I have repaired many of these, and the culprits on this is the clutches are so far worn that the clutch pack cannot be pressurized, so the transmission slips..

The speed sensors fail and you get this hard limp mode 2nd and reverse-No slippage

Zulfigar is right, but the one o-ring that cooks hard is the torque converter lock-up solenoid....

If you want to fix this cheap, u need to do some parts swapping and inspection.

I would drain all your fluid on a new clear paint pail from the home center...Inspect that fluid and the pan

I would drop the valve body. Inspect the the solenoid seals.. The hard one will be the torque converter one-You cannot buy these seals by them selves. You need to buy a transmission rebuild seal kit for $90 bucks to get the seals, or new solenoids...Under the two larger solenoids are screen filters, some do not have the filters...I have never seen plugged filters...

Now, a failed speed sensor(s) will look like this-Post35-it will be ruptured..

http://www.peachparts.com/shopforum/tech-help/325722-722-6-transmission-refresh-diy-questions-3.html

The intelligent way to test the speed sensors is with a dual trace oscilloscope for the odd pulse.See attachment

Go to the self serve junk yard and get the lockup solenoids, and torque lockup...

Inspect the springs in the valve, but the regulation spring is critical...They break..

Photo DIY- 722.6 Conductor Plate and Regulator Valve Spring R/R - Mercedes-Benz Forum.

Hope it helps,

Martin

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