Arduino Not Working With External power

[arduinopi]

I have a ping sensor hooked up to my ardunio with a servo And all of this all works accordingly when the USB cord is plugged in.
when I plug in a external power supply (ac adapter) to ardunio only servo works with the code the ping sensor does not blink and no I don't think it's a power issue the adapter is 5 volts at 3 amps I heard somewhere about a pull-down resistor or something, about there being a flaw with the board.

thanks for the help

and I have a ardunio duielimove

[CrossRoads]

You have the 5VDC going into the barrel jack? You won't get 5 V out that way, the regulatar will drop 1.5-2V on you, so everything will be underpowered.
The 5V will need to to into the 5V header pin.

[MarkT]

Before you connect it to the 5V pin, do measure the voltage to make sure its 5V.  Anything over 5.5V could fry the Arduino.

[arduinopi]

my ac adapter is plugged into the barrel adapter and the 5 volt out put reads 3.8

wait what are the 5V header pin? the pins near 3v3,reset,vin pins if so you can put the voltage in there?

now If I where to use batteries how could I use those? 3 double A's are 4.5 volts,  4 double A's are 6 volts  how would I go about doing that?

[CrossRoads]

Yes, you can connect your 5V source, or three 1.5V batteries, to the 5V pin on the power header.
The 16 MHz arduino will run well on 4.5V. 
6V is too much.

[arduinopi]

is the barrel plug regulated? so you could put for example 9 volts through it?

[wildbill]

is the barrel plug regulated? so you could put for example 9 volts through it?

Yes, it is - see Crossroads reply #1

[CrossRoads]

Yeah, what he said