Powering Servo

I am using this servo: http://www.pololu.com/catalog/product/517 for an automatic drink dispenser.

When I power it through a USB port (tried 2 different computers), it works like a charm. I then tried switching over to powering it with a 5v usb wall wart rated for 700mA and the servo wouldn't move. Then tried a 2A wall wart, and it moved for a second, then no response. Finally tried a 2.3A 5v mac wall wart, and the servo started jittering. It was doing something similar to what I programmed it to do, but very unresponsive.

I know it's a big servo, but there's no way it draws over 2.3A under no load. I'm also thinking this is not the issue as USBs only put out 500mA AFAIK. Unfortunately, I don't have a multimeter to diagnose the problem, or a higher rated 5v power supply. Has anyone else had this problem with wall warts? Is it that they just don't put out what they claim, or is it something I might be doing?

Could it be that your Servo is not sharing a ground with the Arduino?

Could it be that two of the servo wires got swapped?

The wires are fine, but ahh, I never connected the grounds. When I was powering both the arduino and the servo with the PC, I guess the grounds were connected internally to the CPU. Now that i'm using two different sources, I have to connect them externally? Just by my (limited) knowledge of electronics, I don't see why it would matter, but thanks. I'll try it now and report back.

I'm guessing the differing performance depending on the wall wart was just a fluke.

dberm22: Just by my (limited) knowledge of electronics, I don't see why it would matter, but thanks.

The Arduino output signal going to the servo is relative to Arduino ground. Without that ground connection the signal is just floating.

Woohoo, it's working! Thanks for the answer and the explanation. It even worked with the 700mA wall wart. Not sure why I got different results using different wall warts, but now that it's working, I don't really care. Thanks for the help!