Not functioning as expected with external power

When it works When I'm running the blinking led sketch and powered by USB, the LED connected to 13 works as expected as flashes every 1 second.

When it doesn't work When I power the board with an external 9v 1000mA wall-wart, and run the blinking led sketch, the LED connected to 13 flashes very rapidly and definitely not 1 second (like 5x faster).

Problem discovery I realized the problem after I was unable to get my WiShield to connect when powered by external power.

My Setup The arduino is brand new. I'm running Arduino 17 on Mac OSX. Last week, the IDE crash and was unable to upload sketchs with a programmer error (i think). The problem appeared to work itself out after a restart and reset of the arduino.

Plead for help Has anyone else ran in to this? I'm not really sure what to search for and have come up empty handed with searches like, "rapid LED flickering" and such. Thanks for any help!

If you change the sketch to have an LED on some other pin blink, does that also exhibit the odd behavior?

Have you connected the grounds together:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

If you change the sketch to have an LED on some other pin blink, does that also exhibit the odd behavior?

I switched the blink sketch to use pin 9 and it seems to wait the correct amount of time to blink, but then only flashes briefly instead of staying on for 1 second.

If you change the sketch to have an LED on some other pin blink, does that also exhibit the odd behavior?

I read through the page you linked, but I think the wall-wart is already grounded. I'm definitely missing something there.. should I be doing something other than just plugging the wall-wart in to the arduino to get it use the wall-wart's power correctly?

just plugging the wall-wart in to the arduino to get it use the wall-wart's power correctly?

Could you double check voltage (and if DC or AC output) and current output ratings printed on your wall-wart.

Lefty

Could you double check voltage (and if DC or AC output) and current output ratings printed on your wall-wart.

Just checked (should have done that first) and the 9v wall wart is not registering anything on the multimeter. The only other one I have atm is a 6v and I just read that the arduino could be unstable with that low external power. My project actually requires a 12v so I'm off to try source that.

Thanks for getting my noobness in the right direction! I'll shall never fail to test the output FIRST before posting again.