John Deere L120. Engine dies when PTO switch is pulled.

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.

A possible cause is an open seat switch (operator presence). Locate the switch under the seat & unplug the switch from the harness. Place a small jumper wire into the female socket leads to the wire harness. If the PTO engages without engine shutting down, replace the seat switch.

Check to see if the mower deck is plugged or a piece of wood/debris is wedged between the deck and a blade, or some wire/rope is wrapped around a spindle. If the blades don't turn by hand, it'll stall your engine when the electric PTO clutch is energized.

Lastly, a discharged battery will cause an engine shut down if there's not enough reserve when the PTO is engaged. Measure the battery voltage with a meter. If the voltage is below 11.5 volts, you may have to replace the battery.

check reverse safty switch or engine rpm to low to run pto

The electric clutch draws current while engaged. It is quite hungry on power which means that if the battery is low, the generator may not be making enough to charge the battery and hold the clutch in too. This is why the batteries on these machines are so large relative to say an ATV battery. I have several times experienced units with low batteries which started the engine OK but would not hold the clutch in. The battery may be getting old and not holding the charge. Try charging with a mains operated charger. Make sure to check water level in battery cells. Running them dry can ruin the battery.

Most of these mowers have a safety switch between the PTO switch and clutch. A safety switch or relay is most likely the problem. Hope this will help ya.

check the fuse if ok check the safety switches

You have several safety switches on your unit. The engine should not start if the PTO safety interlock is engaged. Be sure PTO switch is off.
If the sarter Does Not Work Or Will Not Turn Engine Brake pedal is not depressed.
or the PTO is engaged.
Battery terminals are corroded. or Fusible Link in starting circuit is blown - (Not likely unless bad wires) Here is a link to your wiring diagram (maybe your model)

http://www.postimage.org/image.php?v=Pq1tOfiS
Notice how the starting circuit goes through the brake pedal (must be depressed) and the PTO switch (must be disengaged) and that the PTO only works with operator present. Print that page while link still works.

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

Try a new seat switch,or the rio switch on the right rear of the trans-axle.

Check and adjust your RIO switch{reverse implement option}near the trans if so equipped.What happens when you press forward pedal then engage blades?Would also check the lower bearing on your pto clutch to see if its seized up.