Alexpops,
"Sure sounds like bad cables, loose clamps, corroded connections or an internally shorted starter. " Clamps are not loose, I am getting the full 12.8v through the cable to the 100amp fuse and main connector in the line, doesn't that mean the cable it good? These are within 12" of the battery itself. When I do get the full voltage and try to jump the starter using the screw driver technique nothing happens now.
"Any smoke coming from the starter or cables when you jump the solenoid? do the cables get hot? No way a brand new battery should draw down to <4v on an initial start with new battery & new starter." No smoke. Cables don't get hot. Correct on the battery draw down, the battery retains its full voltage, its the down stream components that show the voltage drop like the ignition switch.
"Does it do the same thing on a bench test? As I remember (might be wrong as Ian's flooding destroyed multiple vehicles' starters so I was working on several different ones) there were two grounds one to the block & one to the frame. Pulling that starter should be easy (unless your Cab is in the way). two bolts, 12 or 13 MM IIRC an extension for the ratchet & flexhead or swivel to thread your way in to the back (block side) bolt & if it won't pull easy after that, a mallet or deadblow with a couple taps to break the corrosion at the bell housing. Could be the bell housing connection giving you the resistance. wire brush before re-attaching, At least that way you can test the starter standing up comfortably instead of bent over or on your knees in the cold & see if it's the starter or something on the way to the starter." I have not bench tested it yet. The reason why I have not is because I am not getting the correct voltage down to the S terminal on the solenoid as I should. When I try to jump the solenoid using my screw driver touching the battery terminal to the the S terminal, one time I got it to react for only a second then nothing, now when I try it I get nothing. I figured until I get the full voltage on S terminal there is no point in removing it to bench test because there is a reason why its not performing as it should, its not getting the voltage it needs. Thats just my logic but I could be wrong, the thing that also makes me doubt its a starter caused issue is the sporadic behavior of the instrument panel. If it was just a starter problem I don't think the instrument panel would be acting up the way it is.
Summary, i have full voltage to the battery terminal on the starter but no signal/voltage to the S terminal to activate the starter and the reason for that is because the voltage coming into the ignition switch is 3.9v when it should be 12v and the instrument panel is out but occasionally operates correctly until I try to start the tractor, then it shut offs. Yet the battery output is 12.8v. Something between the battery and the ignition switch is failing but I don't know what those modules or parts would be. I'm going to study the schematic you gave me, thank you, to see if I can figure that out.