The relay and a temp sender in the engine control the light. Having your injectors checked for spray pattern would not hurt, and could give you a bit more power. Nailing is often caused by a poor spray from old injectors.

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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