Are the glow plugs cycling correctly? I read recently in the FSM that it may be necessary to cycle the glow plugs several times, without starting, to charge a capacitor in the glow plug relay. Placing a charger on the battery to get, and keep, it at top charge seems to help with this. Originally, I thought the glow plug relay had failed on mine but going through this procedure brought the engine back to life. It's worth a shot!

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Sam

84 300SD 350K+ miles ( Blue Belle )