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I'm still a little confused about what you're measuring. Are you measuring one glow plug wire at a time? Or an entire bank at a time? Or all 8 at once.
If you're clamping over the wiring bundle going to one bank, for example, you're seeing the amperage of 4 glow plugs. So 85 to 45 amps is an average of 21.25 amps initially, down to 11.25. This doesn't sound like a problem to me. When the glow plug is cold it's going to draw more current than when it's hot (see post #9).
As cheezit said, though. The correct way to test the glow plug is to unplug them and measure their resistance individually when cold.
Thats fine I understand that, but this doesnt tell me if I have something else going on in the GPCM.