Agree with Lou. When in doubt, I put a voltmeter lead on the starter stud...the end of the copper stud protruding out the front of the starter proper, not the solenoid. I put the other lead on the aluminum case of the starter proper. Set to 20v range and have someone attempt to start it while you monitor.
If you aren't getting 10V at the starter where measuring, since you have a new (fully charged you checked yourself) battery, your problem is a high resistance somewhere in your interconnect cabling. Do not forget to check where the battery negative cable attaches to the tractor proper. If under a starter mounting bolt (which needs to be removed and that interface needs to be cleaned then reinstall the bolt and tighten accordingly) the chances of a bad connection are minimized. If under some other engine/tractor frame bolt, good place for the problem...corrosion.
The copper in the starter proper and copper in the interconnect cabling is the lowest resistance as temperature decreases so this is in your favor (good). CCA of the battery drop with temp drop (bad). Engine resistance per se drops since the steel contracts and interface gaps get larger (good). Engine oil gets thick and is a big deal (bad) if in a cold climate and you aren't using a good, clean, cold weather rated oil....like Shell Rotella T 5W-40 full synthetic (top of my list). Even using their dino 15w-40 can thicken up on you, and if using straight 30w or higher....forget it (really bad). If you get a measurable starting advantage from any crankcase heater, you probably have oil fighting you....and old dirty, black, diesel oil, is a culprit. If you find a viscosity chart for engine oils vs temp, you will find that going cold the viscosity increases at a very fast rate as the temp. drops.
Last is engine compression. Compression is what generates the heat that explodes the diesel mist in the cylinder. Low compression or slow engine rpms during cranking can prevent this. If, when you get it to light off, if you see a lot of white smoke that takes a few minutes to clear out (not half a minute or so at worst) you have a sign of a cold cylinder and condensation from unburned diesel due to low compression caused by worn rings, or slow cranking rpms.
I bought a used 3000 and fought the idea that compression was my starting problem for years (had no diesel compression tester) until I had exhausted every trick I could think of and did an inframe. The pistons were ruined. Top ring slot was worn completely out. Had to have new pistons along with the other normal O.H. parts.
Thermostart, I will say, is a must for cold starting on older tractors with "low compression" and really cold temps. Even down here in N. Tx. when I had a low compression tractor, I had to have it and then some to get a light off. As of today, none of my Fords require TS on the coldest days here in N. Tx.
TS takes about 30 seconds for the self contained coil to turn a hot orange color, heat from such causes the diesel fuel plug contained therein, to unseat and let diesel dribble down over thse hot coil wires igniting the droplets into glowing embers. Rolling the starter switch on over to Start, which also keeps the TS working, the engine rpms suck the glowing embers into the combustion chamber igniting the diesel mist in the combustion chamber. The colder the temp, the longer the TS process.
