ECM Blower Motor Not Working at Night

So, I have had this problem going on for about a month and a half now with my A/C. Throughout the day, the A/C will work fine. However, there will come a time in the middle of the night when it stops working. When this happens, the outside compressor does turn on, but the blower motor does not and the house temperature never cools to the desired setting. I eventually turn the system off to prevent the coils from freezing. This doesn’t happen every night, but does most. It also doesn’t happen at a consistent time.

When the problem occurs, I have tried turning the A/C off and on, but it seldom matters. I also tried testing the fan on the "non-auto" thermostat setting, but even that doesn’t turn the blower on. I have already had two different HVAC technicians take a look, but they both say that they are unable to troubleshoot the problem unless it is happening when they arrive. Since this only happens in the middle of the night, I am out of luck and starting to lose lots of sleep.

I have resorted to some troubleshooting on my own and here is what I find when the problem occurs in the middle of the night. I go to the control board and verify that the thermostat is sending the low voltage signal on the “G” fan wire. I have also verified that the low voltage signal is being sent from the control board to the ECM motor on the correct tap for the A/C. I finally verified that the control board is sending the high voltage to the motor. To me, it all seems that the motor is getting what it needs and should be running fine just like it does all day long. Hmmmm…so why does this problem only happen at night?

There is one interesting thing that I noticed. It might have nothing to do with anything, but perhaps it’s worth considering. Throughout the day, the AC voltage that I measure from the outlets in my house is usually in the range of 120 – 122 volts. It seems that at night, the voltage drifts a bit higher. When the problem happens, I usually find it to be around 123 and change. This certainly seems reasonable and should not cause a problem, but it is the only thing I see that appears to differ at night. Perhaps the motor is going bad? Since this is an ECM type motor, there is no start capacitor to check.

So, my question is this. I’m about to pull the string and buy a new motor, but before I do, I was wondering if all signs point to a failed motor. Is there any other troubleshooting that I can do? A new motor is pretty pricey and I would hate to spend the money and still have the same problem. Thanks in advance for any help.

Model Numbers:
A/C: Carrier 59SP5A080E171216
Blower: HD46MQ134