See drawing 108:
https://www.startekinfo.com/StarTek/outside/12253/program/ETM/126td83-85.pdf

Seems the SD has the alternator wiring going to a common terminal that connects down to the starter motor. Make sure everything is connected, if all else fails, check continuity from the alternator plug to the battery positive terminal.

__________________
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]