Which transmissions are '89 420SEL "Plug & Play"?

[goldstone]

Just discovered this forum... A treasure for sure!

I am hopelessly in love with my '89 420SEL. She has provided me with unbelievably reliable service for over 200,000 miles, and still handles like the day I purchased her for the most part. WHAT A MACHINE!!!

The tranny is starting to get uppity though. It chirps and squeals intermittantly at highway speeds and is starting to shift a little more roughly than usual. While it may be a simple bearing or seal that's needed, I'm curious about which Benz transmissions are compatible replacements (should it come to that.)

I frequently see decent 126 trannies up for auction on eBay and would like to know which would be "plug & play" with my 420. (I know there were improvements made to the 126 Benz transmissions in subsequent years, but don't know whether these units are backward compatible.)

I will probably wind up letting the Mercedes folks do the swap (if necessary, but I'd sure like to be somewhat educated before I go in there.)

Thanks to any of you in advance for taking a moment to provide some guidance!