My air conditioner only works spuraticly I'm wondering what switch is not letting it turn on: pressure switch or some other safety switch. I replaced contactor and still only works occasionally. I'm getting 24 volts to unit so I know its not tstat. What are the two 24 volt wires going directly into top of compressor? What is the humming small box connected to the small copper tube where it goes into the furnace? What is a small circuit board that the 24 volt wires connect to on the a/c unit? Need help, thanks.

There have been 15,432 different models and brands of units made in the past 20 or so years so it could be almost anything but if you supply a EXACT model of the outside unit we might be able to help.

The ac unit is a Bryant model 563an036-a serial# 2094e18144

Here is your wiring diagram.