I have a john deere z225 (2004).  I have had problems before with the pto, so I replaced it (2011). Recently, I replaced the engine (1 week ago) with a new B&S EX1900.  The engine runs perfectly and I have mowed with it in total approximately 1 hour.  Yesterday roughly 30 min. into mowing, the mower completely died. I started the mower again without issue and as soon as I engaged the pto, it died again. I have cleaned every connection and ran continuity tests in everything including the pto switch, ignition switch, solenoids, pto harness, and all 4 safety switches.  I traced all the wires from battery to solenoid, solenoid to harness and back again. I cant find or solve the issue. I need a little direction please!  Any ideas????

Can you by pass the seat switch?
 
Also, the last zero turn I worked on had the same issue, which turned out to be the PTO switch itself.
 
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Thank you for your reply.
 
Yes, I isolated each safety switch then bypassed each one.  I made sure to remove each switch and depress manually and then bypass with a jump wire. All of them worked.  I did check/clean the PTO switch and harness.  I am receiving continuity for both on all connections. When the key is on, I can engage the PTO and audibly hear it click over.  Is there a way to check the PTO itself? 

You can un-plug the PTO altogether and see what happens. 
 
You can also put 12 volts to the PTO ALONE (disconnected) and see if it activates W/o issue. 
(The machine can be off or on)
 
I don't know if you have a relay for the PTO.
 
Now you can have a switch showing good (via meter) but contacts inside crappy/not carrying the 
actual current and then causing issues..   
 
That's what happened on the last one I had( PTO switch itself).  Checked everything, threw the parts cannon at it (cheapo E Bay unit, OKed by my customer) and that fixed it... 
 
 

Thank you for your reply.
That is what I was leaning towards doing next (PTO off).  Before I start the barrage of parts lol.
I'll isolate the PTO and put 12 volts to it to see if it engages.