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Original Post: AC will not come on

OptsyEagle Member

This is my first house with central air and I am having a problem getting it to come on when I want it to. It is an outside air conditioner with a Robertshaw model 8625 thermostat.

When I turn the T-stat from off to cool, the fan starts and then stops immediately even though the current temperature is 2.5 degrees C above the set temperature. It is set for a 1 degree C differential. I thought if it was left for five minutes it might come back on but it doesn’t.

The T-stat is programmable and I would prefer a manual one where I just set it and it stays there until I change it but I guess this is progress. Anyway, it has an AM1,AM2,PM1 and PM2 settings for weekdays and for weekends.

I have everything set to the same temperature but for some time in the early morning like 3am, 3:30am, 4:00am, etc.

The temporary hold feature should allow me to set a temperature for 3 hrs but it doesn’t seem to work any better.

Anyone have any thoughts on what my problem might be?

Bob14525 Member

If you change the fan switch setting on the thermostat from auto to on, does the inside fan (blower) come on and stay on?

OptsyEagle Thread Starter

Yes, the fan will blow air through the registers when switched to the on position.

Another observation is that when the AC is on a little fan is lit up on the screen of the thermostat. With my system now the little fan on the T-stat is turning but the actual fan on the air conditioner is not and therefore no cool air comes out of the registers.

If I turn the t-stat from cool to off, there is a couple of clicks and then the little fan on the t-stat goes away. If I turn the t-stat from off back to cool, you get a couple clicks and the fan on the t-stat starts turning but again, nothing from the actual air conditioner.

Bob14525 Member

Do you have a multimeter? If you do, there are some voltages you can check to help determine what’s wrong. Set your meter to AC volts and, with the thermostat calling for cooling, check to see if you have 24VAC control voltage coming in to the outside unit.

These tests are all looking for 24VAC, which comes from the air handler/furnace, so if you wish, you can turn off the power to the outside unit (there should be an electrical disconnect near the outside unit.

The control voltage will come into the outside unit via small diameter wires, similar to thermostat wire. It attaches to the outside unit internal wiring using small wire nuts. Remove the wire nuts and check for 24VAC between the two wires that had nuts on them. Let us know what you find.

PJmax Group Moderator

Good advice from Bob there.

It would appear to be a condensor related problem.

You may have a low refrigerant issue keeping the condensor from starting.

Checking the voltage on the control wire to it will confirm.

OptsyEagle Thread Starter

It seems to be an intermittent issue. Sometimes it works and sometimes it does not. Right now, after about the 7 or 8th time trying it, it is now working, so I suspect checking the voltage right now would not mean anything.

Would a low refrigerant issue be intermittent and can it be fixed by a DIY guy? I should also add that this AC is about 19 years old.

When it goes back to not working again, which I expect since I have been dealing with this issue for 3 or 4 weeks now, you are saying that if I turn the main power to the air conditioner off, I should still get 24VAC at the outside unit?

I don’t recall seeing an electrical shut off outside, and not even sure what it should look like, so would the breaker for the AC be OK to shut off for this test?

PJmax Group Moderator

With the service disconnect open…. you will still get 24vac as it comes form the air handler inside.

The low pressure switch in the refrigerant line monitors the pressure which will change based on ambient air temperatures.

OptsyEagle Thread Starter

OK, so it can end up being intermittent if the refrigerant pressure is close to the edge. It probably did get a little hotter outside and perhaps that is why it finally decided to work.

Just so I understand what I am looking for. When my problem returns and my AC does not come on when it should, I can turn off the service power to the AC and then I can check the voltage coming into the outside unit.

If it reads 24VAC, then I am guessing it is telling me that the T-stat is working fine and my unit is not working well (perhaps a low refrigerant condition). If it reads 0VAC then it is telling me that the T-stat is not turning it on as it should. Is that correct and about what I am looking for?

Bob14525 Member

As Pete mentioned, it could be a low pressure condition due to being low on refrigerant. It could also be a bad Time Delay Relay (see the fifth post on this forum thread for more info) if your system has one. Using a voltmeter to check voltages is the easiest way to determine the problem.

OptsyEagle Thread Starter

OK, so my problem returned. When I turn on the AC by switching from off to cool, the t-stat clicks, a little fan starts up in the left top corner of the LCD display and the fan on the outside units kicks on. Then about 15 seconds later the fan on the outside unit kicks off and that is the end of my AC. The little fan on the T-stats display continues to spin.

