Need help with AC, no 24VAC

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Old 06-01-14, 05:34 PM
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Need help with AC, no 24VAC

I've been reading through the forum and still need help finding a solution to my AC problem.

Problem: Outdoor A/C unit will not turn on when set to cool on t-stat. The indoor fan blows, but the outdoor A/C unit does nothing, not even a sound. When I first took off the service panel I saw that there was an old beehive inside, so I cleaned it out and after reading some stuff online I decided to replace the contactor. A/C still wouldn't run after replacing the contactor, it does work however if i push in the plunger manually. So i tested the voltage at the contactor, I would get 240VAC to the contactor, but i wasn't getting the 24VAC needed to engage the plunger.

So I visually checked all the low voltage wiring from the circuit board in the basement to the outdoor unit and everything seemed intact. I checked the thermostat by jumping some of the cables. I first jumped R and G and the indoor fan came on, which is what its supposed to do i think. Then I tried jumping R and Y, the indoor fan comes on but the outdoor A/C unit still does nothing.

I then checked for a high pressure switch reset button in the outdoor A/C unit, doesn't seem like it has one.

I checked the drain pan in the basement, there was nothing there, and it doesn't seem like my system has one of those float safety switches either.

I also tried hosing down the outdoor unit, perhaps the coil was just really dirty. I checked the evaporator coil in the basement to see if that was dirty, it was clean. And I also replaced the air filter. Still no luck.

Some history: Last winter was EXTREMELY cold (some days were -50F) and we had a lot of snow. We've been finishing the basement so there's a lot of construction/dust down there. The A/C worked fine last summer and the heat worked fine during the winter. (Don't know if any of this is helpful but I figure more info would be better)

I think at this point, I should be checking the circuit board to see if there's an issue there, i'm just not sure how to check all that stuff, so some guidance would be nice with that.

The outdoor unit is a Lennox 13ACD.

Any help would be much appreciated. Thanks everyone.
 
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Old 06-01-14, 06:00 PM
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Welcome to the forums.

Go to outside unit and see if you have 24vac where the wire enters the unit..... not where it connects to the contactor.

Let us know if it's there.

The service manual says there may be a manually resettable high pressure switch.
 
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Old 06-01-14, 06:11 PM
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Deleted... already pointed out by PJ.
 
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Old 06-01-14, 06:20 PM
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Thanks for the response. How do i check the voltage on the actual wire? When I checked on the contactor I put the probes of the multimeter at the terminals where the wires are plugged into. I'm not really familiar with electrical stuff… Currently the 24VAC wires that come from the house into the unit (two small red and white wires) are separated and each one is connected with a wire nut that both seem to go to the condenser deeper in the unit. Do I just unscrew the wire nut and touch both probes to the 24VAC wires?
 
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Old 06-01-14, 06:37 PM
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Do I just unscrew the wire nut and touch both probes to the 24VAC wires?
That is exactly correct. ..................................
 
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Old 06-01-14, 06:44 PM
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Well, just my luck, it started raining out. Guess that will have to wait until tomorrow. Anything I can check inside?
 
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Old 06-02-14, 05:15 PM
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OK PJmax or anyone out there, I tested the wires at the outdoor AC unit, the red low voltage wire that comes in from the basement has 27VAC and the white (I think its called the common wire) had 0 volts.

So, seems like there's some kind of open switch that goes to the compressor??
 
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Old 06-02-14, 05:22 PM
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Please measure from the red to the white.

Red to frame = 24vac
*red to white = 24vac
white to frame = 0v
 
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Old 06-02-14, 05:37 PM
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24VAC from red to white.

Thanks again for the response.
 
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Old 06-02-14, 07:41 PM
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Ok. You'll probably find one of those wires goes right to the contactor. The other one is going to pass thru at least the low pressure switch and possibly a high pressure switch. You can follow the path visually and use the meter to check the 24vac as you move thru the circuit.
 
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Old 06-02-14, 08:24 PM
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Can't check right now because it's pretty dark out, but from what I remember last time. I followed the low voltage wiring and one of the wires was wire nutted to a black cable that connected to a green cap which was connected to a copper pipe that lead into the condenser. Another black cable came out from the green cap and lead back up to the area near the contactor and I believe it was adjoined (via wire nut) to the other low voltage wire from the basement.

