Usually a/c problems are electrical problems.

The most common type of electrical problem is the wires themselves. Where ever there is a connection to a wire there is a potential problem.

By connection I mean the factory crimped on connectors that attach to all the components. Over time the spot where the wire and the connector meet look for corrosion, heat damage, rust, ect.

The next most common electrical problem is the switches such as the main contactor (relay), fan relay, ect. They can stop working when the internal switches burn and pit. You need to use an electrical tester to check these.

To start off with you must check to see if the power is on at the outlet where the cord plugs in (window units) or if its a central air conditioner you must check inside the box (the circuit breaker on the side where the power cord comes out of the unit), the breaker box has a removable cover around the breaker.you must use an electric tester to make sure that the voltage is correct. Or if their is no meter use some kind of tester that emits a light.

Below is for window units, below that is for central units.

All window units and central air conditioners have a switch of some type that the main power cord goes directly to after it enters the casing of the unit. On a window unit the power runs through a wire that is plugged into the wall outlet then passes through the metal cover. Inside the cover the cord is split, the ground wire (either green or a bare wire is screwed to the metal covering of the a/c).

You will need to remove the casing of the a/c to get to the internal wiring.

MAKE SURE THAT THE POWER IS CUT OFF OR THE UNIT IS UNPLUGGED, THEN CHECK AGAIN !! you can never be to safe.

The other 2 wires are split up (we will call them #1 black, #2 white). #1 black wire goes to the selector switch on a window unit, this is the switch that has (off, high cool, low cool, high fan, low fan). From the selector switch things get buisy, looking at the back of the selector switch you will see many wires hooked to it, this is where the #1 black wire coming into the selector gets sent to the vital parts, compressor, fan motor.

The #2 white wire goes directly to each of the components (fan motor, compressor). Again on a window unit one of the wires, in our case the white wire comes in from the plug to inside of the unit after entering the unit the white #2 wire is connected to the wire connector on the back of the selector switch. The wires all are common to each other because they are the same leg (leg= each wire #1 and #2 are called #1 leg #2 leg)****SEE THE IMAGE BELOW****

Since the #2 white wire is allways connected to the fan and compressor, since it doesn't go through a switch. BUT in some cases as stated above, All that needs to happen to get the a/c to work is to get the #1 black wire to send power through the switch to the compressor and fan.

The selector switch is the way the power from #1 black wire gets to the fan and compressor. So when you turn the switch to cool the power that came in from the cord that connected to the selector (#1 black wire) goes through the switch to the fan (now the fan has both wires with power so the fan comes on).

BUT!!! The selector switch also sends the #1 black wire through the switch to the compressor, heres the BUT! But after going through the switch the wire connects to the thermostat (the thermostat acts like an on off switch depending what temperature you set it at)THE COLORS REPRESENTED ON MY IMAGES ARE FOR UNDERSTANDING THE BASIC WIRING OF THE GROUND, NEUTRAL, AND HOT WIRES ON on actual a/c's the wires are different colors.

Central Air Conditioners.

On central units the thermostat on the wall sends 24 volts (24 volts comes from a transformer in the blower housing) to relays and contactors that switch the power on and off to the compressor and motors.

The relay and contactors are like any other electric device, it needs both wires to work. Inside the blower unit (the unit that is inside the house on split systems) the transformer reduces the voltage from 220 down to 24 volts. The transformer has 2 wires coming out with 24 v. one of those wires goes to every relay (switch). The other wire (red) goes to the thermostat, then the thermostat sends the power to the relays (switches), as the temperature goes up and down the thermostat turns on and off the relays that power the compressor and fans.

The reason its only 24 volts is so if you touch a live thermostat wire you will not get electricuted.

Below is a diagram of a central a/c commonly wired Below that is a window unit commonly wired

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