Check your grounding, in fact recheck all your connections to starter carefully, especially the terminal connections on the body below the turbo that lead to the starter. Make sure your battery is good too. The glow plug system draws a lot of current, so if there is even a little resistance in the circuit to the starter solenoid you will see a voltage drop there. Look carefully at where the wires are crimped to the terminal lugs. I had really bad corrosion on mine, it's a wonder it worked at all! What's the resistance across the starter solenoid?