So tonight I checked all the battery cables with an ohmmeter, from the negative terminal on each battery to the top of the AC compressor was 0 ohms, and from each positve terminal to the small wire on the fender solenoid was also 0. (This was with the positive cables disconnected from the batteries.) Dug out my spare starter, looks like it's quite possibly an original starter, filthy dirty, connections were rusty, mounting surface corroded and caked with a scale of some sort.

Cleaned it all up, and put the two side by side, and grabbed my booster cables to do some testing. The one I took off? It turned so slow you could hear the reduction gear teeth 'chattering'. The spare starter almost torqued itself off the bench! Tested the solenoid on the spare before putting it on, (nothing worse than getting something on and it not work!) and seemed ok.

Fought to get it back into position (wouldn't an extra 1/2" of room lengthwise be a whole lot nicer??) and got it bolted in. Hopped in the truck, let the glow plugs cycle a few seconds, and it fired right up!

So.... someday next week that starter will be going to visit the rebuilder again...... I do have to wonder if maybe there is something dragging in the gear reduction side though the way it sounds.