If you read my last post on “What is an ECM Motor?” you should have a basic understanding of how an ECM motor works. As promised, we will now talk about how to troubleshoot equipment with ECM motors. This will be presented in 3 parts so make sure you don’t miss any of them.
Troubleshooting the ECM motor can be very simple if you just remember that it not just ON or OFF. There are basically 4 problems that will not allow the motor to run:
- There is no input power to the motor controller (high voltage input).
- There is improper or no communication to the motor (low voltage inputs). This problem could be the CFM Programming control or board or the low voltage connector (wiring harness).
- The motor controller (module) has failed.
- The motor has failed (least likely of all the problems)
Too often, technicians “assume” the motor has failed because it is not running.
So let’s take each problem and see how to check that out.
- No power to the motor — We need to verify that the motor and module are getting the 110 (or 220) volts to it. The power comes into the module on the 5 pin plug. If there is a jumper between pin positions 1 & 2, the motor is set up to operate on 110 volts. If there is no jumper between pin positions 1 & 2, the motor is set up to operate on 220 volts. First, disconnect power to the system. Then remove the 5 pin connector from the module. With the plug removed, turn the power back on. now, check for proper power at the plug
With the main power restored, you should be able to read the power at the 5 pin connector. (Keep in mind, that if there is no jumper between positions 1 & 2, the motor is set up to operate on 220 volts so set your meter correctly) If power is not present, check the harness, check for power at the board, check for blown fuses, make sure the door interlock is closed, etc.
After all the input voltages have been confirmed or corrected, you need to turn off the power to the unit and reconnect the 5 pin plug back onto the module. The plug must be connected all the way and locked into place to prevent arcing. If it is not fully connected and the locking tabs on the end are not “clicked” into place, vibration could cause the plug to move and break the power connection to the module.
Also, to ensure proper operation and safety, the motor must have a properly grounded connection from the 5 pin connector to ground. This is verified by reading from pin 3 (ground) and the unit chassis. You should read 0 resistance with an ohm meter.
- Now we have verified that we have line voltage to the motor but is still in not running. The next step is to verify that we are getting communication from the unit control or CFM Programming board. This involves checking the 16 pin harness to the module. In this part of the troubleshooting, you need to know the “orientation” of the plug. This is done by looking at where the “index” tab is on the plug. Now you can start to check to see if the control or CFM programming board is sending the proper signals.
ECM motors used in “standard” equipment operate in “thermostat mode” meaning that 24 volts is sent to various pins on the 16 pin harness plug based on the system mode of operation. By this, I mean, what is the thermostat calling for — heat (1st or 2nd stage), cool (first or second stage), fan only, heat pump cooling, dehumidification (reduced blower speed), etc? Once you know what mode of operation you are in, you can now see if the board or control is sending out the proper communications to the motor module.
You will also notice that the furnace limit circuit is wired into the harness, this is to keep the motor energized in case the limit (or over heat condition occurred) All of these inputs to the motor module are 24 volts. So, how do you know which pin to choose if you are checking for proper communications? Here is the “decoder ring” for the plug:
Note the “indexing tab” on the right side middle. This will give you the proper orientation to check the pins as needed.
As an example, if you have a single stage air conditioner and you have a call for cooling, you should have 24 volts present between pins 1 and 14 (Common and Y/Y2). If you do not have 24 volts there, check the harness, check the connection at the board. Remove the harness from the board and see if you have 24 volts at those same pins on the board. If you do, the problem is in the harness. If you don’t, the problem is in the board.
Keep in mind that pins 4, 5, 7, and 11 are not mentioned above. That is because these are not necessarily 24 volt outputs to the module. These 4 pins are used for the DELAY, COOL, ADJUST, and HEAT jumpers on the board that let you set up the programmed CFM. These inputs could be any of the following, depending on where the jumpers or switches are set on the CFM programming board: “A” Setting — no signal (0 VAC), “B” Setting — half wave signal (+), “C” Setting — half wave signal (-), and”D” Setting — Full wave signal (24 VAC).
As you can see from the diagram above, these can be 24 VAC or nothing or anywhere in between. None the less, these are the basic checks for the control board and inputs to the ECM motor module. It does take some time to check these. Attached at the end of this post is a sheet you can use to help record your voltages on the pins while you’re laying on the floor in the blower compartment trying to check out the motor/module.
Hopefully, these checks will aid you in properly diagnosing ECM motor problems. Keep in mind, that nearly 40% of all ECM motors returned under warranty have nothing wrong with them, they were mis-diagnosed. These checks will help you make sure you have found the problem and not just “assumed” the motor was bad and replaced it.
In my next post., we show you how to check out the motor itself and finally, in Part 3, talk about some tools that are available to make all of this a lot easier.