AC - Low Voltage Problem - Control Board? (2025)

04-03-15, 08:39 AM

I am trying to troubleshoot my AC, as it is not functioning at all. I thought the contactor was bad since it was not getting the 24V across and it looked a little rough. New contactor was installed and still no luck. I tested the transformer near the control board and found it to correctly output 24V to the board. So, then I tested the board and measure the voltage across the wires while the thermostat called for cooling. The board has the following wires:C G R W Y.
The 2 wires running to the outside unit (contactor) are Y and C. When I measure directly on the board with a lead on Y and C, I get 0 volts (while the thermostat is calling for cool). If I measure across any of the wires, with a voltage lead on R, I get 28V.

When the thermostat is calling for heat, I am measuring 28V across W/C, as expected. Looks like just having issues with low voltage on cooling call for Y/C.

Based on my logic, the transformer is good, the thermostat is functioning (and it functions just fine for heating), so I am leaning towards replacing the control board. There is not a fuse on the board or at the outside unit.

Can someone verify this seems logical and let me know if there is any other testing I should be doing before replacing the board? Thanks in advance!!

Furnace model: TUE1C100A9481AA
AC Condening Unit (2TTB3): 2TTB-SVN01A-EN

Last edited by kansascitysteve; 04-03-15 at 09:25 AM.Reason: clarification

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04-03-15, 09:00 AM

W---> heat
Y---> cool
G---> fan
R---> 24vac
C---> common for thermostat use and to compressor outside

You should always measure 24vac between R and C.
You should measure 24vac between W and C when calling for heat.
You should measure 24vac between Y and C when calling for cool.

The wires running to outside should be on Y and C.

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04-03-15, 09:22 AM

OK - Thanks for pointing that out, as I need to correct my initial post. You are correct that it is Y/C running to the contactor outside. I measured again across Y/C while calling for cooling and there is still 0 voltage...

I measured across W/C when calling for heat and is measure 28V....

So, still sounds like a bad controller board? Maybe a replay or something not triggering the voltage when calling for cooling?

Thank you!!!

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04-03-15, 09:47 AM

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04-03-15, 12:09 PM

Disconnect the W, Y and G thermostat leads from the control board. With the thermostat calling for cooling measure between the C and other wires individually. You should see the 24 volts on both G and Y but not on W. If you do not get these readings then either the thermostat is toast or you have a broken conductor between the thermostat and the control board.

To test the wiring, temporarily disconnect all wires from the thermostat and with the wires still disconnected at the control board connect R and Y at the thermostat together. You should then see voltage between C and Y at the control board. Disconnect the Y from the R at the thermostat and temporarily connect R and G. This should give voltage between C and G at the control board. If any of these tests fail you need a new cable between the thermostat and control board.

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04-04-15, 09:12 AM

Excellent post Furd . That is pretty much how I would have tested it , but would probably not written it as well .

Wonder if he will reply ? If he found the defect ? If he fixed it ?

God bless
Wyr

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04-04-15, 10:35 AM

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04-05-15, 04:42 PM

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04-05-15, 05:11 PM

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#10

04-16-15, 06:04 PM

So, I am back.... The new thermostat somehow fixed the problem for a few weeks, but the low voltage problem is back. I went back to the transformer and confirmed it was getting 28v. The plug going into the control board is also getting 28v. Now, we are back to no voltage across Y/C when measured directly at the control board. Weird thing is now the inside unit fan is blowing air, which makes me think the new thermostat is definitely calling for cooling, otherwise the blower would not come on. I priced a control board and it ran $200 and I really dont want to buy and install if that seems far fetched or not the problem. Once I install it they will not let me return it, if that is not the problem. Hoping you have some ideas, but thought I would update the situation and get your thoughts.

I am going to test the wires per FURD below and see what those results look like, I am just baffled since the transformer is sending 28v to the control board, but I cannot measure the 28v across Y/C, which is what is going to the contactor outside.

Thanks!

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#11

04-16-15, 08:39 PM

When the thermostat calls for A/C.....

There is 24vac across Y and C for the compressor and
there is 24vac across G and C for the blower fan.

Many thermostats have up to a five minute compressor turn on delay. That would delay the 24vac on Y and C.

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#12

04-16-15, 09:32 PM

So, I set the thermostat to call for cooling. The blower kicked on immediately and I let it run for 20+ minutes. Then I went and measured across Y/C and G/C directly on the control board.

Voltage across Y/C is still 0v, while G/C is measuring 28v (and blower fan is running). I also confirmed the transformer output was still 28v, while running.

