You may want to determine if the engine seems to have compression. if so, actually replace the spark plug if you haven't already. Still nothing? I would pull the flywheel and check the key. It is possible to have spark and a bad coil btw.

Hey folks, so I'm posting this follow up in my original thread because I think I have a related problem. Last year, I replaced the PTO drive belt and the mower blade drive belts on my inherited JD STX 38 Yellow Deck. Today I took the mower out (NOT the first time this season) and had some serious issues. First of all, the grass is tall - 18-24 inches. I had the mower height set to the tallest height. The grass may have also been slightly damp - although not wet. I was able to cut basically one path, then I realized that although the PTO was engaged, the blades weren't cutting the grass. I was going at the slowest speed (first gear) and even tried cutting using only 1/2 of the mower deck width at a time - no help. I took the mower back to my driveway, cleared out lots of old clumped grass from the blades, and started the mower and the PTO. The blades were definitely turning. I turned off the mower again, and noticed that the PTO drive belt is looking terrible - it's all pitted and slack - it's not tight around the PTO shaft! So, my theory is this: the PTO drive belt is shot again (Although not broken yet) - so the blade spin when I'm not trying to cut anything, but when I come in contact with any sort of grass of substance, the slack belt is "slipping" and resulting in the blades not cutting? Does this make sense? Solution: replace the belt AGAIN???? A few more things to note: 1) I tried cutting shorter grass and drier grass and had very little luck. 2) why would my PTO drive belt, just replaced a year ago, be so shredded already? That's worrisome... any advice would be appreciated. thanks.

***"Mounted in plastic"*** Unless Deere has included some sort of "ring terminal" to the mounting, there is no need to separate the PTO switch from the plastic mounting panel. You said that the brake switch had been defeated before you acquired this mower and now the wires are taped to the brake pedal. OK, I don't see anything in the diagram where a defect in the brake switch could cause a problem in the operation of the engine that that occurs ONLY when the PTO switch is turned ON. By this I mean that any problem in the brake switch that could affect how the engine runs..........would affect the engine even without pulling the PTO switch to the ON position, so we can move on from that and go to the PTO switch itself. After studying this diagram at length/depth.......I believe the problem (engine wants to die when PTO switch is turned ON) can only be due to a defective PTO switch. And I think it must be because the PTO switch is bleeding some voltage into the magneto kill circuit which then upsets the normal spark making function of the ignition system. The way I see it, there are 3 ways you can test this: Option A, disconnect the wire pigtail from the PTO clutch and connect 2 independent, temporary wires to the clutch, be sure to maintain the correct polarity, Blue wire is the hot side, Black wire is the ground side. With the temporary wires in place, and the engine running, touch the temporary wire for the hot side of clutch to the "Battery hot" post of the starter solenoid (safer than the battery hot on battery). If the PTO clutch engages without affecting how the engine runs...........defective PTO switch. Option B, Disconnect (or cut) the White wire lead that serves the PTO switch (if you choose to cut, make sure you cut so it can be restored). With this white wire out of the equation, start the engine and turn the PTO switch ON. If the engine runs OK now, replace the defective PTO switch. Option C, Replace the PTO switch and hope for success.

The PTO clutch on the tractor is electro-magnetic and takes a lot of current to operate. If you have a low battery condition this will stop the tractor because you don't have enough voltage to run the ignition system with the PTO turned on. First, check the battery voltage with the tractor off. You should have about 12.75 to 13+ volts (depending on the age and condition of the battery). If the battery voltage is low you could have one or more of the following problems: 1. Old battery that will not hold a charge - needs replacement. 2. Bad voltage regulator - battery is not being charged and electrical system is running on low voltage when the engine is on and the PTO draws enough to shutoff the tractor. 3. Bad stator - battery is not being charged. Of the three possibilities - 1 & 2 are the most common. A bad stator is the least likely but, you can easily narrow this down. Start the tractor and check the battery voltage. If the stator and voltage regulator are working correctly, you should have 14+ volts at the battery. If you do not have 14+ volts with the motor running, then the battery is not being charged - and there is probably not electricity being supplied to run the electro-magnetic PTO clutch + ignition. You can sort this out by starting the engine and pulling the connector from the voltage regulator and looking for AC voltage using a multi-meter. I do not know the exact terminals on that engine - but, you should be able to find a service manual on line in electronic format for about $8.00. Otherwise, you can do this by a process of elimination. Put the probe in the top left contact and check the other contacts sequentially. What you're looking for is about 30 Volts AC from the stator. Then go to the second contact, repeat, etc. If you don't find the voltage - bad stator. This really isn't as likely as a bad voltage regulator. Voltage regulators have diodes in them and other semi-conductors and after being continually exposed to heat, vibration, etc. - they go bad. So, check the voltage regulator with a multi-meter on the Ohms setting. Most of the resistances should be in the 4.5 mega-Ohm area. If you find opens or low Ohm readings between contacts on the voltage regulator - it needs to be replaced. My guess - it's the voltage regulator and you will find several "open" conditions. I just replaced the voltage regulator on a John Deere garden tractor because the PTO would not stay engaged and the battery was always low. If you find the voltage regulator is bad, in your case I would replace both the voltage regulator and battery if the battery is over 2-3 years old as constantly running it in a discharged state has probably compromised the battery's ability to hold a charge.