Some high efficiency air conditioning systems use air handlers with three speed fan motors. A three speed AC fan motor's wires connect to a capacitor and a fan relay in the air handler's control board. Some control boards start the fan motor at low speed and then increase the fan's speed after a set amount of time. AC fan motors running at high speed cool air more quickly but remove less humidity than fans running at medium speed. AC fan motors running at low speed remove more humidity but cool less efficiently than fans running at medium speed.
Disconnect the electricity to the AC unit at its circuit breaker. The correct circuit breaker has a "Blower," "Furnace" or "Air handler" label.
Inspect the three speed fan motor's wiring chart, found on the side of the fan motor's case. The wiring chart lists each colour-coded wire and its function. The fan motor mounts in the air handler's blower housing.
Examine the air handler's wiring diagram, found stuck to the inside of the unit's lid or in a packet placed in the electrical box. A key on the wiring diagram lists the air handler's parts and gives an identification code. Find the fan relay---often labelled "FR"---and the blower motor---often labelled "BM"---on the wiring diagram. The lines coming from the blower motor represent the fan's wires and the word above each line represents the wire's function. The numbers on the fan relay identify its pins and their functions.
Loosen the blower housing's green screw with the correct screwdriver, often a 1/4-inch hex head screwdriver. Slip the wire connector on the fan motor's green wire under the loose screw, then tighten the screw. If the green wire uses a round wire connector, remove the screw, slip the screw into the round hole and then replace the screw.
Route the three speed fan motor's capacitor wires, usually brown, to the capacitor mounted on the blower housing. The capacitor looks like a silver metal cylinder with two wire terminals exiting its cap. Press the fan motor capacitor wires' terminal connectors onto the capacitor's wire terminals---one wire on each terminal---with needle-nose pliers.
Route the three speed fan motor's four speed wires to the air handler's electrical box. A hole with a rubber grommet in the bottom of the electrical box provides access for the wires. The electrical box contains all of the air handler's electrical control parts and its terminal block.
Press the terminal connector on the three speed fan motor's common wire, usually white, onto the terminal block's common side, usually labelled L2. The wiring diagram identifies the terminal block's L1 and L2 sides. The wires from the circuit breaker enter the unit and connect to the terminal block.
Press the terminal connector on the three speed fan motor's high speed wire onto the fan relay's high speed terminal pin with needle-nose pliers. Often a three speed fan uses black insulation on its high speed wire.
Press the terminal connector on the three speed motor's medium speed wire onto the fan relay's medium speed terminal pin with needle-nose pliers. Often a three speed fan uses blue insulation on its medium speed wire.
Press the terminal connector on the three speed motor's low speed wire onto the fan relay's low speed terminal pin with needle-nose pliers. Often a three speed fan uses red insulation on its low speed wire.