Furnace blower motors are used to push air through duct systems to heat a home evenly. When the furnace blower stops functioning, heat can build up in and around the furnace, and warm air won't reach the rest of the house. Fortunately, a replacement blower motor can be wired in to correct the issue and restore heat to your home.
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Obtain a Replacement Motor
If the motor is malfunctioning or not functioning at all, the problem may be due to weak spots in the windings, which can be remedied by replacing the blower motor. The replacement blower motor must match the specifications of the old one for voltage, horsepower, amperage and speed ratings.
Locate and Label All Wires
Make sure the motor is not connected to any power source. Locate the blower motor's wire identification label, which will be located on the motor's housing. If it is unreadable, look it up online using the manufacturer model number. The wire ID label will provide the insulation color and function for each wire.
Using the wiring diagram for the unit that holds the blower motor, it will be possible to identify the fan relay as well as the various speed terminals and terminal locations if the motor is multi-speed. If the blower is using a multi-speed motor, the settings will be labeled as "High," "Medium" and "Low."
Locate and Identify the Capacitors
The speed wires connected to the blower motor are connected to a fan relay, which uses a low voltage signal to activate a high voltage switch. Constant voltage for the blower motor is maintained via a capacitor supplying voltage across its secondary winding.
The blower motor capacitor mounts to the outside of the blower's housing. In many models, it will look like an oval cylinder, while others may resemble a round-shaped housing. Each capacitor will have two sets of wire terminals that extend from its top.
Disconnect the Old Blower and Wire the New One
Take a picture of the old motor in place before disconnecting the motor and setting it aside. Secure one blower motor capacitor wire, usually insulated with a brown coating, to each terminal using needle nose pliers to ensure a secure connection.
Next, identify the power source wires to the unit. These wires, usually insulated with a black coating, run from one side of the terminal block to the fan relay and supply a hot wire to all other electrical parts. On the other side of the terminal block is where the common wires, usually insulated with a white coating, are located. Locate the common wire on the new blower motor and use needle nose pliers to secure it to the terminal block connector on the common side.
Finally, secure the blower motor's speed wires onto their corresponding fan relay terminals. Attach the wire marked "High" (black insulation) to the terminal marked "High." the wire marked "Medium" (blue or yellow insulation) to the terminal marked "Med," and the wire marked "Low" (red insulation) to the terminal marked "Low." The remaining speed blower motor wire will be secured to the only empty fan relay terminal that remains, for the "Off" position.
Test your fan blower. If it operates correctly, the issue will be resolved, and you can return to enjoying warm air forced through the ducts to heat your entire home.