Why does your John Deere mower die after engaging the blades seat safety switch is by-passed?

I believe you have a defective seat switch. The switch is designed to sense if the operator is setting on the tractor when the PTO is engaged. For instance, if you stop moving and step off the tractor with the PTO engaged, it will kill the engine for safety reasons. When the switch is defective or has a disconnected/broken wire, the switch will not send a signal that there is a person on the seat.

The switch is located either on the tractors seat pan under the seat or may be located in the seat itself. It is easy to test the switch with an ohm meter, if the switch tests good, work your way back the wires until termination to look for failures.

If you are a person of light weight, the switch can be adjusted for sensitivity by loosening the lock nut and raising the switch higher on the seat pan or further into the seat.

Other things to check;

1. while setting on the seat with the key on but not running, you may hear a click when you turn the PTO switch on.

2. If the above is true, remove the spark plug wire(s) and attempt to turn the blades by hand, they should move freely.

3. If the above is not true, something is binding the deck/blades which will cause the engine to die.

4. Remove the belt the goes from the PTO to the deck and try turning on the PTO as normal, if the engine dies, it is likely the seat switch. If not, it is likely that the deck is the cause.

Hope this helps.