An air conditioning system uses the outside condensing unit to force liquid refrigerant through a metering valve, found in the inside unit's evaporator coil, and then it sucks the resultant vapor refrigerant back to the compressor where the cycle starts over again. Two sets of wires connect to a contactor, a type of relay, inside of the condensing unit. High-voltage wires provide power for the fan and compressor and low-voltage wires, coming from the thermostat and the inside unit, turn the outside AC condenser on and off.

...
Outside AC condensers use high- and low-voltage wires.

Step 1

Turn off the electricity to the air conditioning system at the circuit breaker box. This will require turning two breakers off, one labeled "furnace" or "air handler" and one labeled "AC" or "air conditioner."

Step 2

Unscrew the outside unit's control-panel cover with the correct size of nut driver, often 1/4- or 5/16-inch. The control panel's cover, located above the spot where the copper refrigerant lines enter the unit, usually has five screws with two on each side and one in the center of the top. Set the screws in a safe place for later installation.

Step 3

Pull the control panel's cover straight down to remove it from the condensing unit. Set it aside.

Step 4

Push the high-voltage wires from the condensing unit's disconnect box through the wire clamp, located under the control panel but above the copper refrigerant lines, and into the control panel. The high-voltage wires have two insulated power wires and one bare copper ground wire. The disconnect box, usually mounted to the building within arm's reach of the condensing unit, provides a safe way to disconnect power to the condensing unit during service calls.

Step 5

Connect the bare copper ground wire, from the high-voltage wire set, to the grounded terminal block with a flat-head screwdriver. The grounded terminal block has two or three flat-head screws that hold wires in place.

Step 6

Strip 1/2 inch from the end of both insulated high-voltage wires with wire strippers.

Step 7

Push the stripped ends of the high-voltage wires into the contactor's high-voltage wire terminals. The contactor, a black box-shaped component with spring-loaded bus bars and a low-voltage operating coil, acts like a high-voltage switch. The white and black wires from the disconnect box connect to one end of the contactor. The fan and compressor wires connect to the other end of the contactor and come pre-attached from the manufacturer. The white high-voltage wire from the disconnect box connects to the side—also known as the leg—of the contactor with white wires connected to it, and the black wire connects to the side—or leg—with black wires connected to it.

Step 8

Tighten both of the contactor's high-voltage terminal screws with a flat-head screwdriver.

Step 9

Push the low-voltage wire set, from the inside unit, through the wire clamp and into the condensing unit's control box. The low-voltage wire set from the thermostat will go to the inside unit where they combine with the inside unit's low-voltage wires before exiting the house. The low-voltage wire set will have two strands inside of a plastic sheathing.

Step 10

Remove 2 to 3 inches of plastic sheathing from the low-voltage wire set with a razor knife.

Step 11

Strip 1/2 inch of insulation from each low-voltage wire strand with wire strippers. The low-voltage wires usually use white and red insulation, but the colors do not mater.

Step 12

Twist either low-voltage wire from the contactor to one of the inside unit's low-voltage wires and secure them with a wire nut. Twist the other two wires together and secure them with a wire nut. Wire color combinations do not matter.

Step 13

Tighten the wire clamp with a flat-head screwdriver.

Step 14

Screw the condensing unit's access cover into place.

Step 15

Turn on the power to the AC system at the circuit breaker box and test the unit.