You can not assume the glow plugs are good just becasue they have voltage..you need to unplug the harness from the GP relay and measure resistanc to ground...they can either short or open up..but the resistance will be fairly low but fairly equal between them..

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche