Under some circumstances, lack of a glow plug light indicates bad plugs. You need to test the resistance of each plug with an ohmeter. Digital multimeter works best. Each plug needs to be less than 1 ohm. More than that and the plug is bad. Plugs can sometimes be bad if old and ohm out adequately. Sure fire test is to remove the plug and hook them up to a battery with jumper cables and see if they glow bright and hot quickly. It's useful to have a good new plug to compare the response of the old ones.

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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