Todd,
You should get the TM1590 which contains all the wiring diagrams and the theory of operation of each section, plus lots of the mechanical information you will need to keep your tractor working properly. I sent you some wiring diagram excerpts to get you started on last Tuesday the 4th, did you get them?
Below is the schematic of the PTO section.
If there is not any voltage at the PTO switch on the 415 purple wire check the F1 fuse. If you do not have a rear PTO on your tractor, there should be a jumper in X5 that connects the 730 purple wire to the 780 purple wire. The voltage on the 780 purple wire when the PTO switch is off is what initially closes the relay in the TDCM that provides power to the 760 blue wire going back to the second set of contacts on the PTO switch, and this is the source of power to the PTO coil when the PTO switch is on (it comes through fuse F2.) The 750 blue wire from the switched side of the PTO switch should go directly to the PTO coil, and that terminal of the switch should also have the 751 blue wire that illuminates the PTO lamp on the dash when the switch is on and the circuit is working. If the seat switch is off (unoccupied) for more than a second, the TDCM cuts off the PTO.
There should be no yellow wire on any terminal of the PTO switch on a 318.
Keep us posted on what you find, we can help you get this back to working condition...
Chuck