Problems with the pneumatic system can cause closing assist and door lock issues.Google+
Car Problem Reports
Mercedes-Benz S320 Closing Assist/Door Lock Issues Due to Pneumatic System Problems
Mercedes-Benz S320 Problem
Average Mileage: 135,698 mi (40,000 mi - 246,887 mi)
Visitor, 1998 Mercedes-Benz S320, 155,000 mi
AUTOMATIC LOCKS DON'T WORK AND CAN'T LOCK DOORS USING CENTRAL LOCKING SYSTEM
Visitor, 1999 Mercedes-Benz S320, 165,871 mi
can not unlock any door. levers only go up half way but not fully up it almost feels like not enough air to push em up. trunk opens fine
Visitor, 1996 Mercedes-Benz S320, 144,215 mi
Intermittent door/trunk closing assist. Found that pulling fuse and re-installing would resume proper function. Currently researching specific cause. Suspect relay or switching device in system.
Visitor, 1999 Mercedes-Benz S320, 160,000 mi
Closing Assist, drivers Door Issues Due to known reason.
Visitor, 1999 Mercedes-Benz S320, 246,887 mi
I have to manually push all rear and passenger doors locks down. Then use the ignition key to lock the driver's door. I got to manually slam the trunk down and then lock trunk w/ ignition key.
Visitor, 1995 Mercedes-Benz S320, 89,000 mi
Problem began with a battery that does not hold a charge.
Visitor, 1996 Mercedes-Benz S320, 95,000 mi
Easy fix for this problem. Access pump at back of trunk and check air lines for tighness. Can adjust amount of tension applied to doors and trunk. Search web for exact fix but very easy fix.
Visitor, 1994 Mercedes-Benz S320, 120,230 mi
power locks don't work and trunk,Closing Assist/Door & trunk
Visitor, 1995 Mercedes-Benz S320, 154,332 mi
door locks dont work with key or button trunk wont open with button inside car rear backup bars in rear quarter pannels wont go down
Visitor, 1994 Mercedes-Benz S320
Doors will open atfer being unlock with key or remote