On a 1986 300SDL, what is the NORMAL operation of the glow plug relay? My car was built July 1986 if it makes any difference (I know there was some sort of update to some of the electrical systems in "mid 1986").

Since I've had the car, the glow light has never worked properly. It'll literally blink on and right back off when the key is cycled (basically a flash). Up until recently, it would run the glow plugs for ~15 seconds then shut them off (I assume this car doesn't have afterglow). Car would always fire up and run fine since it doesn't get that cold here.

For the last week or so, I've noticed the car running really rough during the day's first startup, so I've been paying attention to the cycle. It seems on the first cycle of the day, I only get about 5 seconds of glow, subsequent cycles get about 3 seconds. I'm having to cycle the key about 4 or 5 times to get enough heat to get a smooth start on the first start of the day now.

Is there something I should be checking? Sensor or something? Or is the relay just kaput?

I've checked all 6 plugs. Bosch plugs, replaced about 6 months ago. All of them read .8 ohms to ground with the harness disconnected from the relay, so I doubt seriously there's a bad plug.

I also doubt fuel system problems. The engine usually catches on the first or second compression hit, even when stone cold. If I cycle the key enough times, I can get a smooth enough start, but up until this past week, it was doing fine without requiring any key cycling at all.

I'm not worried about being stranded somewhere since the car will start even without a glow cycle, but that grouchy start in the morning is getting old...

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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]