On the 123s it is a straight shot from key to starter (through the NSS). Only the later 124s and 126s had a starter interlock relay from the security system.

No real electronics to speak of on the 616/617, the later years got some emission control help but nothing that would stop the engine.

The glow plug relay is electronic and probably would fail open but that is easy to jump out if necessary.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech