Hot Topics: AC Repair

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Original Post: AC is working sporadically

Pdr983 - Member

Hey everyone, My AC unit has been acting really weird lately and it started about three months ago. It works and then stops working. I have done different things which will cause it to start working again so bear with me on the long post :-).

When I have the thermostat set to cool, the outside fan and compressor will not come on, but the blower on the furnace will so it will be blowing hot air. We had a technician come out to look at it, but of course the unit was working when he came out. He checked both the furnace and the outside unit and said there was no shorts and everything looked okay with proper voltage going everywhere and nothing clogging the drains.

Of course it went out and assuming that maybe it is just over heated, I would shut the system down from the switch at the furnace and start it up at around 8pm and it would work throughout the night. The next day, it would shut down again. I decided to change the thermostat, capacitor and the contactor outside, even though the voltage was showing correctly when tested. This did not help.

I called the technician out again and now it was not working. He saw the yellow wire that was connected to the AC circuit board on the furnace was not completely under the screw. He adjusted it and the outside unit started back up and worked fine for a month. The outside unit recently started acting up again so I would go upstairs and unscrew the yellow wire and re screw it in and the AC unit works fine.

I am tired of having to do this every time the outside unit will not come on, and I always check to make sure the wires are completely under the screw each time. Does this sound like the AC circuit board might be bad?

roughneck77 - Member

What outdoor unit do you have? Is there a circuit board in the condenser?

Geochurchi - Member

Hi, when it happens again, try removing the yellow wire from Y1 at the Tstat and see if you get the same results, post a pic of the control board.

Pdr983 - Thread Starter

I do not know exactly what brand it is but I will find out when I get home. No, there is not a circuit board at the condenser. I will try removing that Y1 at the Tstat tonight to see if I get the same result. Here is the pic of the control board in the attic.

ac wiring board

roughneck77 - Member

Removing the Y1 wire will cause the air conditioner not to run at all.

Is there a condensate overflow somewhere?

Pdr983 - Thread Starter

There is, it is below this to the right but the cut off switch to that is in the normal position and there is no condensation/water in the overflow pan. The condensation is dripping outside.

Geochurchi - Member

Hi,roughneck77 I was not thinking of leaving it off, just removing it the same way he was removing it from the board, to see if it produced the same thing as removing it from the board—could be another issue.

Pdr983 - Thread Starter

The outside unit is a Lennox Elite. It has not gone off yet so I have not had to try it from the Tstat.

Pdr983 - Thread Starter

Okay, so the AC stopped working again this afternoon so my wife went up and shut off the power to the blower. When I came home, I disconnected the Y on the Tstat and put it back in. I then flipped the power back on to the blower and the AC fan and condensor came on so I think it worked this way as the same wad unhooking the y on the furnace circuit breaker.

Dkodgis - Member

If you are being tortured and it sounds like it, and you spend time on the board, I think it is a wise choice to change it. You do not know what it looks like on the other side. It could be there is something going on that is causing intermittent function. I have seen this before. I swapped out boards and that was that.

clocert - Member

You may want to use a meter to check to see if you have 24V at the yellow wire on the board when this problem occurs, if not, check the yellow wire at the T-stat, If you do have 24V at the T-stat, then board or wire issue, if not , T-stat issue. If you do have 24V at the yellow wire on the board, then check the contactor terminal and find out why the contactor is not working. Hope this will get to the bottom of the problem.

Geochurchi - Member

Hi, when this happens you do have 24 VAC at the contactor coil? I believe that’s what you said in a earlier post.

Pdr983 - Thread Starter

When it started up, I tested the original contactor, it was showing 240v coming in, 24v at the coil. I replaced it anyways just to make sure there wasn't something "phantomly" wrong with it. After the replacement, it was showing them same readings with the meter, 24v across the coil.

I am going to check the board once it goes out again.

Pdr983 - Thread Starter

Right after hitting reply, my AC went out. I checked the voltage at the Tstat with the red and yellow and it read 26 but when I went to the circuit board at the furnance, the red and yellow did not read anything. The red and c read 26v as well as the red and fan wire.

roughneck77 - Member

0V between R and Y means your unit was calling for cooling.

24V between R and Y means your unit was NOT calling for cooling.

0V between C and Y means your unit was calling for cooling.

R and Y are the same circuit when cooling is running.

Pdr983 - Thread Starter

Thanks, but the weird thing is, the yellow wire is screwed tight on the circuit board when it stops running but starts back up when I loosen the screw and retighten it.

roughneck77 - Member

There should be two separate conductors on the Y terminal. One coming from the thermostat. The other running to the condenser.

The board in the furnace as a whole has no bearing on turning the condenser on and off, besides providing 24V to the R terminal. The thermostat handles on and off functions of the condenser.

Pdr983 - Thread Starter

Yes, there is a wire also going from the y terminal to the condensor. I am going to try to crimp the two wires since they might be vibrating and loosen the connection to the y terminal, maybe this will fix it.

roughneck77 - Member

One wire on each side of the screw head. Tighten down till neither wire can be pulled out by hand.

If there’s that much vibration going on you have other problems.

Pdr983 - Thread Starter

@roughneck77 that sounds better so I am trying that. I was over thinking this. I do not think there is a lot if vibration because I watched it turn off, turn on and run. Maybe since the two wires were twisted together to connect to the y terminal, the screw could not evenly clamp down?

Geochurchi - Member

Hi, that terminal on the control board is probably just a junction point, I would try taking the two conductors off and just wire nutting them together.