Really need some help. I was working on the wiring of my home air conditioning and managed to mess up the low voltage wiring. I was installing a wifi thermostat and opened the panel to add a blue wire. Discovered a lot of corrosion on the terminals. I took a picture and proceeded to clean each terminal. Well long story short. My photo wasn't clear enough for me to get all the wires back where they belong.

I'm pretty sure I got the thermostat wires correct, what has me baffled is the low voltage wires to run the outside compressor. My thermostat has power, the fan has power, but I can't get the compressor outside to run.

I did some testing, and got the compressor fan to turn on, but it's not making cold air. So I undid the wires.

Question is. I have two low voltage wires that go to my outside compressor. What should the connect to on my terminal panel.

I've attachéd a photo. Notice the two wires on the side to my compressor. It's a trane that's at least 10 years old. It's a electric furnace and single stage ac, in south Florida.

Thank you.enter image description here

  • Can you post a model number or wiring diagram for your furnace? May 23, 2018 at 11:41
  • Is this a heat pump or conventional furnace?
    – Tyson
    May 23, 2018 at 12:05
  • Can we see the original picture?
    – mike65535
    May 23, 2018 at 12:44
  • you need to post the brand and model or part number which can be useful for identification. Otherwise, we can only guess which is bad if you put the wrong guess. Dec 28, 2020 at 10:10
  • There should be a wiring diagram stuck to the air handler wall or the back of the cover or in an envelope sitting on top or nearby. If not, the nameplate with model number. Please post GOOD pictures of those. Judging by the condition of your terminals, the sticker and nameplate may both be ruined but let's try.
    – jay613
    Aug 25, 2021 at 12:04

1 Answer 1


I am guessing B is common in your case. In that case Y and B. enter image description here

I have drawn a line though the terminsals you will not be using since you have straight cooling and are using the air handler like an electric furnace. R goes from the thermostat to the air handler but not the condensing unit as does W1 which is your heat strip. O is not used since you do not have a reversing valve. Y goes from the thermostat to the air handler then carries on to the condensing unit. Note the common 24 volt is labeled B or B/C (this is typical with Trane products). The B is likely labeled C on your wifi thermostat. So C from the thermostat goes to B on the air handler and then on to the condensing unit. It does not matter which wire from the condensing unit goes to B or Y. I hope this edit clears up any confusion.

  • 1
    How good is your guess?
    – fixer1234
    May 23, 2018 at 8:08
  • If it’s a heat pump it’s likely a bad guess.
    – Tyson
    May 23, 2018 at 12:07
  • Heat pumps do not have 2 wires. Most of the older Trane/American Standard units called the common B. They went to calling it B/C for a while and now mostly use the common C designation.
    – user76730
    May 23, 2018 at 14:17
  • While O has become more standard for heat pump reversing valves there are a lot out there that use B. When B is/was used it was usually reverse logic of O. I’ve never seen a C terminal labeled B— I have seen the blue wire used for C plenty of times. Additionally the other terminals make this look like a heat pump, but the is no O in use. My opinion is B is the reversing valve and this is a heat pump, but OP hasn’t given the model number or wiring diagram yet, so it’s just a guess at this point.
    – Tyson
    May 23, 2018 at 19:23
  • 1
    As stated in the question it is a Trane electric furnace with a single stage air conditioner. There are only 2 wires coming from the condensing unit and the furnace, fan and thermostat are all working. That is telling me that is all hooked up correctly including the B being the common wire. The T is for an optional temperature sensor located in the condensing unit. For Trane units O is always for heat pump reversing valve. If there is a schematic that says different then I will change but everything in the question so far leads me to believe the condenser hooks to Y and B on the furnace.
    – user76730
    May 24, 2018 at 0:15

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