting, tucked the fittings into the boot, and then re-zip tied the electrical wiring (had to cut the zip lines to bring it down enough to solder). Here's the finished product:
After cleaning up, I hopped in the car and just put it into gear (reverse first, then drive). Both engaged which is a good sign. But I think this is used for shifting from 4 to 5? Will have to re-read the thread to understand more about this (any info appreciated). Would be great to have a way to test this and verify it works (or doesn't work!)
Will update once this has a chance to bake for a while
