Problems with the pneumatic system can cause closing assist and door lock issues.Google+
Car Problem Reports
Mercedes-Benz 400SEL Closing Assist/Door Lock Issues Due to Pneumatic System Problems
Mercedes-Benz 400SEL Problem
Model Year Affected: 1993
Average Mileage: 211,559 mi (178,000 mi - 271,000 mi)
driverside door assist does not work,only driver side door will lock auto.
Visitor, 1993 Mercedes-Benz 400SEL, 178,000 mi
The Pneumatic close assist system is very problematic. I have repaired this at all 3 San Diego MB dealers with no success..