This is that little vacuum valve that always gets it nipples broken off... It is located right behind your thermostat housing on the 1981 to 1985 turbo diesel engines. It controls vacuum going to your EGR valve. When it breaks it will effect the operation of the EGR and it may make your transmission shift hard. This is a genuine OE part.
1981 to 1985 Turbo 300D 300CD 300TD and 300SD
617 Turbo Diesel EGR Thermal Valve