Hi Everyone,

I have been having an issue with my central air conditioning system for the last couple of summers and haven't been able to track down the issue. I'll give you the back story as concisely as possibe so you know what repairs I have had done.

We replaced our central air conditioner in August of 2012. The system ran great all the way until May of 2014. In May of 2014, when I went to turn the AC on for the first time, nothing happened. I called a technician and he determined there was no coolant in the system. He recharged it and added a dye pack. It turned out there was a leak in the evaporator. It was replaced under warranty and it seemed to run fine for about a month.

After a month, I started to notice that the condenser would turn itself on and off at random while the blower fan in the house continued to run. Sometimes, it turns itself on and off 20 times before finally staying on. It would come on for 5-10 seconds, shut off for 5-20 seconds, and repeat that until it stayed on. I called the technician again and he determined that the TXV was bad. It was replaced under warranty but a couple of weeks later, the condenser started turning itself on and off at random again. A few times, I have come home from work and the house has been super hot, like the AC wasn't running at all. This has happened a few times, but is not anywhere near as frequent as the condenser turning on and off.

I called them out again to take a look at it a couple of times this summer but they haven't been able to figure it out. It never seems to happen when they visit. When they check the coolant levels and voltages, everything seems to be fine. I thought I had it narrowed down to the thermostat. I noticed one day that if I didn't have the thermostat slider switch in just the right spot for the cool setting, the condenser wouldn't stay on. I replaced the thermostat last week, but a couple of days later, the condenser started turning on and off at random again.

The condenser turning on and off happened on Sunday morning and I took the cover off of it. I noticed the contactor is pulling in and out when it is turning on and off. I don't know if the contacter is bad, or if it's just doing what the voltages it is receiving are telling it to do.

My question is, has anyone else run into this issue in the past? I've been googling all summer trying to find someone with a similar situation, but I haven't found any forum posts or blogs that describe this issue or what fixed it. Is there anything I can do to troubleshoot this while it is happening since I can't reproduce it when the technician is here? I'd like to save the system if possible. When it turns on and stays on, it cools just fine. I just can't seem to track down what is preventing it from staying on.

If it helps, below are the model number of the condenser, evaporator, and thermostat.

Condenser: Ruud UANL-031JAZ
Evaporator: Ruud RCFL-HM3617CC
Thermostat: LUX TX500Uc
Furnace: Heil fan assisted gas (I couldn't find the model number, but I can check closer if it helps)

Thanks!

 

Do you have water in the secondary pan?

Do you have a float switch from on the secondary pan or on the primary drain line?

I have seen a float switch cause intermittent cycling of the condenser because the drain was partially restricted.

 

Thanks for the quick response!

I don't know if I even have a secondary pan. There is single PVC drain line that comes from the box that holds the evaporator and nothing else. I opened up the box with the evaporator on sunday and didn't see anything that looked like a float switch.

The drain seems to be working OK. It just drains to a sump in our basement.

If you have any other suggestions, I'll be happy to try them

Thanks again!

 

That constant cycling must be corrected ASAP as it is not healthy for the compressor.

Do you have a voltmeter ?
You'll need to check the 24vac signal where it comes from the house and connects to the condensor. There should be a junction box where the brown thermostat cable enters the unit. There should be two wires in the thermostat cable that connect to the unit with wire nuts.

If you are losing this voltage.... then the problem is in the house.
If the voltage is steady at 24vac then the problem is in the condensor.

My guess.... a low pressure or low pressure switch issue.

 

@PJMax, thanks for wading through my post and responding! I've been concerned about the compressor killing itself from the constant cycling also. It doesn't cycle on and off every time the system runs. Most of the time, it turns on and runs like normal. It's been frustrating that we haven't been able to narrow down what the problem is. I'm wondering if the TXV was ever actually an issue when it was replaced last summer.

I do not have a voltmeter, but I have been considering getting one after reading a few other threads here. I'll pick one up. My question is, when I take the cover off the condenser, where should I check the voltage for the thermostat wire? Should I do it at the contacter or at the wire nuts? I wish I had a voltmeter right now. I woke up to my blower fan running and the condenser not running at all.

