Mark,
Welcome to the forum, and I imagine you are not in the USA as the result of your E240. Fill out your profile.
Getting back... I have some questions for you.
1) Shifting from 1 to 2 okay, and shifting 2 to 3 not okay(sometimes) what about the other gears?
2) "Drives like in Neutral" I never though I could not accelerate from neutral, so what you are saying it goes into neutral? during the shift of 2 to 3, correct?
3) what about the other gears after the shift of 2 to 3 ?
4) How long was the time between the last transmission fluid change to when you started having problem.
5) Lucky 5, five times!! How long were the fluid change intervals in between these 4 to 5 times? To get ALL the fluid out. You need to do a fluid exchange to get all of it as the 2000+ 722.6's have not torque converter drain plug. On E320's this is how you get all the fluid out, see post 11:
http://www.benzworld.org/forums/w21...eveal-torque-converter-drain.html#post5406702
What I would do is what is outlined in post 2. Check the Transmission Computer for intrusion of transmission fluid as the fluid will "wick-up" the transmission electrical control cable and make it up to the transmission computer. If you find fluid at the top in the Transmission Computer area, you will need to remove the computer(disconnect the battery) and clean it with electronic contact cleaner. Clean the connector too. Allow everything to dry completely before assembling.
Look in the stickies section for the replacement of the transmission connector and the o-ring if you find fluid in the Transmission Computer. Buy a factory new part from the actual dealer, no aftermarket parts!!!
Now, my guess providing no fluid is found up in the Transmission Computer area. To check the resistance of the shift solenoids since it is an easy test to do. The test is performed at the transmission computer connector at the computer area. You need an ohm meter, see P23 attachment.
Now, if the fluid has just come out brown in all the fluid changes then the clutch packs are just worn-out, and your transmission needs to be rebuilt. The transmission has nothing to grab onto, so it is just slipping?
The 2-3 shift is the Achilles heal with all 722.6's. You could have the solenoid not working right, not making good contact, 2-3 piston(very worn-common), spring in the valve is worn or broken, pressure regulator valve can cause this, but you have problems in all gears. THE BROWN fluid will not be affected by anything in the valve body. I would swap the 2-3 shift solenoid with the 3-4 solenoid-see the 722dot6 document.
The valve body is heavy and must be torqued-on. Here is a video of valve body update. I would buy this Sonnax kit, but it will give you some insight.
I just did a fluid-exchange on 722.6 S500 two months ago The fluid had never been changed in the life of the car, and the car had 185k Miles, and after four-gallons(15 liters) of fluid. The car shifted excellent, but before the car would slip in all gears, and the fluid came out chocolate-brown.
Let us know what happens.
Best of luck,
Martin