Our central AC will cool for hours, then randomly stop working for several hours, then works again. My husband replaced the capacitor and it did not fix the problem. The thermostat is calling for cooling, but the compressor is not responding (not even humming). The AC has been running intermittently for a couple weeks now. Any ideas on what to check next?

 

Welcome to the forums.

First off... be careful. There's 240vac in there.

There is a contactor inside the unit that connects the compressor and fan to the incoming 240vac line. You need to see what the contactor is doing.

If the contactor is closing you could have major problems if you've already changed the compressor capacitor.

If the contactor is not closing you need to check for 24vac on the coil. If you are low on refrigerant that will keep the condensor from starting. When you check for 24vac.... turn power off to the outside unit. The contactor is powered from inside.

This is a dual pole contactor. Two moving contacts. Some only have one. The 24vac is pretty much in the same place.



As I re-read your post.... you may also have a compressor overheating problem. A typical problem there would be the fan slows down or stops and the compressor cuts out on thermal overload. It would take anywhere from 1-3 hours to cool off. BUT... the fan would be working while the compressor was in overload unless the fan motor is bad.

 

Thanks PJmax. Neither the fan nor the compressor responds to the thermostat call for cooling during the "down times". We will have to google what a closed vs open contractor looks like and check that next.

 

PJmax, is the picture you included how it should or should not look (open or closed)?

 

I modified my picture. The circle is around two poles. Your contactor may only have one pole in the center. When that's out... it's off or normal. With 24v it gets pulled in.... or you could press it in carefully with a clean stick and that should turn on the compressor and fan for testing only.

 

Ok, got it. Are we turning off the indoor and outdoor power sources before checking, or leaving them on? I've seen videos of people testing them removed from the unit, which makes me think no power is necessary, but to see if it responds by pressing it in manually the power would have to be connected.

 

We are trying to check the contactor on our intermittently running AC. The contactor is not engaged/pulled in while the thermostat is calling for cooling so we know this can be a cause of low voltage to the contactor. To push in/engage the contactor to test if the compressor will kick on, do we leave the power supply on and manually push it? It seems like you would have to, but is it safe to push that in while power is available to the system? If it is low voltage, will replacing the contactor fix that problem?

 

OK. I joined your thread. We'll keep them in one place.

There should be a disconnect near the outside condensor for servicing it. Open/turn off that disconnect to remove the covers from the condensor.

To test the contactor manually..... reconnect the disconnect. Push in the contactor with a non conductive stick.

To test for 24vac to the contactor.... open the service disconnect. Now you can check for the 24vac as it comes from the inside unit. Check it at the contactor. If not found.... check it at the wire nut connections where the thermostat wiring connects to the condensor.

The contactor would be replaced if you measured 24vac at it and it didn't pull in or the visible contacts were extremely burned/pitted. Some black arcing is normal. The contactor would not be replaced if you didn't find the 24vac there.

The 24vac value is not written in stone. It could from 22vac-29vac.

 

I'd like to ask a question since I'm having the same problem. If my compressor is in thermal protection mode and not turning on, does turning off the system at the thermostat for a few minutes reset the protection and allow the compressor to kick on? That's what mine does. Same symptoms as the OP (condensor fan on but no compressor) so I'll turn off the system and when I turn it back on, I have compressor again.

 

A compressor goes into thermal overload when it gets too hot. There is no way to manually* reset it. The compressor needs to cool back down to a lower temperature and the internal overload resets automatically.


* Actually there is.... you can spray cool water on the compressor to cool it quicker. It's something I don't like doing as it creates thermal shock in the compressor. It's like pouring cold water over a hot pan from the stove... it can warp and bend.

 

Is a thermally overloading compressor getting ready to die?

 

Possibly but it's more likely clogged fins in the condensor causing it to run hot or the fan is slowing down or stopping.