I had a similar problem with my 95 S600. The ADS caution light would come on as soon as the steering wheel was turned, but only when the car interior was warm. Turned out to be the Steering Angle Sensor. The good news is that your system is defaulting to firm when the caution light comes on. That at least means that the system is still functional in "soft" and "firm" modes. I'm not liking the fact that you can't pull a fault code. There is a way to manually check all the components in the system, but you need a special MB breakout box to do this. Or, if you're a crazy nut like me, you can make your own and test each component. There is a very good test procedure listed on Alldata. I attached a link to a page in my web site that covers ADS. You may find it useful...
http://v12uberalles.com/ADS.htm