At least with my SDL, when the plugs weren't working, it took quite a few more revolutions to start than when the plugs were fixed. I figure the starter has only so many revolutions in its lifetime, so those extra revolutions everytime it starts served to shorten its life. Maybe it is flawed logic, but I would rather pay for glow plugs than the starter.

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Keep everything as simple as possible-but no simpler--Albert Einstein