This part appears to be known by many names:
Pressure Regulator Valve, Vacuum Modulator Valve, Pressure Transducer or Change-Over Valve Helpful link to diagnostics: http://www.mercedes.gen.in/mercedes-sprinter-turbo-limp-home-los-diagnosis-fault-finding/
Black polymer/plastic, with three vacuum connections and a two pin electrical plug.
Only for these two model years.
Not for 2004 and later