I took off the outside cover. The only wires I saw with wire nuts were the two yellow ones that are now outside the box. A small black and white wires were connected to them respectively.

After the quick cycle above there was 0 volts AC across those two wires. This was with the outside fan off but the t-stat set to cool and the little fan on its display was spinning.

What does that tell me?

Bob14525 Member

The contactor is a relay (black rectangular item in your picture). When the thermostat calls for cooling it sends 24VAC to the contactor coil, energizing it, which turns on the outside unit (both compressor and condenser fan).

Since you say you’re seeing the condenser fan turn on for ~15 seconds and then turn off, the contactor is apparently closing for ~15 seconds. My guess is that your system is low on refrigerant and the low pressure switch is shutting down the outside unit to protect the compressor.

You would have to have a qualified AC technician diagnose the problem and repair it. I don’t think it’s something simple that you can fix yourself.

OptsyEagle Thread Starter

If the low pressure switch shut down the outside unit should I not still get 24 VAC on those two leads I probed?

Bob14525 Member

If you have a low pressure switch (not all units do), It would be between where the 24VAC comes into the outside unit (wire nuts) and the coil terminals on the contactor (thin wires).

With the 240VAC turned off to the outside unit (there should be an electrical disconnect located near the outside unit), follow the wiring from where the 24VAC comes into the outside unit at the wire nuts to where it finally gets to the contactor coil terminals.

If you don’t have any pressure switches (low or high) or Time Delay Relay, the wires will lead directly from the wire nuts to the contactor coil. If there are one or more “things” connected between the wire nuts and the contactor coil, you need to see if they are open when the outside unit isn’t working.

OptsyEagle Thread Starter

It’s hard to tell where everything goes. The two wires I was probing were connected by the wire nuts. I would think that this would be my signal from the T-stat and it would be 24VAC. Since it read 0 VAC I was thinking that maybe this might indicate that this is a T-stat problem.

I would have thought that if something is turning off the condenser and fan then it would happen AFTER the 24VAC that comes into the unit.

Am I missing something there?

As for the contactor. At the bottom two terminals I get 245VAC and the top two terminals I get 0 VAC. I imagine this is because something has switched the unit off, I just don’t know what.

Bob14525 Member

Sorry if I misunderstood what you were saying. I thought you were saying that you were seeing 24VAC at the wire nuts even after it shut off. If you’re seeing 0 volts at the wire nuts, then the problem is inside the house.

Some air handlers have a switch that disables the outside unit if the condensate from the evaporator coil backs up (doesn’t drain properly). If your unit has such a switch, that could be turning on (if your condensate isn’t draining properly).

OptsyEagle Thread Starter

If inside the house I would assume it would be in the T-stat. Is there a way to test the T-stat?

Bob14525 Member

If you remove the thermostat from it’s backing plate so you can get at the wires connecting to the thermostat, connect the wire going to the R terminal to the Y and G terminals. R is the 24VAC from the transformer, Y is the wire going to the outside unit, and G controls the blower.

If you connect those three wires together, you will bypass the thermostat and be requesting cooling. You might want to turn off the power to the furnace/air handler before you play with the wiring to eliminate the possibility of shorting out something. After the wires are connected, turn the power back on.

OptsyEagle Thread Starter

To clarify, so I don’t connect the wrong wires, I am to connect the wires going to terminals Y,R&G, NOT the red green and yellow wires?

My unit has a wire going to Y1 and nothing going to Y2, so I assume I use the wire going to Y1.

My unit looks like it has a red and black wire going into the slot R. Does that make sense and do I connect both wires to the wires from Y & G or just the red wire?

Bob14525 Member

Usually, the colors of the wires correspond to the terminal designations, i.e. Green to G, Red to R, Yellow to Y, etc. However, there is nothing that says that the wire colors have to match the terminal designations.

Since you have two wires (red & black) connected to the R terminal, use both of them (chances are the red wire is the 24VAC and the black wire powers something else). And yes, use the wire connected to the Y1 terminal. Your thermostat can handle a dual stage condenser unit, hence the Y1 & Y2 terminals.

OptsyEagle Thread Starter

OK, so I connected the 4 wires together the two in R, the G and Y1 and my air is on and seems to be running fine and has not shut itself off. I tried running it the regular way just before connecting those wires together and it wouldn’t even come on at all.