As for the low pressure switch, the technician mentioned that when he was out about a month ago, but he couldn't confirm it since the system seemed to be running fine when he was here. He checked the coolant levels and voltages then and he said everything looked normal.

I'll pick up a multimeter tonight and if the system is cycling on and off, I'll check the 24V wires from the thermostat.

Thanks again!

 

One more update. After noticing that my condenser was not running this morning. I turned the AC off and back on. The condenser didn't even try to turn on. This has happened in the past. Usually, just turning off the AC at the thrermostat for an hour or 2 seems to "fix" it. I thought I would share that in case it helps you guys. It might be important, or it might be a red herring.

 

Jason, you when you get a voltmeter, I would suggest that you check for 24VAC at the contactor first (side terminals, small diameter wires). If you don't have 24V at the contactor, then check for voltage at the wire nuts where the thermostat wire comes into the outside unit. If you have voltage there (wire nuts), then you likely have an open pressure switch (low or high) in the outside unit. If you don't have voltage at the wire nuts, you either have a problem with the wire from the house to the outside unit (intermittent break in wire), or a problem inside the house. When the A/C isn't turning on, you should check for 24VAC inside the house where the wire going outside connects (probably on your furnace control board). If you have voltage there, but not at the outside unit, it's the wire going outside. If you don't have voltage at inside end of wire, then it's possibly a bad control board (further investigation required).

 

@Bob14525, Thanks for the tip. I just picked up the multimeter. It's a Klein Tools MM100. I hope that is a decent multimeter. If this is not adequate to test this let me know before I open the package.

My wife turned the AC off this morning. When I got home, I turned it back on and it seems to be running fine so far. The condenser is running along with the blower in the house.

I'll get the multimeter ready tonight so the next time it starts acting up, I can run out and take the cover off the condenser and check the contacter and 24V lines.

I appreciate everyone's help so far!

 

Jason, the Klein is a good meter. It's more than adequate for what you'll need.

 

The issue with the condensor turning off while the blower in the house stayed on happened again.

I checked the 24V lines by placing the probes of the multimeter on both sides of the contacter. I didn't see any voltage and the conatctor was NOT pulled in. The bar on the contactor pulled in on its own after a few minutes and everything started to run. The voltage at the contactor read 24V after the bar pulled in. A few minutes after that, the compressor turned off but the condensor coil fan kept running. The compressor was pretty hot to the touch. I put the cover back on and went grocery shopping. By the time I got home, the compressor was running again.

I'm not sure what is happening now. Is the compressor supposed to turn off while the condensor coil fan keeps running?

 

No, they should both turn on/off together. The contactor is a relay. When 24V is present on the side terminals (coil), it pulls the contact down and sends power to the compressor and fan. Since you're intermittently losing 24V at the contactor, the next place to check (when it's acting up) is where the 24V comes into the outside unit (small diameter cable). It connects to the outside unit wiring using wire nuts. If you have 24V at the wire nuts but not at the contactor, then you likely have an intermittent low or high pressure switch or time delay relay (these in series with the 24V, before it gets to the contactor).

 

I figured they were both supposed to be on at the same time, I just wanted to confirm.

I'll check at the wire nuts the next time it acts up. I saw where that hooked up to the brown wire from the house. If I recall correctly, it looked like the brown wire had a red and white one inside of it.

One more question. Should I just stick the probes in the wire nuts to check this or do I need to unscrew the wire nuts and place the probes on the bare part of the wires? Sorry about the noob question, I've never done this before and I want to make sure I don't damage anything or hurt myself in the process.

I know I've said this a bunch of times already, but thanks again for guiding me through this!

 

If you can make contact with the wires without removing the wire nuts, that's the safest way to do it. However, often there's not enough bare wire to get the probe on. If you have to take the wire nuts off, be very careful not to let the two wires touch each other or any metal (frame).

 

I got home from dinner tonight and the condenser was off while the blower in the house was running. I did a reading at the wire nuts and it showed 24v. It was 0 at the contacter.

