At work we don't do ohms tests on glow plugs. We just scratch a jumper from the battery to each plug (unplug the connecter of course) if it sparks..good injector, if no sparks...bad injecter. On my car the light would not come on after 2 glow plugs failed.
Not sure if it's my controller though but lately the glow plugs work great in the morning but after the car sits in the sun all day they won't work and the car cranks a bit before starting. I'll look into that another day.

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83 300sd
3 Fords