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I have a John Deere D140 I have had for around 5-7 years. It's been great. Last year I started to notice when I turned the key I got nothing on the start sometimes No click. Nothing. Like a dead battery. Not even lights. I check battery and all fine. I noticed that when I hard shut on/off the PTO for the blades I could then get it to start. Then I was cutting grass and all of the sudden the blades stopped but tractor ran right in the middle of cutting. I took tractor to garage and shut down and then we are back to will not start. Thinking it's PTO I replace it. Still no joy. This wreaks of pure electrical issue but not sure where to look next? Any thoughts? Fuse? Wiring somewhere?

Test Seat & PTO switches with multimeter/

The PTO is brand new oem part so not seeing that as a problem. The link does not show how to test seat switch. I need some more help.

If a seat switch is bad will that also not allow the lights to even come on with key set with lights on? I could have brakes engaged and sitting on seat and key no only will not turn over engine but not even any lights.

If you follow the Schematic I posted you will see that the headlights doesn't go thru the seat. Just goes thru the main and ignition switch. And when the engine is running there power comes from the alternator. The seat switch only controls the magneto kill circuit.

ok so seeing rule out seat switch as the issue if I can't even get lights to come on with a key. So where do I go now to find the issue? It is literally like the battery is dead. Could it be ignition switch? What is the explanation for why I was cutting grass and blades decide to just go off and I can't get them on even with flipping PTO? I just need some direction of what to look at.

check for loose neg battery cable where it bolts on to frame had a similar problem with a simplicity mower

Start at the battery. Should measure 12+ volts. Check both cables for tightness at both ends. Then go to the fuse, should have 12v both sides, then to the key switch, 12v on the B connection, red wire. Turn the key to Run+Lights, should have 12v at the L connection, red\white wire and lights should be on. Rotate to Start position and 12v at the S wire. From here all the safety switches come into play. Quit where you don't have the 12v. For me, common switch problems are seat and PTO, but a loose\broken wire\dirty connection could be intermittent and anywhere.