How would I tell if the SLS is working?
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Open the tailgate and have one or two people sit there. Then start the engine, and if the rear slowly raises to level the car, the SLS is working.

In the left front corner of the engine compartment, behind the left headlight, there is a round container with the word "OEL" on the cap. This holds the special hydraulic fluid used by the SLS system.

The parts that are most likely to fail seem to be the hydraulic struts in the rear and the Nitrogen spheres just behind the rear axle. The former are much more expensive and harder to fix than the former.

If the system does not work, the rear of the car will tend to droop, and the rear suspension will feel rigid.

The AC is always a pain on older Mercedes, as is the radio antenna.

You can adapt a sedan antenna to fit the TD if the parts are there to swap.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)