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#13

04-17-15, 05:01 AM

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#14

04-17-15, 06:03 AM

OK - So I ran the wiring tests recommended by FURD below. Here are the results...

1. Disconnected W/Y/G from control board
2. Set thermostat to call for cooling
3. Measurements
C/W = 13v
C/Y = 22v
C/G = 28v

4. Then I left the control board wires disconnected and continued...
5. Removed all thermostat wires
6. Connected R/Y at thermostat
7. Measured voltage across C/Y at the control board
C/Y = 28v

8. Disconnected R/Y at thermostat
9. Connected R/G at thermostat
10. Measured voltage across C/G at the control board
C/G = 28v

So, can anyone help me interpret these results? I am guessing there is an issue with the C/W measurement at the control board in step #3, since FURD mentioned the voltage should be 0v, but I am getting 13v...

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#15

04-17-15, 08:06 AM

So, crazy enough.... When I connected the R, Y, G together at the thermostat, the AC ran perfect both outside and inside. So, I am assuming this means the brand new thermostat is bad (Honeywell RTH6580WF)? Is there something that would be causing my thermostats to go bad? Its a little odd that I have gone through 2 now in 2 weeks....

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#16

04-17-15, 09:33 AM

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#17

04-17-15, 10:23 AM

Yes, I am measuring 28VAC between R and C at the thermostat.

Transformer is measuring 125VAC on the line side and 28VAC on the control board side.

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#18

04-22-15, 09:32 AM

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#19

04-23-15, 04:34 PM

Many thanks GEO for doing that! Weird thing is that I went and exchanged the thermostat and now it is all working again. While that is great, I still am afraid it is only a matter of days/weeks before this process repeats itself.

Is it possible there is some weird voltage variance or fluctuation going on? Or is it possible there is still an issue with the control board? Seems super odd to me that the same issue presented itself twice and each time the solution was replacing thermostat.

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#20

04-23-15, 07:08 PM

A little tight for time but a few questions....

Is your transformer fused...... is there a fuse on the control board ?
Has the fuse blown ?
If there is a fuse and it's a 5A...... replace it with a 3A.

If something, like the contactor, is drawing too much current it can burn up the tiny control relay they use in the thermostat.

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#21

04-23-15, 08:27 PM

You most likely have a problem with the cable between the furnace/air handler and the thermostat itself. Most likely is a wire broken inside the insulation that sometimes makes contact and sometimes does not and that is most likely the yellow wire.

Disconnect all but the red wire at the control board. Disconnect all the wires at the thermostat. Connect the red and the yellow with a small wire nut and then connect your meter securely to the C terminal and the yellow wire at the furnace/air handler. Do not rely on just holding the test prods on the wire or terminal, make secure connections. With the furnace power on you should see your 28 volts on the meter, Now have someone wiggle all the thermostat wires right up to where they come out of the wall while you observe the meter. ANY fluctuation in that 28 volts is a problem. If it passes this test then wiggle all the wires of the cable at the control board and the entire cable as far as you can while still observing the meter.

I suspect that you are using a digital meter and if so the 13 volts is probably a "phantom" voltage and can be disregarded.

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#22

05-02-15, 09:11 AM

So, I knew it was just a matter of time! The new thermostat had worked great since my last post. Now, this morning the thermostat is calling for cooling and only the internal fan is blowing, NOT the unit outside. So, I will try to do the wire test Furd mentioned and post back the details. AC still runs great when I manually press the contactor in.

My unit does not have a fuse anywhere on the board or transformer. I am guessing I either have a wire short between the stat and board, or something is causing the tsats to go bad.

Thanks for all your insights!

Last edited by kansascitysteve; 05-02-15 at 09:28 AM.

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#23

05-02-15, 10:20 AM

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#24

05-02-15, 11:19 AM

Add a 3 amp blade type fuse between the transformer and the furnace when you replace the transformer.

This is a good video on locating shorts with 24 volt LED.
https://www.youtube.com/watch?v=SZCXbQLgjaI

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#25

05-02-15, 11:47 AM

If none of the disconnected wires at both ends were touching anything then this means that the cable in the wall is damaged and somewhere the yellow is touching a "grounded" object like a metallic water pipe or something. Did you have the wires from the A/C condenser unit also disconnected?

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#26

05-02-15, 12:01 PM

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#27

05-03-15, 08:56 AM

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#28

05-03-15, 09:54 AM

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#29

05-04-15, 02:31 AM

Will possibly need to choose a new location for the tstat . Not close to a supply register . Close or by the return air register .

Put a blank over the location of the old tstat .

God bless
Wyr

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