I have '85 300D 617 and the on dash glow plug light works intermittently as does the relay. I can hear the relay popping appropriately when it's working but more and more often the relay isn't working and and the dash light doesn't come on either. I tested the relay according to the great tutorial on Dieselgiant.com and everything checks out. I tested the wires for excessive resistance and the pulled the glow plugs to see if they lit up. Everything works. Except... except when it's cold (30*F or less). More than likely when it's cold the glow plug system won't perform. Honestly, the system works famously whenever it's warm but not when it's cold. I'm at a loss. Question: do relays go out by performing sporadically for a while before they finally die completely? I was under the impression they would just quit, not die a long, lingering and frustrating death. I don't mind buying a new relay if I knew that would fix it. Thanks everybody, I appreciate any help or ideas you can offer.