I was under the impression that the glowplug light coming on as an indicator of a bad plug occurred immediately upon release of the starter and not minutes later.
Does anyone know if the 'glow plug light on as indicator of bad plug system' means that the glow plugs are actually energized when the light comes on? I had assumed that only the light was coming on and that the glow plugs were not being energized.
There have been a number of instances reported on here of glow plug relay's sticking on after the timer should have turned them off but I don't recall another instance of a relay randomly energizing in the manner your report. But then on other other hand, few people have a voltmeter that would tell them that a random glow plug light appearance meant the glow plugs were actually being energized as opposed to just the light flashing on. I'd hypothesize a problem in the ignition switch but that's just a random shot.

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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