drop in voltage
It would help if you filled out your profile and let us know where you are located, and what your driving (year, model, etc.).
However, in lieu of those missing items, you might want to review the recalls and TSB's for your w211 at the following site:
http://www.alldata.com/recalls/index.html
Alternatively, take your car to the dealer and let them diagnose your problem. What's worse, a few bucks out of pocket, or not being able to charge your customers?


