Pto on Johns Deere 7800 won't engage. Pto light comes on very briefly then goes out. Pto doesn't turn at all. Brand new switch and pto sensor. Could it be selenide?

https://farmergrows.com/john-deere-pto-problems/
This site may give you some help.

Try the following test. For safety purposes remove spark plugs. Now with engine off and key in run position turn PTO on. You should hear the clutch engage (clunk). Grab both sides of the clutch pulley and try to rotate the pulley. If the clutch is properly engaged you will not be able to rotate the clutch pulley without turning the engine.

Likely you need a new switch. Check that cabling and connector are in good shape. These tend to overheat from failure and you have to repair the cable connector when replacing the switch.

Part Number LVA21438
Description
SWITCH, SWITCH, PTO (RIO,3SYM,NEW B
Remarks
SUB FOR AM127393; PTO
Package Qty 1

https://partscatalog.deere.com/jdrc/sidebyside/equipment/67312/referrer/navigation/pgId/11592590

hmm strange are the belts driving the blades ok if so then the PTO could be damaged but if there's no sign of damage it maybe a bit frozen up/locked up have the engien off but turn the key on and engage the PTO switch and manually turn the PTO (not by the blades please....dont want to have your hand taken off) turn by the PTO itself and if you hear any clicking sound disengage the PTO and engage a few times to ensure your hearing the clicking sound of the PTO turning on and off and it should be back to normal also check any connections going to the PTO before doing the manual work of repairing the PTO

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
Best Manuals

Based on the combination problems involving the coil kill wire and the PTO activation, I would suspect the PTO switch is bad.

I do not have a schematic for the 300x, however I most of the functions are comparable to the LA 145, 165, 175.

The PTO switch is a multifunction switch. It will have 3 to 4 different functions internally. On the LA 175 it serves 3 functions. These 3 internal functions are independent of one another. Electronically they are known as single pole/single throw switches. In other words even though they are all housed inside a single unit they act light 3 separate light switches in your house.

One function ("light switch") controls your cranking system. When the key is turned to Start, power flows from the Key switch (purple wire) to the PTO switch. With the PTO switch off it flow out (purple/white) and goes to the Brake switch. If the Brake is set the power flows through to the starter solenoid and your engine cranks.

Another "light switch" inside the PTO switch controls the Blade Clutch. Power flows from the Key switch, in run position, (yellow wire) to the PTO switch. With the PTO switch on it flows out another yellow wire to the Blade Clutch. A DC volt meter should show 12 volts switching on and off as the PTO is turned on and off.

The third "light switch" completes a Ground Path. This ground path is a totally separate pathway running through the PTO switch. The ground system is fairly complex because it operates differently under different scenarios.

To produce a spark, the key switch must be in the start or run position. While starting the PTO must be off and the brake set. The seat switch does not matter at this point.

Once the engine is running the seat switch plays a part if the PTO is activated or the Brake is released. Someone must be in the seat.
The Kill system is activated

1. When the key switch is turned off.
2. When the operator leaves the seat without setting the brake and turning off the PTO.
3. When the machine is reversed while the PTO is engaged and the back up switch (RIS) is not pressed.

So there are three different paths to ground; any of which can kill the engine by grounding the coil. Except for turning off the key, all other grounding functions flow through the "grounding light switch" in the PTO.

Because you have a combination problem, the Coil kill wire is grounding the engine and the Blade Clutch won't activate the blades; both of which are affected by the PTO switch, odds are you have a bad PTO.

Obviously there are too many complex functions to just bypass the switch. You will have to replace it. The part number you need is AM135131. You can do a site search to locate and order this replacement switch for about $26.00 including shipping at this site: GreenPartStore Parts for John Deere Lawn Tractors John Deere Lawn Garden...

These John Deere Garden Tractors have a RIO (Reverse Implement Option) on the PTO. If your PTO is engaged and you back up, the PTO will automatically stop. To reset it, depress your PTO engagement switch to the OFF position. Take your foot off the reverse pedal. Pull up on the PTO engagement switch. The PTO should restart. To use the RIO feature, pull up on the PTO switch for a second or two just before you step on the reverse hydrostatic pedal. The yellow RIO indicator light should come on your dashboard and you can now back up with the PTO still running. Works great for snow blowing when I don't want to keep resetting my electric PTO clutch to keep restarting my snow blower.

If your PTO will not restart, you may have a PTO switch starting to fail, or a likely logic problem in your electric PTO control module, or one of your forward/reverse indicator switches, which is linked to your PTO control module along with the overtemp circuit, is stuck/faulty.

look at the fuses there maybe a blown one

Try replacing the starter solenoid. If that doesn't do it, try replacing the whole starter.

Park brake switch is the most likely culprit,any other would cause the tractor to not turn over in the crank position.