Hi, I'm trying to make a bluetooth-controlled cart with the hc-06 module and a motor shield in the mega arduino, I'm having trouble receiving the android commands through hc-06, when I connect the arduino in the computer's usb everything works perfectly but when I turn on a 9v battery the arduino gets letters and random symbols. I do not know what else to do, can anyone help me?
Probably because you are using a 9v PP3 battery. Try using a proper battery, like 6xAA. You need power, a little, PP3s don't have any.