Is there anything else I should test in the meantime?

 

That means that the problem is limited to something in the outside unit. Many, but not all units, have either a low pressure switch, high pressure switch, or both in series with the 24VAC (between the wire nuts and the contactor). The pressure switches protect the compressor in case of abnormal pressures. Some units may also have a Time Delay Relay circuit in series with the 24V line. Unfortunately, I couldn't find a wiring diagram for your outside unit, so I don't know what's in there.

You may have a wiring diagram label on the inside of the service panel (cover you remove). If not, you'll have to trace the wiring from the wire nuts to the contactor (one side is probably direct, while the other side will pass through the devices mentioned). Once you've found the pressure switch/switches & TDR (if present), the next time the system acts up, measure the voltage across each of the devices. The voltage should be 0V across each device when working normally (they pass the control voltage to the contactor). If you find one with 24V across it, that's where the problem lies.

 

The unit has a high and low pressure switch as far as I can tell. They look like they are soldered to the high and low pressure lines. I attached a photo of the wiring diagram on the inside of the cover if that helps. Hopefully it is readable.



The diagram also mentioned a time delay control.

I didn't see any obvious bare copper for either of these switches. Is there a particular spot I should place the probes of the multimeter to get a reading?

 

Jason, unfortunately the picture of the wiring diagram is too small for me to easily read. However, since you say that you've found the pressure switches and TDR, the only remaining thing is how to tell which one is "open". How are the wires from the pressure switches connected, either to the switch itself or to wherever the wires lead to? Personally, I'm suspicious of the TDR, only because the one in my system went bad about 5 years after the unit was installed. I didn't replace the TDR, I jumpered around it and replaced my old mercury bulb thermostat with a digital thermostat. All digital thermostats have the time delay function built into them.

When the system runs, does it seem to cool normally? If so, then it's doubtful that the pressure switches are opening up because of low or high pressure, again making me suspicious of the TDR. If you have a digital thermostat you could try jumpering around the TDR and see if that solves the problem.

 

It looks like the bulletin board shrinks the images when you upload them. Here is a direct link to the original photo I took if you want to take another look. You should be able to see more detail.

http://jason-baker.com/img/originals/WiringDiagram.JPG

I'm a little confused by the diagram. I see the time delay control on the diagram, but I'm not sure where it is physically on the unit. I'll have to do some more investigating to see if I can locate it and if there is any way to get a reading from it.

Thanks again Bob!

 

Jason, thanks for the link. That is very readable now. It appears from the diagram that there will be a brown wire that goes to the R1 terminal on the TDC, although the diagram shows the TDC as being optional, so there's no guarantee your unit has one.

It appears from the wiring diagram that of the two 24V wires coming into the unit, one passes through the TDC, while the other passes through the High/Low pressure switches on their way to the contactor. It also appears that the Yellow wire from the Terminal Block/wire nut goes to the pressure switches, while the Brown wire goes to the TDC, again assuming that they're both there.

If the two wires (inside the unit) connecting to the thermostat wire coming from the house at the wire nuts are yellow & brown, then it appears to be wired as shown. Also, it appears from the wiring diagram that the two 24V wires connecting to the contactor are also yellow & brown. If this is so, then there should be a relatively simple way to determine which side of the circuit is opening up when the unit is acting up.

When it's acting up, check voltage from the yellow wire at the wire nut to the yellow wire at the contactor, same for brown wire at wire nut & contactor. When acting up, if you measure 24V between brown wires, the TDC is the problem, if 24V between yellow wires, pressure switches are cause.

If you want to take some pictures of the wiring inside the unit and post them in the same folder or provide links, I can take a look, however if you confirm that wiring inside unit matches diagram, there's no need for pictures.

 

Bob, thanks for the detailed explanation! Since I'm not 100% sure what I am looking at, I will take some photos of the inside of the unit and post links to them here. I'll annotate them to help clarify what is what. I'll try to post them tomorrow evening. It's dark here at the moment and the unit seems to be running OK right now.

