I don't recall a single post on this forum describing a bad glow plug light and I've been on here 8 years. Anyone else recall a bad light?
The light doesn't work if there is a difference in the amount of current drawn between plug #1 and the others. There are two coils in the relay that conduct this comparison when the glow plugs are energized.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13