An air conditioner uses relays to switch the system's high-voltage parts on and off. The relay contains a low-voltage coil and a high-voltage switch, sometimes called "contact points." When the thermostat switches on, a low-voltage signal energizes the appropriate relay. The low-voltage signal passing through the coil creates an electromagnetic field, which closes the relay's contact points. Once the relay's contact points close, high voltage passes through the relay and continue on the electrical part the relay operates. Replacement relays must match the original's function, voltage and amperage.
Turn off the air conditioning unit's circuit breaker or disconnect switch. Most units have a disconnect box within 3 feet of the unit's control panel that contains a circuit breaker-type switch, or a pull-out buss bar. If not, turn off the system at the circuit breaker panel.
Compare the new relay to the old one. The relay's physical shape, design and electrical ratings must match. A sticker on the relay's housing gives the electrical information. Heat sequencing relays must have identical time delays, as stated on the sticker.
Wrap each wire connected to the relay with a piece of masking tape. Write the relay's terminal identification numbers, found next to each terminal, onto the tape. Some relays, like condensing-unit contactors, do not have terminal identification tags. In this case, write a different number on each wire's masking tape, then draw the relay on a piece of paper and write the wire's numbers on the drawing at their respective terminals.
Pull each wire's terminal connector off of the relay's terminals with needle-nose pliers. Grip the connector, not the wire, with the pliers.
Remove the screws holding the old relay in place with the correct screwdriver. Usually air conditioning units use 1/4- or 5/16-inch hex-head screws.
Lift the old relay out of the air conditioning unit. Set the new relay in its place. Secure the new relay in the air conditioning unit with the screws that held the old relay in place.
Push each wire onto its respective relay terminal, using the tape tags, or the drawing, as a guide. Use the pliers to push the connector over the relay's terminal. Each wire connector must slide completely over the relay's terminal.