I can follow the low voltage wires again tomorrow but i'm not sure where I would be able to test, the only exposed part of the low voltage wiring is where it's wire nutted to the black wires that lead toward the condenser. This is where I measured voltage today.

I tried pulling out the green cap or pushing on it to see if that was a pressure switch that had some kind of reset but it was stuck in place.
 
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Old 06-02-14, 08:28 PM
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Here's a picture of the schematic. It does look like there is a low and high pressure switch along with some kind of timed out control? Not really sure how to read a schematic diagram.

https://www.dropbox.com/s/20qrfc4e0d...528_153745.jpg
 
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Old 06-03-14, 05:57 AM
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Per the schematic, it appears to me that the pressure switches (low & high) as well as the timer circuit are optional. Some units have them and others don't. Follow PJmax's advice and visually trace the circuit out. Follow the wires from where the control voltage (24VAC) comes in and see what it passes through to get to the contactor.
 
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Old 06-03-14, 08:11 AM
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That one green item sounds like the low pressure switch shown as S87/S24 on the schematic. The low pressure switch is never an option. The high pressure switch and the timer are optional. You'll need to connect one probe of meter to C.... the thermostat wire that goes directly to the contactor. Use your other probe to check both connections on the switch. Should be 24vac on both connections. If not.... try temporarily jumping around that switch and see if contactor closes.

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Old 06-03-14, 05:26 PM
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OK guys after looking more closely to the schematic I think and I HOPE that i may have found some sort of issue that can be easily fixed.

I noticed on the schematic that the C wire should go directly to the contactor and the Yl wire (which i assume is the red low voltage wire) should go through the low/high pressure switches).

When I looked at how it currently is set up, it's the other way around. The Yl wire (red low voltage) is nutted to a black wire which goes directly to the contactor and the C wire (white low voltage) is nutted to another black wire heading through the low/high pressure switches.

Should I just swap positions of the red and white low voltage wires? I didn't want to do this without any guidance, wasn't sure if this would mess something else up.

Or does this not make any difference whatsoever…
 
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Old 06-03-14, 05:29 PM
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The voltage is A/C..... polarity/color makes no difference.

Put your probe on the Y1 wire/red and move the other probe thru the circuit.
 
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Old 06-03-14, 05:39 PM
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I tried to follow the wire and it ends at that green cap. Here's a picture of the inside of the AC unit. If you look closely at the hole where all the wires enter, there's a small black wire that's flagged with blue tape, that's the wire that is nutted to the low voltage wire. If you follow it down it ends at that green cap (low pressure switch?). You can also see another black cable that leads up and out of the green cap that goes directly to the side of the condenser. Unfortunately I can't access the area of the condenser where all those wires connect to, it's too deep into the unit.

https://www.dropbox.com/s/68avccejrm...603_190708.jpg
 
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Old 06-03-14, 05:53 PM
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Those wires go towards the contactor area.... or at least that's where the compressor ones end up. The wiring is made to be repairable and replaceable.

We can't follow it for you. You have to do that.

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That is the low pressure switch in green.
 
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Old 06-03-14, 06:21 PM
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ok now I'm really confused… sorry for my ignorance.

here's a photo of the contactor area.

https://www.dropbox.com/s/466ndwlddq...603_190316.jpg

When I follow the low voltage wires. It originates from the basement and it enters the unit from the small hole on the right in the contactor area. The white wire is nutted to a black wire which leads deeper into the unit and ends at the low pressure switch. From the low pressure switch another black wire comes out of it and heads back up into the contactor area and it connects to the contactor. The red low voltage wire that comes from the basement is nutted to another black wire which is directly connected to the contactor.

I'm not really sure how else to follow the circuit. There are no other connections/terminals/exposed wire to test at besides at the contactor area.
 
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Old 06-03-14, 06:24 PM
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OK....so that means the only item inline is the low pressure switch. You can try temporarily bypassing it to see if contactor activates. It's sounding like you are low on refrigerant.
 
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Old 06-03-14, 06:36 PM
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Thanks PJ,

Last question. How would I wire it to bypass the low pressure switch.
 
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Old 06-03-14, 06:45 PM
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Follow the black wires to their end and use a piece of wire to connect them. It almost amounts to connecting the other 24vac wire to the contactor.
 
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