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ECM “constant torque” fan motors can be easily identified by noticing their spade terminal plug ends. However, make sure to read the motor's rating plate to verify this. These fan motors may also be referred to as ECM multi-speed fan motors. Technicians may be most familiar with the Genteq X-13 model or Broad Ocean models. The ECM constant torque fan motor is now the base standard for all gas furnaces and air handlers in order to meet electrical efficiency standards for the equipment. Higher efficiency and higher cost systems may have an ECM variable speed (constant airflow) motor instead.
Though these constant torque motors are an ECM type, their troubleshooting procedure is fairly straight forward compared with ECM variable speed (constant airflow) motor types. If the motor is not starting even though you suspect it should be, there are several steps that can be taken.
(This image is courtesy of our friend JD Kelly @student_of_hvac)
With the power off to the system, turn the blower wheel to make sure it free spins and is not bound up. If there is a door switch to the blower compartment, temporarily keep that switch closed with a magnet during the next steps. Next, make sure to keep your hands clear and turn the power back on to continue the diagnosis. Because the power to the unit must remain on while troubleshooting, HVAC technicians should take extra caution during testing.
Using a multimeter, measure to see if the correct AC line voltage is present at the blower motor's main power terminals. If the unit is powered with 120V, then 120V should be measured on the motor’s main power terminals. If the unit is powered with 240V, then 240V should be measured on the motor’s main power terminals. This line voltage will be present any time the HVAC unit has power (as long as the door switch, if equipped, is closed) even when the motor is not called to be on.
(This image is courtesy of our friend JD Kelly @student_of_hvac)
Next, turn the fan on at the thermostat. With a multimeter, measure for a 24V signal on the motor's low voltage terminals. The 24v signal tells the motor to turn on at a specific airflow speed (specific programmed motor torque). To read the voltage, measure between the C terminal (the low voltage common) and one of the five 24V speed taps. The system’s wiring diagram should note which speed tap number or color should be powered for during air conditioning mode, heating mode, and fan mode. Sometimes fan mode is uses the heating mode fan speed. Keep in mind that not all five 24V speed taps on the motor may be programmed for a specific speed by the factory, but some may still be set to an unknown default speed.
Place one multimeter probe on the C terminal and the other probe on the speed tap that is supposed to be powered with 24V.
(This image is courtesy of our friend JD Kelly @student_of_hvac)
If 24V is not measured, then there is no call for the motor to be turned on. This is due to either a low voltage wiring problem, thermostat issue, or circuit board issue and is not the motor’s fault. It could be that the thermostat is simply not turned to a mode which requires the fan to turn on. To double check if any 24V is present, keep the one probe on the C terminal and move the other probe to each of the other low voltage speed terminals, one at a time. This verifies that you are not accidentally measuring the wrong speed tap. If there is still no voltage measured, there is no call for the motor to be turned on and the motor is not at fault.
If 24V is measured between the speed tap and C, and the motor does not turn on, then either the module or the motor is bad. (This is assuming that you do have line voltage power to the motor as mentioned in a previous step.) If the fan does not turn on, it is likely that the module is at fault. To verify this, disconnect the module from the motor frame and simply measure the electrical resistance on the three pairs of windings from the main section of the motor. If these resistance readings match and they are not shorted to ground then that verifies that the motor itself is good. The only thing left is the motor module. This must be bad. Of course, on the module, you can visually see if any capacitors have popped or if the current limiter inside is burnt. You can also smell if there is any burnt rubber or plastic odors, indicating a problem.
The long and short is that the ECM motor with its module and motor, should be replaced. Usually a constant torque motor module is not sold separately. The new motor must be programmed for the model equipment that it is being installed in. If an aftermarket motor is being installed, be sure to measure airflow during heating and air conditioning modes and set it on the correct speeds accordingly.
Disclaimer: Before performing troubleshooting procedures on any live HVACR equipment, make sure to go through a proper state approved apprenticeship program and take an OSHA 10 or OSHA 30 course to make sure you are aware of the electrical hazards present on these systems. Remember that electrical hazards are not visible to the eye or other senses so be careful!
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Published: 9/6/23 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel
