Supposedly, ASD was introduced on diesel engine cars as an option beginning in 1991. It is described in the owners manual as follows:

"The automatic locking differential engages when there is a difference in speed (slip) between the front and rear wheels of over 1.2 mph and the vehicle speed is less than 15.5 mph. It will disengage at speeds above 16.2 mph.

The yellow function indicator in the speedometer comes on whenever there is a difference in speed between the front and rear wheels of over 1.2 mph warning of slippery road conditions."

Herb
'82 240D
'87 300SDL
'92 300D 2.5 Turbo