Ford 3000 diesel--starter won't spin

Was using the tractor the other day, turned off the engine and now the starter will not spin. It will very easily start if I pull it off with another tractor or vehicle but not by turning the key on to engage the starter.
This is what I have done so far:

Checked the neutral start switch 4-5 times -have continuity with lever in neutral position but not in High or Low Range.

Have continuity and voltage running through switch when put in start position.

Removed the starter and took it to a repair shop for testing--
everything tested Ok.

Replaced starter and re-attached all wires. Have continuity and voltage from the positive battery post to all the wires on the starter and solenoid with the key off or on. But when the key is put in the start position I lose all voltage to the starter and solenoid.

There is a part underneath the ignition switch about 2-2.5 inches in diameter that has a cable from the positive post of the battery to a terminal on this part and another cable that runs from a second terminal to the starter solenoid. There is a pin that sticks through the sheet metal underneath the key that will turn left and right slightly. Assume it is some sort of a relay but don't know why the pin on it has 2 positions. At any rate I have continuity through this part whether the key is in the off or run position. Sorry but I can't remember about in the start position can check that again tomorrow.

Sorry about this being so long but I hoped if I put everything I did in the first post it would help to rule out some issues that could be causing the issue.

Thanks in advance for any help!!

Just because you have continuity through the various switches during testing, that does not mean they will support any significant current
flow that the starter needs to operate.

Here's what I like to do when troubleshooting starting system issues like yours: Take a heavy duty remote start switch and directly power
up the starter engagement terminal (the small male spade) and see if the starter cranks normally. MAKE SURE the transmission is in neutral
before you do that. If the starter cranks normally (do this several times), that means your starter, solenoid, battery, and cables are fine
and the problem is in your starter engagement circuit.

The hook your starter wire back up and take that same remote switch over to the shift cover area. Separate the safety switch wires, and
power up the male terminal in the main harness, not the switch harness. If it cranks, then reconnect that single wire to the safety switch
and then power up the male terminal of the switch harness and see if it cranks with the trans in neutral. If it cranks, the problem is
further up. I hope you get the point and can take it from there.

If you were one of my students I'd have you check the voltage drop at various points along the circuit to find the issue, but that takes
quite a bit of explaining to fully understand before you do it, time I'm not going to spend here.

Ok, everybody look at me and say you are so stupid!! Had a friend come over today to see what I might have over looked. After about 20 minutes of talking about what the tractor wasn't doing and what I had done he started checking voltage.

He started at the battery and was working towards the starter, I started at the starter and was working my way back to the battery.

After about 5 minutes he decided that I wasn't getting enough voltage / amps out of the battery, like next to zero. Pulled the cable off the positive post to clean the terminal. Put the post cleaner on the battery post and nothing happened, took some pliers and scuffed up the terminal by rotating them on the battery post until it was shiny again. Hooked up the battery cable, turned the key and the starter started to spin, he said the area between the battery cable and battery post had crystallized and prevented a connection.

I guess the moral of this story is start at the power source and work to the non-operating item.

This post was edited by poapony on 07/30/2021 at 07:19 pm.