If it was a refrigerant issue would the system turn on when I connected those wires together? What do you think observation tells me about my problem? What should I do now?

Bob14525 Member

Connecting the wires together as you just did simulates the thermostat calling for cooling. If the system works properly and doesn’t shut off by itself with the wires jumpered, that indicates that the thermostat is the source of the problem. Before replacing the thermostat, have you tried putting in a fresh set of batteries?

OptsyEagle Thread Starter

My thermostat does not take batteries.

I do wonder why it sometimes would turn on for 10 seconds or so and then shut itself down. Today it wouldn’t turn on at all, when asked to. Not sure what in the T-stat would make it do all that.

OptsyEagle Thread Starter

OK, so I think it’s time to replace my T-stat and I am looking at a few products. I think I would prefer digital non-programmable. I don’t need the programming options.

The Honeywell one I was looking at (Model RTH5100B1033) says that if I have six or more wires that this one will not work for me. I have seven wires but one is in the “C” slot, which they tell me not to count and two wires are in the “R” slot that my current manual says is 24VAC, return. I am not sure if I should count that as two wires or just one.

I have a gas furnace and as far as I know, no other heating source than that. I also have an air conditioner. I do not believe I have any 2nd stage heating but on my T-stat I do have a blue wire in a terminal labelled W2/E. My manual says this is for 2nd stage heating or emergency heating for heat pumps which I am pretty sure I do not have. I am guessing the installer of my T-stat just stuck that wire in that terminal and that it probably doesn’t do anything. Could that be the case?

If that wire is not used and the C wire is not counted, would that mean I could use this T-stat that wants five or less wires? Any idea why I would have two wires (a black wire and a red wire) going into the R terminal (24 VAC, return). I have a yellow wire going into the C terminal that they call 24 VAC, transformer common side.

Appreciate any help. This is all a new language for me.

Bob14525 Member

A thermostat needs power to run. Many thermostats use batteries to power themselves, however you mentioned that your current thermostat doesn’t take batteries. That’s why you have a C (common) terminal. The transformer supplies 24VAC between R and C. This voltage not only controls the furnace and outside AC unit, but also powers the thermostat.

It’s hard to say, without knowing the Make/Model furnace you have, whether the Blue wire going to the W2/E terminal is needed. The W2 terminal is normally used for a two stage furnace, controlling the second stage.

The E terminal is normally used for Emergency heating (often resistive heating) and is usually used with a heat pump. Since you have a gas furnace, it seems unlikely that you have a heat pump, however you may have a two stage furnace.

OptsyEagle Thread Starter

Would there be a quick way to tell if my furnace was a two stage furnace?

I think the T-stat that I was thinking of acquiring uses batteries so would that mean I would not need the C wire. If so, do I cap it or put electrical tape on it. Not sure what to do with any extra wires.

Bob14525 Member

What is the make & model of your furnace? That’s the best way to tell if you have a two stage furnace. As for unused wires, there are a couple of things you can do. You can tape up the exposed end or put a wire nut on the wire.

PJmax Group Moderator

“Would there be a quick way to tell if my furnace was a two stage furnace?”

Open the lower blower service door and you will see the control board. See if the blue wire is connected to anything or if there is a W2 terminal.

OptsyEagle Thread Starter

Really appreciating your help guys.

So my model is a Goodman GMV80704BXA. I looked it up and they call it a: two stage/ Variable speed furnace. So except for the fact that I don’t know what variable speed means, it seems that I do have a two stage furnace, so I guess that Honeywell T-stat will not work.

Will any “non-programmable” t-stats work on two stages or would only programmable ones accommodate a two stage operation?

Bob14525 Member

You can use a “regular” thermostat with a two stage furnace. I have a two stage furnace with an inexpensive single stage thermostat. The advantage that the two stage thermostat gives you is that, depending upon the difference between the room temperature and set point, the thermostat will run the high or low heating stage. If it’s just a small difference, the low heating stage will run, if large, the high stage will run.

OptsyEagle Thread Starter

So I installed a new White Rodgers Programmable thermostat (Model: UP300) for my two stage furnace and AC. With the help from you guys and the manual it was pretty easy to install. Since yesterday my AC has been working great, oscillating on and off like it is supposed to, so it looks like my problem got fixed.

Couldn’t have done it without you guys so thank you very much for taking the time to help me out. It is very much appreciated.

Bob14525 Member

Glad to hear that everything is now working correctly. Thanks for letting us know the outcome.

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