I'm sorry, it doesn't work quite that way. In a parrallel circuit, if one circuit changes, the curent stays the same in the other circuits. It is the amperage (curent) that makes the glow plug get hot. The total amperage seen by the control module will be less but the amperage in the good glow plugs will stay the same and the amperage in the bad one will be less. Think I(current measured in amps) = E(Voltage) / R(resistance). The voltage to all circuits stays the same, the resistance in the good circuits stays the same, so the amperage stays the same in the good circuits. Only the bad one changes.
The problem is that a diesel will not start as well if even one gp goes bad even if the rest are working correctly.
I hope I didn't confuse you, but it is hard for me to explane things.

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84 500 SEL (307,xxx miles)