I should have said I don't think it could be the thermostat, because it works fine until dark.

Not trying to be smart but do you have a programmable thermostat that is programmed wrong or the time is wrong. I am just making sure you don't overshoot something.

Countryboy, yes i do have a programmable stat, but It is definitely calling for cool. If it's not calling for cool at night, I can turn the temp down until it does and the fan will kick on, but no compressor. Then eventually, a few hours later, it will kick on.

During the morning and daytime, I have no issues at all.

You may not be programming the t-stat correctly.

Disconnect any power going to the furnace & thus to the T-stat, then take the battery pack out of it, then replace the battery pack reestablish the power & see if it works during those hours when it wouldn't run, even though stat indicated it should be calling for cool air. Let us know the outcome.- udarrell

Udarrell, I'll give that a try and let you know.

Thanks.

Ok, I tried that and again tonite, the compressor is not coming on. The stat has these terminals:

RC which is jumpered to RH and has a red wire coming off of RH which goes to the furnace I assume. The other terminals that are being used are: W Y G, with corresponding wires going to the furnace I assume once again.

Can I take the stat off of the wall and check continuity to make sure its working corrrectly? IF I remember right, when calling for cool, I should have continuity between R and Y. Is this right? I'm just trying to find a way to eliminate the stat.

Thanks.

On a call for cooling R closes across G to bring the indoor fan on and simultaneously R closes across Y to bring on outdoor unit (R to W is heating)

Any possibility that you have a malfunctioning POCO installed device that limits operation of the unit during certain hours of the day?

You cant run continuity because there is voltage present...24vac. What i would do is pull the stat off the wall with the 4 wires exposed. Take the red wire(voltage) and touch it to a bare spot on the green wire(fan)or the wire that is on the "G" terminal of the stat & you should hear the indoor fan com on. Then take the red wire again and touch it to the yellow wire(a/c) or the wire thats on the "Y" terminal on the stat. The outdoor unit should come on. If everything works by doing this, replace the stat. Get a good one like Honeywell or White Rogers and not on that you picked up at the hardware store.

Hope this helps

Touching all those wires sounds a little dangerous. Maybe you should call an air conditioning repair company.

Fluffy, Thanks, I'll give that a try

"Touching all those wires sounds a little dangerous. Maybe you should call an air conditioning repair company."

--- I completely agree. The original poster needs to have an experienced tech over to do a diagnosis. Not a job for a homeowner.

Its low voltage to the thermostat and if he wants to... he can turn the power off to the indoor unit, then twist the wires together, then turn power back ...to test it. Anyhow...he did post on this forum for our advice and he did talk about running continuity on the wires...which means he has a meter... so i figured he is mechanically inclined.

If not... my apology

Fluffy, I got home today, and my stat was calling for cool, and the fan was running, but no compressor. So, I jumpered R to G and R to Y. I got fan, but no compressor. So, at least this eliminates the stat.

Any other ideas?

FYI, I am an electronic technician, I just don't know much about HVAC, so, I can at least do some basic checks.

Com'on folks, it's low voltage they stat stats in home stores don't they?

A person an isle over can also purchase a line voltage ballast for self install, now that's about as dangerous a person can get to the edge.

Sounds like you have eliminated the t-stat... so now would be a good time to call a tech. It could be a bad contactor or maybe even a timer in the outdoor unit thats is acting up....either way, call someone.

Hope this helps

"Com'on folks, it's low voltage they stat stats in home stores don't they?"
Yeah but if the guys a little unsteady of hand or he jerks just a little everytime he see's a little arcing from those low voltage wires that contactor could be banging that compressor on and off like crazy. Not good!
C'mon you guys, sharpen your brains here! You got a DIYer trying to diagnose a problem here,,listen to what he's saying, "runs fine all day, won't run when the sun goes down but comes back on later at night" He's banging his head against the wall trying to figure this out without any tools or meters. Here's the key gentlemen "then sometime during the late night hours it will start up again" Hmmm, does the term OPEN INTERNAL OVERLOADS ring a bell? Thats the direction I would lean. Something is overheating in this unit and when it finally cools down, Bang, the unit starts back up! Get a tech to check your system out before you kill it. It might be somethng simple like a partially plugged coil, undercharge, overcharge, whatever, but it's not a t stat problem. Hey maybe it's afraid of the dark, is that why it hums?????:-0 :-)

Thanks for the help Fluffy. Time to call a pro.

how old is your unit might be over heating and kicking in when it's cooled down might need to put a hard start on it to help it kick on

If it's heating up, what's a hard start kit going to do?

JTW please do share the results of the pro's diagnosis if you don't mind. You've got some of us curious. :)

Has anybody mentioned to check whether the relay at the compressor is pulling in when he has that condition and if not then checking for 24 VAC at the relay might be useful to verify voltage or relay contacts?

No harm done in checking his T-stat out right?
Better than wasting money on a new one to find out that wasn't it.

Yes time for a tech with knowledge and experience

I'd suggest after running all day and then not coming back on at night that he feel the compressor shell. Might be as hot as a frying pan.

I bet there's an energy managment system installed by his electric company that's turning it off. An a/c with an internal problem will most likely show it's problems during the day when it's hottest and most stressed. OOps, I forgot to say "COME ON PEOPLE"

OK genius or is it guinnesses? Why would an energy management system let the system run when one can assume most people would not be home but then shut it down when people would be home? You must be a second shift worker!

energy companies around here cycle systems off during peak loads...when people are home. Generally that would be the evening

Some savers switches will cut only one leg of the 220 for some reason, if he happens to have a contactor that allows 110 to flow through w/o 24 volt power the motor may be on this same lead, which would explain the condensor fan motor running.

Hi jtwtech - did you finally find out what was the issue?

my ac is having the same problem. It is only 2 years old. It works fine during the day but at night it won't run.

Thanks

Could it be the coil freeze at night and it won't start?

I would measure across the relay control on the outside unit if it shows near 24V when there is a calling for cool.