The fact that the glow plug light comes on is not adequate evidence that all the glow plugs are working. You need to confirm voltage to all plugs when the key is in the glow position and that each glow plug is actually glowing properly. A rough and ready test for glowability is the ohm test for each plug but it is not definitive.
These tests can be performed in under 5 minutes using a simple digital multi-meter. Haror Freight will sell you one for under $5.

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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