Efter ungefär 4 sekunders sökande på google fick jag fram detta, sen kan du precisera dig lite mer i topic som typ "Defekt ASD på mercedes 300" sen är inte årsmodell skit dåligt att veta heller.
"I found a post on a German messageboard that described the diagnostics
procedure, I thought I'd share it in case someone encounters the same
problem, it seems common.
There's a diagnostics connector just next to the battery. If you remove
the cover, you find 8 terminals in the following arrangement (they're
supposed to be numbered, mine weren't):
To start the diagnostics connect terminal 1 to terminal 5 (ground) for
about one second, with the ignition on. The ASD light on the dashboard
will go out for two seconds and start blinking. Count the blinks for the
1 No error
2 ASD control unit defective
3 Brake light switch defective
4 Front left wheel sensor failure
5 Front right wheel sensor failure
6 Rear wheel sensor failure
7 All wheel sensor failed
8 Magnetic valve or brake light switch failed
Mine blinked 8 times. The valve thing seemed much more expensive and
difficult than the brake light switch, so I replaced the latter for 14
EUROs. I connected the 1 & 5 terminals for 15 seconds to clear the error
code. The light stayed off after that and when I tried the ASD
(Automatic Slip Differential) in a mud pool it seems to work well."