The engine(s) are described in a paper by John Blazer:

THE ENGINE

The problem engine is a 3,449 cubic centimeter displacement in-line
6-cylinder turbo-charged diesel engine. Diesel engines were first used
in the W-116 300SDL S-Class beginning in 1978, continuing through 1980.
That engine was a 3 liter 5 cylinder turbodiesel, model 617. The W-126
S-Class debuted in 1981 with that same engine, which was changed to a
3.0 litre 6 cylinder (the 603.96x) in 1986-87. This same 3.0 liter
engine was also used in the W-124 300D and 300TD in model year 1987.
These engines are NOT the problem. That engine block and head, however,
were the starting point for the 3.5 liter version. For comparison
purposes it will be helpful to know how many of these 6 cylinder 3.0
liter versions were in use:

Year Body Model Engine Model # Sold

1987 W-124 300D/300TD 603.960
6,022
1986-87 W-126 300SDL 603.961 13,830

Source: Robert Nitske's book "Mercedes-Benz Production Models Book,"
Motorbooks International.

MODELS INVOLVED

The problem engine was first installed in 1990 and, with minor
modifications, was used through 1995. The models, with number of cars
sold in the U.S. and the list price, are as follows:

Year Body Model Engine Model # Sold $ MSRP

1990 W-126 350SDL 603.970 855 56,800
1991 W-126 350SD/SDL 603.970 3,914 53,900/57,800
1992 W-140 300SD 603.971 1,131 69,400
1993 W-140 300SD 603.971 1,005 69,900
1994 W-140 S350 603.971 672 70,600
1995 W-140 S350 603.971 425
65,900

Source: MBUSA Web Site, 12-Year Model Overview