Hi folks. A thorough search turned up some threads discussing this, but the configurations were different from my '91 500SL. I have a leak in this area in the front driver's side area of the trans.



I replaced the O-rings in the kickdown solenoid, but I then discovered the leak is coming from above, around the overload protection switch. It seems to be running down the back side of the plate that the switch is mounted through.



The switch itself is fine, it doesn't leak between the plastic and the case.


I replaced the little O-ring on the end of it, but I'm still getting fluid in this hole that the switch came out of.



The fluid is then leaking past the seam where the threaded part meets the plate. The seam doesn't have any kind of seal, so I'm assuming it's not meant to hold back pressure, and there isn't supposed to be fluid in that hole.





The diagrams I've been able to find online and the dealer parts counter has don't describe what's going on in here. If I pull out the lock ring, and unscrew the hex nut, is there a seal in there that could be causing fluid to get in there? I'm a bit out of my depth here, and don't want to mess up some adjustment or release a bunch of small parts that will turn my leaking trans into a broken trans. Does anyone know if this is a possible point of leakage, and what is required to fix it?

It looks as though ATF is getting past the o-ring in your third picture. It would never make it to the seam or the threads if the o-ring was containing it, right? I'm thinking that the o-ring either isn't thick enough or you may not be getting it to seat for some reason.

Thanks for the reply. I replaced the O-ring with the official Mercedes part from the dealer, it's thick like the old one, and fits perfect. It goes in to a huge notch, and would be impossible to install wrong. That's why I'm thinking the fluid has to be finding another path, and the only thing I can come up with is something to do with that hex nut with the hole where the switch and it's O-ring goes in.

What series transmission is it? I may have a service manual that covers it. From what I see in my manual, the body of that block is held in place by the hex nut. Inside, there appears to be 1 or 2 more O-rings to seal the block to the transmission.

Thanks very much. It sounds like I can remove that hex nut and check without opening a can of worms. That block is actually secured by the overload switch itself. It's loose at that seam, and being held in place by the tubing.

It's a 722.3; The exact MD is 722353 03 538026. The parts counter guy said it looks like there was a mid-year change to the overload switch, which is probably why the one I purchased is wrong. But like I said, the switch doesn't look like it's the problem, and the O-ring wasn't it either.

I have a detailed diagram that shows everything but the block (that only came on some models). However, I don't know how to convert a PDF file to Jpeg so I can post it here for you.