ASD locks the diff up to 19mph, above that speed the warning light will signal the driver of a difference between front & rear wheel speeds, but it doesn't lock above 19mph. It functions as a normal limited-slip diff above 19mph. Full details here:

http://www.w124performance.com/docs/mb/other/ASD-ASR-4MATIC.pdf

Note that as the internal clutch pack wears out, the 100% lock will gradually reduce to near zero. Refreshing the clutch pack will restore functionality but it's a big job to take apart the diff to replace the clutches. The clutches are usually shot by 100-150kmi or so, but the lifespan will vary depending on how the car was driven.