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oldmandan 07-02-2012 05:26 PM

A/C will not come on unless blazingly hot??
I have a heat pump that I am having a significant issue with. When it is very hot outside (~85 Deg F or above), the unit runs fine and cools well. However, when it cools off slightly, the A/C will not run even though the stat is still calling for cooling. What happens when it is cooler is that the contactor will close very briefly (just a click) and then open. It repeats this cycle about every minute or so. Once the outside temp gets hot enough, the contactor will close and stay closed, and the unit will cool.

Any help or thoughts would be appreciated. ??

scottmcd9999 07-03-2012 06:13 AM

I've seen low pressure switches cause this, but generally the cycle on that is the unit runs for a minute or two, shuts off for several minutes, cycles back on for a minute or two, etc etc.

Other than that, it would almost have to be some form of electrical issue, but I can't recall ever seeing an electrical issue that was related to outdoor ambient temperature. I suppose it could happen - the circuit board in the unit perhaps has a solder joint that fails below a particular temperature, for example - but that's going to be a difficult one to troubleshoot!

Can you verify that this ALWAYS happens below a particular temperature?

Does the indoor FAN run during this condition? In other words, are you getting air movement through your ducts when this occurs?

When the outdoor unit is clicking, does the unit actually come on and run (i.e. the outdoor fan and compressor), or do you just hear the click, and nothing comes on outside?

hvactech126 07-03-2012 06:29 AM

You need to verify your voltage (24V) to the contactor coil. If you have voltage and it is doing this then you need a new contactor. But if the voltage disappears you need to trace the circuit and find the culprit.

oldmandan 07-03-2012 04:56 PM

Still will not run when it is cooler
1 Attachment(s)
Thanks very much for the reply.

In answer to your questions:
- the inside furnace blower works just fine, runs in auto or on, shuts off when in auto when the stat is not calling for cooling.
-When the contactor clicks on, it is just very momentary. I can see a little spark across the contacts when the contacts close, but then they open immediately. The compressor makes a little, very short noise when the contacts close. If I (carefully) manually close the contacts, the compressor runs just fine. The problem seems to be that something kills the control voltage to the contactor as soon as the contactor closes.
-from the circuit diagram, I wonder about the "discharge thermostat" which seems to be in the compressor, or the anti-cycle timer.
I have attached the circuit diagram if anyone can help. The real confusing part is still why it runs ok when the outside temp gets very high, but will just click when it is cooler. I do not let it stay on when it just clicking because I do not want to burn the contacts. I just wait until it gets hot and then turn on the cooling.

Thanks much, again, for the help.

My wife is about ready to divorce me if I cannot get this figured out.


techpappy 08-07-2012 08:03 AM

There is an outdoor coil temperature sensor interlocked with the contactor. I'm not an AC tech but seems to me you could jump it temporarily to see if this allows operation at lower temp. If so, problem solved. Don't forget to turn power off before jumping. i.e., remove wires on sensor and join them together when outdoor temp is below 80 def F. If unit runs then replace sensor.

scottmcd9999 08-07-2012 09:00 AM

The outdoor coil sensor is for defrost control. It wouldn't really turn the system OFF, but it could give a false reading and force the system into defrost.

More likely would be the sensor on top of the compressor. Early generation scroll compressors had a sensor mounted on top of the compressor - two black wires that went into a plastic cap. You can check this to see if it's open when your system is not running. If it is, then you should first determine why it's opening - is the condenser coil clean? That's the most common cause of overheating.

If the coil is clean, and charge is good, etc etc, then you may simply have a bad sensor. Considering the age of the equipment you can simply jump it out, but be aware this does remove some compressor protection.

Or if the system is

oldmandan 08-07-2012 04:33 PM

Problem Solved
Thanks for all the input.

For potential future reference, the problem turned out to be a bad anti-short cycle timer. I verified this device was the problem by jumpering around it, and the unit ran fine. I then replaced the device and all is well. Why this short cycle timer would only work when very warm is beyond me. However, I am including my experience just in case someone else runs into a similar issue in the future. Again, thanks all.

turnermech 08-07-2012 05:41 PM

those controls were very problematic especially after thunder storms.

Missouri Bound 08-07-2012 08:00 PM


Originally Posted by oldmandan (Post 983673)
Thanks for all the input.
, I am including my experience just in case someone else runs into a similar issue in the future. Again, thanks all.

Glad you got it fixed and happy you chose to share the problem and the solution with the forum :thumbup:

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