Why Motor dies when you switch the gear p to r or d?

Could be lots of things What year? What engine? Any other problems? Engine shutting off when shifting out of park is caused by an electrical short in the backup light or automatic power door lock circuits, which are both activated when the transmission selector is moved into or past the reverse position. The engine management computer senses the short and kills the power to the ignition. To confirm this is the problem, disconnect the wire to the backup lights at the park-neutral safety switch located on the drivers' side of the transmission. If a short is the problem, the car will continue to run as you move the gear selector past reverse. You will have to find the short in the circuit and repair it before you can reconnect the wire.