Hi and welcome to FixYa, I am Kelly
The service manual is here:
The wiring diagram is on page 84 of the manual.
Disconnect the positive and negative terminals from the battery and make sure they do not contact the chassis to form a ground. Read for a short with the key in run
and lights OFF
from the positive battery terminal to chassis ground. If you ar noting a short at this time you are literally going to have to chase wires. I would start by disconnecting the red wire from the PTO switch followed bythe red wire on the PTO relay. If the short goes away I suspect a problem with the wires at the PTO clutch as this is were many wiring problem manifest due to foreign objects thrown by the belt hit the wires on the clutch. After that I disconnect the engine connector and check again. Lastly I would disconnect the dash monitor connector and test again. If you still have the problem after disconnecting the engine harness and dash monitor there is a wire chaffing some where against the chassis. You can find it... it just takes a lot of time.
On another note...
You should probably make sure the ignition switch is not causing your problem.
Key switch testing connector removed
Off = M+G+A1 (continuity) Also read each one to the case of the switch
Run W/lights = B+A1+A2 and L (continuity) Read B + A2 to to the case of the switch.
RUN = B+A1 (continuity)
Start = B+S+A1 Read S to the case of the switch. (continuity)
Don't forget to remove the large (+) wire off of the start solenoid when troubleshooting or you will read the starter winding giving you a false ground
Troubleshooting will end up being isloation of one leg of each wire at a time. I usually make a straight line sketch of wire temination points by wire color and connection on scratch paper for each wire I test and draw a single angle line across it after I have tested it.
If you determine that the problem is in the engine alternator, I will need your engine model number so I can get the service manual for that also.
A word to the wise.... if you disconnect a wire... write down color and where it was disconnected from on scratch paper. and line it out after you re-connect it. (experience)
Thanks for choosing FixYa,