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Old 09-07-2011, 09:47 AM   #1
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A/C not working


I removed the thermostat and measured between Red (R+RC) wire and all other wires and did not see 24V. Does this necessarily mean that I have a defective transformer or is there more trouble shooting to be done before I go ahead and replace it. My unit is in the attic. Is the transformer located in this unit or is it in the A/C unit outside the house.
Any advice will be appreciated.
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Old 09-07-2011, 10:13 AM   #2
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A/C not working


When the thermostat doesn't receive the 24v, you need to check first the transformer primary and secondary voltage. If you have 24vac at secondary of transformer the next step is check the wires for open, short or bad connections.

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Old 09-07-2011, 10:14 AM   #3
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A/C not working


Yes, the transformer should be in the attic.
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Old 09-07-2011, 04:19 PM   #4
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A/C not working


First look top see if there is an emergency/over flow drain pan under the unit. And then look to see if its full of water. And if there is a float switch that is doing it job of shutting the power off. if so. Fix the cause of the water, then drain the water out of the pan.
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Old 09-07-2011, 10:43 PM   #5
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Thank you for your input. However I am a little perplexed with what I am finding.
1) Took thermostat off the wall and noticed that there was a red wire connected to R and RC, white connected to W, yellow connected to Y and blue wire connected to G. I checked voltage between red and all other wires and saw none. Then I tested for shorts and found that there is about 2.6 ohms between red and yellow wires.
2) I went outside to the AC unit and saw that the yellow and blue wires go to the AC unit. Everything looks well here in terms of wiring. 250V is also available. Therefore I am not sure why I see such low resistivity between red and yellow
3) What happened to the green wire that should have gone to the fan? This wire was never connected. So how is it that the fan in the unit in the attic turned on when the AC or heat was working in the past?
I am assuming that the blue wire is neutral and the white goes to the heating unit. Correct me if I am wrong.
I have not gone to the attic as yet to check the transformer or wiring since it is awfully hot. But I plan to do it tomorrow.
Any insights from the experts will be appreciated.
Regards,
Veej
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Old 09-07-2011, 11:36 PM   #6
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A/C not working


The blue wire is being used to FAN (G) . You need to disconnect all wires from thermostat and controller to test correctly continuity.

You need to test first the transformer.
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Old 09-08-2011, 06:45 AM   #7
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A/C not working


Stop concentrating on the thermostat for a moment, put back the stat and wires exactly as they were. make sure there is power to the air handler and outside unit, turn thermostat all the way down, go outside and check if it's running, if it's not check the thermostat wires outside for 24 volts, if 24 is present troubleshoot outside unit, if 24 volts is not there follow post #4.

Let us know what's up after that.
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Old 09-09-2011, 09:30 PM   #8
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Finally it cooled off today to go to the attic and test the transformer. It is dead. Removed it and tested on the bench also to conform. So, I'll have to replace it. But when I was in the attic, I found two dead transformers lying around the unit. This tells me that two transformers were replaced between 1995, when the house was built, and 2000, when I bought the house. The question is why so many? If there is a short, it should blow a circuit breaker and or blow the transformer immediately, isn't it? Obviously something else is going on. Any insights from the experts??
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Old 09-09-2011, 11:19 PM   #9
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A/C not working


Check the wires for open, short or bad connections. Also replace the transformer to 50va or 65va
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Old 09-10-2011, 01:48 PM   #10
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So I replaced the 24V 40VA defective transformer with a 24V 75VA transformer. Luckily it has a circuit breaker built in. Here is what is happening now:
1) I see 24V between red and blue and between red and yellow. Blue and yellow go to the outside AC unit.
2) When I move the fan switch from Auto to fan, the fan starts to run. Put it back to Auto. Fan shuts off as it should.
3) Move the cool switch from off to cool position and nothing happens. I go outside and check for 24V, none. Go to the attic and find that the circuit breaker on the transformer has tripped. Reset it and I get 24V back again.
4) Checked all wires from thermostat to controller for shorts and open. None.
4) I have 50A50-112 controller. The red LED flashes 6 times. Obviously it is an error code. What does it mean? What else should I check for?
Please advice.
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Old 09-10-2011, 03:03 PM   #11
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Don't forget there's 2 sets of wires. One runs from the thermostat to the air handler and the other goes from the air handler to the AC. Check the set that goes to the AC for mouse chew,weed whacker damage,insulation damage where it comes out the wall and insulation damage at the air handler and AC cabinet.
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Old 09-10-2011, 03:47 PM   #12
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A/C not working


Thank you for your suggestion. Here is what I did:
1) Remove thermostat completely. I have red, yellow, white and blue wires hanging out of the wall. Check resistance between two in all combination possible. All of them are about 500 ohms or more. Therefore it conforms that there are no shorts in the cable between thermostat and 50A50-112 controller. Physically checked cable between controller and outside unit. Slight fraying of outside insulation of cable but no shorts.
Simply shorting the appropriate wires(red,yellow,blue) physically with the thermostat removed, trips the circuit breaker of the transformer. This also conforms that the thermostat is not the culprit.
Regards,
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Old 09-10-2011, 03:52 PM   #13
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Old 09-10-2011, 05:12 PM   #14
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Thanks for the diagram. I have a feeling that there is a wiring error that has existed for years. Let me explain:
1) I have a digital Honeywell thermostat. It has the following inputs:
B
R RC O W Y G
The cable coming out of the wall was connected like so:
Red wire to R and RC.
White wire to W
Yellow wire to Y
Blue wire to G
The cable also has a green and brown wire but they are not connected to any input.
2) The 50A50-112 furnace control has the following inputs:
C G R W Y
The cable from the thermostat was connected like so:
Brown wire of cable connected to C
Blue wire of cable connected to G
White wire of cable connected to W
Yellow wire of cable connected to Y
The second cable between furnace control and outside AC unit is connected like so:
Blue wire of cable to C of furnace control
Yellow wire of cable to Y of furnace control
On the other end the blue and yellow wires of the cable are connected to the two relay inputs.

As you can see, there is no common between thermostat and furnace control. Therefore the question is: which of the inputs to the thermostat is common??
Please advice.
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Old 09-10-2011, 05:23 PM   #15
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A/C not working


You don't need to have C (Ground) between Thermostat and furnace control. You have to connect the C wire when the thermostat doesn't have battery or the new wifi thermostats

If transformer has tripped you have short wires somewhere.

Disconnect Brown wire on the furnace control.


The contactor coil maybe bad or low resistence....to test disconnect on the furnace control:
Blue wire of cable to C of furnace control
Yellow wire of cable to Y of furnace control
A

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Last edited by JJboy; 09-10-2011 at 05:28 PM.
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