 

Hi Bob,

Here are a few more pictures. The contacter photo might be a little confusing since there are a few wires there. I tried to add some notes, but the annotation software on my machine is terrible. If you can't make sense of what is going on the photo, I'll see if I can find some better software to label this photo.

Compressor
Contactor
Fuse Box
High Pressure Switch
Low Pressure Switch
Outside Overview
Thermostat Wire To Fuse Box
Wire nuts connecting thermostat wires to pressure switches and back up to contactor

 

Jason, thanks for the detailed photos. I think I can follow the wires and understand how it's wired. it appears that you don't have a TDC, as the brown wire that connects to the thermostat white wire goes directly to the contactor (not through the TDC). However, it does appear that you have both low & high pressure switches, and I see what you mean about no easy way to check the voltage across the switches. Here's what I would suggest.

In the contactor image, you can see a yellow & brown wire passing through a small grommet near the bottom of the picture. Those are the yellow & brown wires connected to the thermostat cable (wire nuts) The yellow wire comes out of the small grommet and goes back down through a large hole. It then should connect to one of the pressure switches (can't follow the wire from the pictures). You'll have to trace where it goes by gently tugging on the wire and following where it goes.

Once you've found which pressure switch the yellow wire goes to, find the other wire from the switch (also yellow). That (second) wire should connect to the other pressure switch. What I would suggest you do is to gently trim a little of the insulation on the wire that goes between (connects) the two pressure switches. Trim the insulation on the wire in a section of the wire that is not near any metal. Once you have cut the insulation, you have a way to connect your voltmeter probe and can effectively check the voltage across each of the switches. Put one voltmeter probe on the uninsulated wire, and the other on either the yellow contactor wire, or the yellow wire at the wire nuts. This will give you the voltage across each of the switches. The wiring diagram indicates that the low pressure switch is first in the circuit, so if that is correct, measuring from the uninsulated wire to the wire nut would give you the voltage across the low pressure switch, while measuring from the wire to the contactor would give you the high pressure switch voltage.

When you're all done and have found the problem, wrap the cut you made in the insulation with some electrical tape to re-insulate it.

Good luck!!

 

Thanks Bob! I'm not familiar with reading wiring diagrams like that, so I wasn't sure what all was in the condensor. I'll see what I can do with testing the pressure switches on my own. It's been running OK today. Before I remove any of the insulation from the wires though, will that void the warranty?

Also, is it possible that the pressure switches are ok and I still have a bad TXV? The TXV was replaced last summer, but maybe I had bad luck and got another dud when they replaced it? Just thinking out loud.

 

Jason, good question about the warranty (I forgot that it was new enough that you still had a warranty). I really don't know if it would void the warranty. I would tend to think not, however I'm not an A/C technician, just a homeowner and electrical engineer. As for the TXV, I suppose it's a possibility (again I'm not an A/C tech). It's possible that the pressure switches are good and something (TXV) is causing an abnormally high pressure.

A few years ago a pressure switch in my high efficiency furnace went bad. I got a replacement (under warranty), and the following year it failed again. When I questioned the HVAC contractor who installed the furnace and supplied the replacement parts, he said that he had found out that the manufacturer got a bad batch of pressure switches. My luck, the original and replacement were apparently both from the same bad batch. It's been working fine for the past couple of years with the second replacement switch.

 

I'm going to play it safe and call the company that did my installation and repairs and see if that will void the warranty. If I'm entitled to any warranty replacement parts, I'd rather not risk voiding it.

I sincerely appreciate you taking the time to look at this and walk me through the troubleshooting. It sounds like it is one of the pressure switches or perhaps another bad TXV. At least I can tell the technician what I have observed and measured which may help them narrow down what needs to be fixed, adjusted, or replaced.

I'll keep everyone posted on the outcome.

 

Just for anyone that might have found this post while researching a similar issue, it turns out the TXV on my system was bad. I had that replaced this year and that seemed to fix the issue with the condenser not staying on for the full cycle. I think I just had the misfortune of getting a two bad TXV's in a row, or the technician botched the installation both times.

 

Doubtful. More likely there was crud left in the refrigerant lines when they were connected and a piece of it got stuck there.