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The starter on my Scag Wildcat 52 won't crank. Checked the safety switches, the relay, key switch, PTO switch, all checked out. Additionally, swapped a spare good key switch, no change. Swapped a good spare relay, no change. Removed starter and bench tested, works great. Tested green wire from harness going to starter solenoid trigger spade, turned key to crank and got 9.05 to 11.25 volts depending on different tests. When key is in the run position, the green wire shows millivolts in the 33mv to 115mv (0.033 to 0.115 volts ) on tests done at different times, but curiously the multimeter (mm) shows that the polarity is reversed! The polarity reversal to the green wire only happens in the on position. On the key switch when I test the voltage coming from the battery, it is the same as on the battery 12.75v, when I test the green wire contact on the key switch I get 11.25 volts.

Any suggestions?

Charged the battery to 12.90v. bypassed all four safety switches and tested voltage of safety system with key in the on position and had 12.87 volts, so voltage is flowing through safety system wiring. Connected multimeter to harness green wire that goes to the starter and got 12.86 volts when turned key switch to start. Connected green wire back onto starter solenoid blade. Turned key to start engine, NOTHING happens

PROBLEM SOLVED! It runs again.

I remembered a sort of similar problem on an outboard motor, when I went to start it, the starter would hesitate, turn slowly for one second then it would turn fast, run normal. It occurred to me that the boat ground cable was a smaller size than the positive cable. So, I took an automotive jumper cable and hooked only the negative to the boat battery and the outboard starter body. It started right up with no hesitation.

So, I did the same jumper cable test on the Scag Wildcat 52 mower and the starter cranked right up. Then I tried it with a 16 awg wire and it started right up. I permanently installed a good used black grounding wire 4 awg that had the right size welded connectors on it.

On ZTR's the wire are very exposed so water condenses inside the wire and corrodes through it .
If you cut the insulation off the old one good chances that is what you will find.
BEcause there are a few wires that actually go all the way a resistance measurement will show 0 Ω but under load they will fail

This why when make up battery cables I use no ox paste, tightly crimp using a hydraulic crimper, and then install heat shrink tubing over the connection. This makes as water tight as I can The no ox paste fills in the air spaces making it harder for water to enter the cable at the terminal ends.