I'll have to look at that graph for myself, but in this case, the point seems to be moot as I think the problem is now fixed. Thanks so much for the condescension though...

I opened the relay box and took a look at the strip fuse. Apparently someone has changed it at some point, because it looks brand new, but not to trust looks, I grabbed the screwdriver to pull it out and make sure there wasn't some hairline crack or something. Turns out, both screws were LOOSE! After pulling it and wiping off the little bit of corrosion underneath, I torqued them down tight. Well, so far so good. I ran it this afternoon, stopped for dinner at the old man's house for a couple hours, and upon leaving, it worked as it should, so here's hoping the problem is solved.

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'82 300SD - Somewhere over 220k - Becoming reliable...

Well, after digging through the manual to find the right bit, it looks as though tangofox007 is essentially correct, if a trifle abrasive.

(15-705 Operation of preglow system, part C for 81 and on US models if anyone else cares to read for themselves)
Apparently these models use an internal sensor to determine the time the light needs to stay on, when the light kicks off it activates a timer circuit (referred to as a safety shutoff) that cuts them off if the car doesn't start in 20 - 35 seconds (pretty wide range) At least thats what the manual says...

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'82 300SD - Somewhere over 220k - Becoming reliable...