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Troubleshooting ECM Motors

How do I troubleshoot the ECM motor?

Troubleshooting ECM Motors

When it comes to ECM motors, troubleshooting is pretty straight forward and will allow you to make proper repairs rather than just throw costly parts at the issue.   

Let's start by powering the system off and disconnecting the communicating thermostat connection from the control board and simply jump Red to Green with a black stripe on the legacy thermostat connections.  You may simply twist these two together. Power the system on and wait for time delay.  Pay attention to what the motor does or doesn't do.

The ECM motor in the RHMV air handler (and communicating furnaces) is controlled by model data from various places and the module does NOT contain unit specific software. It does have to be the right horsepower though. Only a 1/2 HP module can be placed on a 1/2 HP motor.

CAUTION: Never remove the line voltage wires from the motor or control board while power is on to the air handler.
   
First, let's verify line voltage.  On the motor module there are two plugs. One is for line voltage and has a Black, Orange and Green wire feeding it. The Black and Orange come from the line side of the transformer to be sure.  You may insert the tips of your meter leads into the back of the connector to make sure you have between 197 and 253 Volts AC.  If you do not, make sure you have power to the unit itself, the heater kit breaker is on or pull out disconnect is properly placed.  Power to the blower motor begins at heater kit if one is present and then to the line voltage terminal block.  An orange and black wire goes to the transformer line voltage side and then jumps to the blower motor.  Remember, ECM motors are powered all the time.  Make sure ground wire is in place.  Verify pins are in correct location and properly seated in the power plug on motor.
 

Once Line voltage is verified, you may move to the low voltage.  This is a 4 wire connector.  Set your meter to read DC volts and check from Red to Brown (the two outer wires). You should measure between 9 and 15 volts DC with the cable unplugged. When measuring the voltage with the cable plugged into the motor, it will be a much lower DC volts. If you do not, check the other end of the harness to make sure you have the voltage coming out of the control board.

You may use a ECM motor tester to check the motor operation.  If you don't have one. Using pieces of wire, apply a known 24 Volts AC to pin 3 (red) and pin 4 which is right next to the red wire.  The motor should run at about 50%. 

If the motor does not run, you may power the system off and wait 5 minutes. 

Remove the module from the motor and  disconnect the 3 pin connector to the motor.  Inspect the inside of the module for any swollen or damaged components such as capacitors.  Look for moisture or indication that moisture has been present. The module will have a lot of potting material, this is normal. If nothing seems out of place or incorrect, check the motor itself.

The motor checks consist of verifying the motor rotates freely. There may be a small amount of resistance or slight cogging feel which is normal and a result of the magnets inside the motor.  Look at the magnets to make sure they are all in place.  Use your Ohm meter and check each of the 3 connections to ground. It should be open or at least more than 100,000 ohms.  Check each of the connecting pins to each other. The windings should be equal.  If the motor passes these tests, then leave it in and you may now replace the module.  Make sure the 3 pin connector is making a good tight connection.

One failure mode of these motors is corrosion on the wiring harness pins as well as the pins on the motor and the control board.  Using a good electrical contact cleaner will help rejuvenate these connections.  You may use a q-tip and clean the pins on the motor module and the control board.

Below is a link to a full Nidec troubleshooting guide.  

ECM troubleshooting
 NidecECM.pdfShare