If you want a real life test to make sure the plugs are all getting hot like they should and you have a good relay path and ground system to the head....Pull all the plugs and reattach the feed wires to them with heavy alligator jumpers from the threaded portion of the plugs back to the cylinder head to support them while they are just suspended where you can see them......
Then opearate your glow system and see how quick and how hot by just looking at them...see if they all compare.....do not crank the engine just get the glow.....

Just do this long enough for a test because the plugs are not in that big heat sink cylinder head......I had two that checked good with an ohmmeter but would quit glowing very quickly when they started heating.....the other two got white/orange hot fast...good eyeball test....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)