400E is correct. You have a faulty sending unit which, if similar to my 300TE 4Matic Wagon, is located in the trunk. Becker, (which is located in New Jersey), will swap you your old unit for a new one at a fraction of the cost for a new unit, which is about $2500. This is a common problem, especially in the wagons due to a design flaw, (gasp!), that put the rear window washer fluid tank next to the sending unit. Over time, spillage from filling and sloshing while driving lands on the unit and corrodes it. When I removed mine, it was practically blue! The radio in the dash may or may not need to be replaced too, but Becker has a swap package for that too. The link to Becker's website is: www.beckerautosound.com.
Their is also a question and answer forum, or you could just call them about your problem. Good luck.
1962 MB 220SEb "Fintail" Sedan
1963 Vespa VNB4T Scooter
1992 MB 300TE 4Matic Wagon
1995 Subaru Wagon AWD