TAC is the acronym for MB's Technical Assistance Center. When a dealership calls to make an inquiry, or requests assistance in diagnosing a problem, a case number is assigned. In your case, 802921. I've made many calls to the TAC over the years. I think I've even spoken with M.B. Doc a few times.

As far as your problem is concerned, are you sure that the tank isn't full? Maybe the gauge isn't reading correctly. I've seen problems like this many times.

If you're certain that the tank isn't filling properly, I'd recommend disconnecting the vent line to the charcoal canister, and see if the tank fills completely. A plugged canister won't allow the tank to vent during fill-up, causing the pump to shut off early. I've seen this happen a few times, most recently on a 2007 S550.

I wouldn't be surprised if the TAC made a similar recommendation.