What diesel engine are you installing? If you're installing an OM601/2/3 engine, you need to bring over the entire EDS system to have that relay operable. Otherwise you can go old-school and manually set the mechanical idle control to ~700RPM.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]