I have an 88 300 SEL and I asked the dealer for some techical info regarding the trans. I was told that the dealers do not repair these units, they replace them with factory rebuilt units. The shop manual only provides a limited amount of info on the trans. tyical of the 126 chassis. There are a number of shops on the internet that offer rebuilt units. If you purchase one from Mercedes you are looking at $2,000-$2,500 excluding installation.

Do you change the fluid and filter every 30K mi.? If I were you, I would drop the pan and see if anything has accumulated in the bottom. The fluid should be a bright red even after 30K mi.