Newbie here,
Just worked out my sketch to eliminate all serial output to the serial monitor and I inserted a 9V 600amp wall wart into the arduino uno and when I pull the usb connector the output stops. I'm transmitting sensor data over pins 2 & 3 using newsoftserial to an xbee radio and reading it with another arduino.
any reason my sketch doesn't run when I don't have a USB? I tried to put a 10K resistor between 0 & 1 (TX and RX) which was a suggestion I saw somewhere else.
Suggestions?
Bill
Measure the 5v line to make sure your 9v supply and regulator are working.
Could it be that the USB cable was providing grounding? Try touching the shell of the USB cable to the shell of the USB connector on the Arduibno board. Then see if a reset gets it going.
I measured the 5V and I got a degraded result (4.4V) so I found a 10V wall wart and now I'm dead on 5V.
I tried the touching ground test and nothing happened. plug usb in it works, unplug with external power it stops.
my external libraries are freqcounter wire and newsoftserial could one of these need a USB?
The xbee is using newsoftserial on pin 2&3 and their is not other reads or writes.
What model of Arduino is it?
arduino uno
I'm doing some experiements and here is what I've found: I've gotten REALLY simple, so I have an arduino uno with an LED tied across pin 13 and ground. I'm turning it on and off every 1000 ms (delay(1000)). I have two wall warts a 9VAC 600mA wart and a 10V 500ma wall wart.
When I plug in the usb cord I get a nice slow blink on the led; unplug and use the 9v wart and I get a furiuos blink, so fast that the led doesn't have a chance to fully brighten. With the 10V wall wart the on light on the arduino looks like its pulsing and again furious blinking. I've also notice pulsing of the on switch with 9V wart but it's more pronounced on the 10V.
Some questions, do I have two bad wall warts? Is there a way to just power the arduino from the USB without computer connections?
I want to eventually go to batteries, but I want to test my setup for a few weeks on power just to get it working, then reduce the power consumption.
Help?
What is the current rating of your two wall warts? Are they both DC output and not AC outout? You must use DC output wall warts rated above 8vdc and around 1 amp is a good max current rating.
Lefty
This is why I'm a newbie!
Thanks! I'm outputing AC! Time for some new wall warts. 
Bill