You see that jumper there between the cap and the ext pwr connector ?
DID YOU REMOVE IT ?
(That jumper connects the arduino +5V to the L293 VCC1. If you have ext motor power connected to the motor shield ext.pwr, that jumper allow the L293 to run off arduino +5V (for VCC1) If you want to run without the USB connection you need to also plug ext pwr into arduino ext dc pwr jack. Without the jumper and without ext dc power into arduino, you need the USB to provide 5V.
l293d.pdf (372 KB)