I added extra ground cables from batt negative to cylinder head/block/frame/alternator....
Also replaced GP wires with heavier cross section stranded copper cable and heavy copper crimped lugs....
That took care of the mysterious changing voltage drops....
I did the above just to rule out current path resistance in my search...
It eliminated my problem....it was the problem..... easy and cheap to do....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)

>>Across lead battery posts 10.4 to 11.6 V

That's a bit low - the battery doesn't sound like it's very strong.

>>V drop from lead battery + post to glow plug 1.6 v

That's a bit high - you have a bad connection somewhere between the battery post and the glow plugs.

I think you've got more than 1 fault here. Replacing the battery will probably allow the engine to start - but, the bad connection will still be there.

As per Yellit's excellent post above, going through each connection between battery post and glow plug, and then the earth path back to the -ve battery post is easy and cheap to do.