PREGLOW System
WARNING: This page is about a different car, the 1997 Mercedes-Benz E300. However, it is still accessible from the selected car via links, so may be relevant.
The preglow time-limit relay module has a short circuit protection feature. If a short occurs in glow plug wiring or glow plugs, current is interrupted. The preglow time-limit relay module is operational again if short is eliminated or glow/starter switch is turned to position "O". A malfunctioning glow plug will be identified by an illuminated preglow indicator light. For symptom diagnostic information relating to preglow system, See Fig 1.