reason i ask is because mine quit coming on. but i get around 12v at all the plugs during a glow cycle also i get continuity between the plugs and the valve cover. car is starting normally ( really quickly ) but ithe ambient temperature is pretty warm right now. where did my light go ?

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000

Its a normal lightbulb. They can blow out at any time without warning just like any other bulb.

And then.... I seem to remember that if one or more glow plugs stop working
it will stop coming on. To sort of let you know.

Check the bulb with a simple 12v source.

Check the resistance on the GP's, or do like I do.

Just take each one out and give it a jolt of 12v and look for red glow.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

As mentioned, they can burn out just like any other bulb. If you feel like getting behind the cluster, pop it loose and clip a voltmeter to the positive wire to that bulb, turn the key on to glow cycle, you should see 12v there for a timed period. Next, make sure the socket side of the bulb has a good, no-resistance connection to ground. That will tell you right off if the light is the problem or if there's other issues.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale