Check ECM motor windings

What is going on

This check looks at the three phase windings in the ECM motor for imbalance, an open winding or a short to the motor shell. It is done with an ohmmeter and the power off. Wait the full 5 minutes after disconnecting power before opening the motor, because the electronics can hold a charge. The windings should all read nearly the same. Anything unequal, open or touching the shell means the motor is done. Keep the readings written down, and handle the module and harness with care.

Likely causes

• Unequal winding resistance between phases
• Open winding
• Winding shorted to the motor shell
• Moisture or contamination in the motor causing a leakage path

Check in this order

1. Disconnect power and wait 5 minutes
2. Disassemble the motor as in the control/end bell check
3. Measure resistance between each phase winding on the 3-circuit harness. The readings should be equal
4. Replace the motor if resistances are unequal, open or shorted
5. Measure resistance from each phase winding to the motor shell. Replace the motor if any winding is shorted to the shell
6. Reassemble and reinstall the blower assembly
7. Restore power only after confirming everything is back together, and verify operation

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)