All central heating and air-conditioning systems use blower motors to push air through the duct system. A blower motor's speed wires connect to a fan relay. A fan relay uses a low-voltage signal to activate its high-voltage switch. A blower motor uses a run capacitor to maintain a constant voltage supply across its secondary windings. A motor with malfunctioning or missing capacitors hesitates when the motor's rotor reaches a weak spot on the windings. A replacement blower motor's ratings must match the old motor's voltage, amperage, horsepower and speed ratings.
Read the blower motor's wire identification label, located on the motor's housing. The wire identification label gives each wire's insulation color and its function.
Find the wiring diagram of the unit that holds the blower motor. The wiring diagram identifies the fan relay and, in the case of a multi-speed motor, it identifies the different speed terminals and the terminal location. The blower speed terminals use "High," "Medium" and "Low" identification labels.
Press the blower motor's capacitor wires, almost always the wires with brown-colored insulation, onto the capacitor's wire terminals with needle-nose pliers. A blower motor's capacitor mounts to the outside of the blower's housing and usually looks like an oval metal cylinder; some capacitors use round-shaped housings. Two sets of wire terminals extend from the top of the capacitor. Press a single blower motor capacitor wire onto each terminal.
Follow the wires that enter the unit from the power source. These wires connect to a two-sided terminal block. The wires, usually wearing black-colored insulation, lead from one side of the terminal block to the fan relay and the hot side of all other electrical parts. The other side of the terminal block holds all of the common wires, usually with white-colored insulation.
Find the blower motor's "Common" wire; in almost every case the wire will have white-colored insulation. Press its wire connector onto the terminal block's common side with needle-nose pliers.
Press the blower motor's speed wires onto their respective fan relay terminals. Press a single-speed blower motor's remaining wire onto the only empty fan relay terminal. Press a multi-speed blower motor's "High" speed wire, usually the wire with black-colored insulation, onto the fan relay's terminal marked "High." Press a multi-speed motor's "Med" or "Medium" speed wire, usually the wire with blue- or yellow-colored insulation, onto the fan relay's terminal marked "Med" or "Medium." Press a multi-speed motor's "Low" speed wire, usually the wire with red-colored insulation, onto the fan relay's terminal marked "Low."