If one glow plug is out, none of them will work, not even the ones earlier in the series because the circuit is not complete. If you put one probe of the ohmeter at the firewall hot wire coming to the first plug and the other on the ground wire at the last plug at the front of the engine, you should have a complete circuit. If that circuit is open, there's either a bad plug or a disconnected wire.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13