In my previous post, we covered checking the power and communication to the ECM motor/module. If all of that was correct, now we need to see if the problem is in the motor, or in the module.
These tests are no different from taking resistance readings on a 3-phase compressor motor. We want to confirm that the “windings” are not shorted to ground and that the resistance is equal when tested phase to phase.
Test A: Measure the resistance between each of the 3 motor leads to the unpainted part of the end shield (winding to ground resistance). Typically a good motor will read infinite ohms to ground on all leads. A grounded motor will read a measurable resistance from any one of the motor leads to ground. For the purpose of this test, the meter should be set to the highest ohms scale. Mega-ohm meters should not be used for this test.
– Test Fails (resistance <100K ohms) – Motor needs to be replaced with an equivalent motor .
– Test Passed (resistance >100K ohms) – If the resistance is greater than 100k ohms, then perform Test B.
Test B: Measure the motor phase-to-phase resistance by checking the combinations of the 3-pin motor connector with an ohmmeter.
- Resistance between lead 1 and lead 2
- Resistance between Lead 1 and Lead 3
- Resistance between Lead 2 and Lead 3
The values for each lead to lead resistance reading should be measured with the meter set to the highest ohms scale –The resistance across any two leads should be less than 20 ohms. All resistance readings measured between any two of the three leads should be no greater than +/-10% of each other. If any of the three resistance measurements are greater than 10% from the other readings, the motor should be replaced with an exact replacement.
The motor must pass both tests to be good. If the motor fails either test, it must be replaced. If the motor passes both tests, the motor control should be replaced (control module) only with an EXACT replacement programmed module.
Replacing the Motor
When replacing a failed motor or reinstalling the existing motor, caution must be taken to ensure that the belly band is not covering any vents and is not installed on the motor control. The motor control does not have a specific orientation, however, the motor does. The three-wire plug must be oriented properly, allowing the plug to reach the motor. This must be aligned properly prior to tightening the belly band.
Always make sure to tighten the wheel key on the flat side of the motor shaft with the blower wheel centered in the housing. If the wheel sits too close to the motor when centered or cannot be centered because it hits the motor, the motor must be adjusted in the belly band.
Replacing the ECM Control Module
1) Lock-out and Tag-out the electrical disconnect.
3) Unplug the 5-pin connector and the 16-pin connector from the motor control.
4) Remove the blower assembly from the HVAC system.
5) Remove the two (2) hex-head screws from the back of the control. BE SURE TO SUPPORT THE MODULE SO IT DOES NOT “DROP” CAUSING THE WEIGHT OF THE MODULE TO PULL THE WIRES OUT OF THE MOTOR. If this happens, you will be replacing the motor as well.
6) Unplug the 3-pin connector from inside of the control module by squeezing the latch and gently pulling on the connector.
Attaching the New Control Module
The 3-pin connector should be inserted into the new control module. A slight click should be heard when inserted properly.
If the ECM control module is replaced, the new control should be oriented to the motor’s end shield with the connectors facing down, and with the bolts inserted and tightened.
The 16-pin connector and the 5-pin connector should be plugged back into the motor. The keyed connectors must be inserted properly and securely until they click.
The blower/motor assembly should be re-installed into the HVAC system.
Now that you know how to perform all these checks by using your meters to properly diagnose and ECM motor, in Part 3, we’ll show you some tools that do a lot of this work for you and explain how to use those tools.