A diode is a electrical one way valve so to speak, it will allow current to
travel in one direction only. use a ohm meter to test you should get a
reading measurable resistance in one direction and infinite when you reverse
the test leads.

the low voltage reading could be cause from a poor connection, loose
connection (loose connections will also cause amp to go up and heat to be
generated) ect.

the test for amp draw you want to place a amp meter in line in other words
disconnect the one line to the PTO clutch and wire the amp meter so the
current will go through the meter start up engage the PTO and see readings.

make sure you don't have part of the meter, your fingers or wires in the
way of the PTO where it may get caught in.

If the PTO is disconnected at the PTO will it still blow fuses when you
engage the switch? do the wires get hot? If so you may just have a power
wire that is partially grounded creating the high amp draw and causing heat
and blown fuses.
hope that helps