Mine was doing that once on a previous W116, and I think the problem was the ground up by the radiator was gone because someone cut it. I reattached it, and suddenly the glow plugs and tachometer worked. Maybe that little 7mm bolt connection is corroded.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles