HVAC System Basics and Tips

Electric Motor Testing and Troubleshooting: HVAC System Basics and Tips

Confirm that the correct thermostat input and ONLY the correct input voltage is present at the interface or main control board on the furnace/ air handler or outdoor unit. Loose or broken low voltage wires are also potential problem areas and can cause intermittent problems.
For the heating and/or cooling systems, use the manufacturers guide to confirm proper demands (heat or cool), especially on multi-stage systems. Use the “Sequence of Operation” charts and the “Thermostat Wiring Diagrams,” found in these guides to confirm proper wiring and operation.

When checking low-voltage connections, always use the C terminal on the board, never ground.

Check the setting of the jumper pins or switches or menu selections on the manufacturer’s control board or system interface. Do not assume they are correct; use the manufacturer’s guide to select the proper airflow, delays, and profiles. Always disconnect the main power to the unit when making these adjustments, unless required for interface operation.
Check all terminal/plug connections both at the furnace/air-handler or outdoor unit control board and at the motor. Always disconnect power to the system before disconnecting and reconnecting plugs. Look for:

Confirm there are no limits, rollouts or safeties tripped. Also check for any fault codes present on the furnace/air-handler or outdoor unit control boards. If fault codes are present, follow the manufacturer’s recommendations to resolve the problem.
If these checks do not solve the problem, or if a fault code reveals a motor problem, read the Tech Tips below.

You may also need to review the motor identification section to identify the motor. Then you can use the corresponding motor section in this guide for additional diagnostics.

Do not automatically assume the ECM motor has failed. Follow this guide completely before replacing it.
When it is necessary to disconnect the power from the HVAC system, it is always good practice to verify that voltage has been disconnected with a voltmeter.
A True- RMS meter is not needed to check high voltage or low voltage (communication voltage) to the motor.
Always pull on the connector and not the wires. Most connectors are also keyed. Reconnecting a connector the wrong way could damage the motor.
When checking any plug connector, the meter leads are most likely larger than the terminals or socket. Using thin leads will prevent the terminals from being damaged by voltage checks. Thinner leads are also available in a 90º angle as needed.

Use this guide to help with motor identification.

The following guides will provide steps for HVAC system motor troubleshooting: