I asked about an orange ASD light on my w124 dashboard a while back.
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):
X 1 X X 5 X X X
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 error code.
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 failedMine 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.
The German post I got my info from:
Fehlercodes:
1 x keine Störung 2 x ASD Steuergerät def., erneuern. 3 x Bremslichtschalter def., Schalter und Leitungen prüfen. 4 x Vorderraddrehzahl VL fehlt 5 x Vorderraddrehzahl VR fehlt 6 x Hinterraddrehzahl fehlt 7 x Alle Drehzahlimpulse fehlen 8 x Magnetventil oder Bremslichtschalter def, ern.Zu 4 bis 7: das Prüfgerät ist auch für 4-Matic. Wenn die Lampe 4 bis 7 mal blinkt, sind Drehzahlfühler gestört und müssen durchgemessen werden (ist aber wieder viel Text), danWechselspannungssignale prüfen, Einbaulage und auf mech. Bescädigungen prüfen, Leitungen prüfen, Evtl. ABS Steuergerät def. wenn ASD und ABS Kontrolle leuchtet.
Hope this helps someone.
Ximinez