The resistors allowed MB to use low voltage glow plugs on a 12 volt system. All plugs and resistors need to be installed and tightly wired together for the system to work. The later system of parallel wired glow plugs is really a better system and the conversion is available but does require some rewiring.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150