Ecm motors or “Electronically Communicated Motors” are all the rave with customers looking to maximize efficiency. They are, however, a little more difficult to diagnose. In this article, I will discuss a couple of different procedures for testing these motors.
As a technician in the field, I come across more and more of these motors. The motor itself is a brushless, DC motor, and it is the module attached to the back of the motor that is the difference-maker. The module takes a 24 volt, DC signal from the furnace control board, via a separate transformer at times, and tells the motor how fast to spin (rpm) based on different factors pre-programmed into the control board by the installing technician to deliver certain cfms at different times. The motor itself runs on either 120 volts (residential) or 240 volts (air handlers and commercial units) delivered via the wiring harness. Each motor has 2 plugs. The 5 pin plug is the high voltage connection. The line voltage must be present in order for the motor to operate. With the furnace calling for heat or cooling, and power on, (be sure the blower door switch is taped down) detach the 5 pin connector from the motor and put your test leads on pins 4 and 5, and test for voltage. If power is good, reconnect and connect a jumper wire between “R” and “G” on the furnace control board. If the motor runs, your problem is with the low voltage wiring to the thermostat or the thermostat itself.
If the motor does not run, disconnect the 16 pin connector from the motor. Put 24 volts to pins 12 and 15, then 24 volts common to pins 1 and 3 simultaneously. If the motor starts, the fault is in the harness and will need to be replaced. If the motor does not start, then replace the module.
It is important to realize that if you ever replace the module and the control board together, you will have to reprogram the motor.
