My 220d salt shaker indicator on the dash does not light when I cycle the glow plugs. However, the brake light dims and the car does start indicating that I'm at least getting some 'glow'. I'm having a hard time finding a troubleshooting procedure for series type glow plugs that includes the dash indicator. It also looks like the online factory manual does not include the 115 section (at least clicking on this section doesn't seem to work).

Does anyone have a good test procedure for testing all components in the series style glow plug system (i.e. measured voltage at each plug should be within xx range)?

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1985 MBZ 300DT
1969 MBZ 220d
1984 MBZ 300TDT
1981 VW Vanagon