Can't believe no one's mentioned the 800 lb elephant in the room.

There is an electrical conductor plate above the valve body that has an external connection to the TCM through a harness and a spacer/adapter/connector located at the right front of the AT.

The connector is known to leak fluid and sometimes it will travel up the harness and flood the TCM, with lots of unhappy results.

So I'd suggest ordering the connector (very cheap) and plan on replacing that after you've drained the fluid (once you drain the fluid there's less leakage) and if the harness end is wet when you remove it from the connector, then go open the TCM and see if there is fluid in there. If so, remove it and clean it, then flush the harness with some electrical spray (with it disconnected below it will rinse out) and let dry before reassembly. Usually the TCM survives.

Additional notes: use MB spec fluid 236.10 or higher, unless you know what you're doing just get it at the dealer (regular auto parts stores will not have it). Buy 4L, you'll lose a bit over 3 when you drop the pan. You also need a dipstick tool, sold on ebay for about $25 with shipping. An inexpensive infrared thermometer will help you know when the fluid is at 80c for final level set.

Only bad thing is that you can't clear trans codes with a regular code reader. You'll end up paying a shop to clear them ($50-150 depending on the shop and market). You might consider changing the connector yourself (and cleaning the TCM, etc.) and taking it in to have the fluid changed, that may save a bit considering you won't have to buy anything.

Outstanding FAQ with details, pictures, links at Benzworld in the W210 forum DIY stickies.

Good luck.