Simplest way to check loop style series plugs is to turn the key to the glow position and see if the heavy squiggly wires between the plugs get hot. They'll glow red if it's dark outside and will burn the skin off your fingers if you touch them. If they get hot, your plugs are most likely working. I'm guessing they won't get hot. If it's just one bad series plug, you can jumper around it and start it with three plugs. Check for voltage to the plugs if the wires don't get hot. There's a strip fuse on the firewall where the wire leads to the first glow plug.

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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