Blower Motor Diagnostic

Testing the blower motor

Use this procedure to diagnose if the blower motor is the source of your furnace problem — after you’ve done basic troubleshooting.

Note: This procedure does not apply to ECM blower motors.

Tools needed

• standard multimeter with setting and clamp to measure AC current

Testing procedure

1. Remove the lower furnace cover to expose the blower motor housing. Usually there is a door switch that cuts power to the furnace when the lower furnace door is removed. Make note of the location of this switch.
2. Locate the multiple wires going from the blower motor to the control board. There will usually be a white wire.
3. Set your multimeter to measure AC current and place the white wire inside the measuring clamp.
4. Depress the door switch and use a piece of tape to hold it down. This allows power to the furnace.
5. Have the thermostat call for heat. The furnace will cycle through normal operations and the burners will ignite.
6. 30-60 seconds after the burners ignite the control board should provide power to the blower motor. Watch the amp meter and note if and when it registers the flow an electrical current. The electrical current should range between 4-10amps.
7. If the blower motor fails to come on and burners cut out, you have tripped your high limit switch.
8. Remove the tape from the door switch.

Interpreting the results

Current measured at 4-10 amps

If you measure a steady current of 4 – 10 amps before, during, or after the gas is ignited, the furnace is attempting to start the blower motor.

If the blower wheel doesn’t rotate, struggles to rotate, very slowly ramps up to speed, or the motor makes loud buzzing noises while current is flowing, the blower motor or run capacitor may be defective. Go to the run capacitor diagnostic to determine the point of failure.

No current measured

If you don’t measure current to the blower motor at any time during the sequence of operations, the control board has a bad blower motor relay and needs to be replaced.

There may still be an issue with the blower motor, but it cannot be tested because it is not receiving power from the control board. A new control board needs to be installed first.