Does it not engage at all or is it hesitating? If it does not engage, you definitely need to see a professional. If it hesitates it just may be that the fluid level is low. Either way, I do recommend that you see a professional and have the problem diagnosed and repaired. Here is a link to help you find a shop: http://repairpal.com/directory
RepairPal Certified shops can diagnose and repair your 1995 Mercedes-Benz S320. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.