just to share with you the technical logic , if you drain the whole fluid including the one inside the torque converter, what will happen is it takes time for the new fluid to reach the torque converter and while the TC is running on dry, it starts to break the o rings and the seal specially the piston which woudl be rubbing agaisnt metal parts in the cylinder. I hope this gives you a little better idea why it is so (as per service manual for all electronic tranny) . No bad blood intended, I am here to promote technical ideas and means for a lot of us who cannot or would want to save $ in maintaining MB vehicles. My companies are based oversea, so nothing to loose for me if I share it with the owners of MB in the USA. I live in NY.