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

  • Not sure what your electrical ability is but before ever changing out a motor, I always disconnect it from the unit and tie it directly to the feed. I've got quite a few patch cords.
    – JACK
    Commented Jul 29, 2020 at 16:16
  • Hello, and welcome to Home Improvement. If you measure the voltage at the motor while it's running during the day, and while it should be running (but isn't) during the night, what do you see? And, you should probably take our tour so you'll know you'll know the details of contributing here. Commented Jul 29, 2020 at 16:20
  • You may want to edit in the make & model of your AC unit. Not sure if it will help, but it certainly won't hurt.
    – FreeMan
    Commented Jul 29, 2020 at 16:59
  • It looks like you now have two separate accounts; you should request that they be merged. Commented Jul 29, 2020 at 17:33
  • With it not turning on at night I wonder what the temps are? Is there something you are missing, I don’t think I have ever had a motor itself that failed only at night but I have had controls that set off alarms at night (full moon) the only thing I can think of would be temp related the thermistor may be going bad (fairly common) replacing that over the entire motor would save quite a bit what is the model #?
    – Ed Beal
    Commented Jul 29, 2020 at 17:39

3 Answers 3


Since all signs seemed to point to a bad motor, I took a chance and replaced it. The new motor has been working fine for several days now. Not exactly sure why it was only failing at night, but I still think it is related to the slightly higher voltage fluctuations I get at night. I did power the old motor directly for high and low voltage inputs (removing the control board from the equation) and experienced the same problem when the voltage moved higher and then it was fine when the voltage came back down. Not sure why. Could a bad thermistor cause this? Not sure. Unfortunately the circuit board in the control module of the motor is potted and I don't see how I could fix it. Too bad. The motor was only 5 years old and it would be great to repair if possible. Anyway, I hope this helps anyone else who may experience similar problems. Thanks all.

  • Yeah, it sounds like something was weird with the electronic controls that caused them to be marginal at high line...glad it's fixed now, though! Commented Aug 11, 2020 at 23:10
  • Thanks for following up. It seems I have a similar problem - works sometimes and not others.
    – Jim
    Commented Oct 4, 2020 at 16:31

YOUR MOTOR IS GOING BAD. I had this exact same problem! Eventually it went out and I had to get a new one. Based on the research I did by Google lol I found that these motors don't do well with heat and humidity. Mine would work fine during the day but then when the temp dropped at night go out and the AC would freeze. This is because when the temp would cool the hot motor it would form moisture on it from the hot humid day. One forum I read had an ac guy who's customer had this problem happen every 2 years and had to replace the motor. They said keeping it at AC on instead of Auto will prevent this problem. If I have this problem again I'll get my ac guy to convert my motor to standard.


Try running the blower on a different power source.. "generator" especially if it is using a ECM motor. The problem usually bad power... the phase alignment is not at 15° angle this is also called the (power factor) If your project is on the end of a power lines or the outer edge of a utility district you will experience this problem more than others. The agriculture community has installed thousands of variable frequency drives and pump motors. This equipment is trashing the power grid causing what is called flat-line power. This will effect your power at different times of the day some are effected all day and night. The utility district will place a huge capacitor banks in the problem areas. They look like huge pole transformers (3) on platforms in the troubled areas. If you see them you know there are problems.... you can test for the problem with a Fluke class (A) power analysis meter "435 series 2 energy analyzer" you can also usa a Fluke Oscilloscope you won't get the total power report the 435 offers. The 435 meter will let you analyze the power by viewing the sign-wave and power factor. "Power distortion". Recording for weeks and sampling in Milliseconds combining the recordings in one analysis report. Utility companies in most cases are not going to help regardless of your evidence. In some cases you will have to put a non-ecm motor in like a rescue 3 speed capacitor start with a quad mounting bracket. You can design simple power filters by using capacitors. As utility conditions change you will have to modify your filters.

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