Have you checked the resistance values of any of the other glow plugs other than #1? I would try that, and also check to make sure that you have 12 volts at each glow plug when you first turn on the key.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine