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I can’t figure out which is the C wire or how to make a C wire and how to connect it to my ecobee 3lite Power extender kit. My old thermostat has wires G,Y,RH, W/E, and O/B plugged in. No "C" wire. It seems like The "G, R, & W" wires coming from the thermostat connect to the three wires that go up into the furnace. All the other wires on that cord going up into the furnace aren't connected to anything.It seems like the Y & O/B cable coming form the thermostat are connected to the grey wire that connects to the outside unit. I cant tell for sure because of all the electrical tape wrapped around those connections.

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wiring inside furnace enter image description here

wiring outside furnace that seems to connect the wires coming from my thermostat to my furnace

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  • Can you post a clear picture of the wiring diagram for your furnace place? Also, can you get us a couple different angles on that second junction, and perhaps denote which cable in that photo goes where?
    – ThreePhaseEel
    Commented Jul 2, 2020 at 3:11
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    I added some more pictures. It appears that wires G, W, and R coming from the thermostat connect to 3 wires that connect to the cord that connects to wires inside the furnace. And it appears that wires O/B & Y connect to a gray cord that goes to the outside unit. I can't say for certain because the wires are wrapped by a bunch of electrical tape and I cant completely see how all those wires are connected under the tape.
    – Krg07
    Commented Jul 2, 2020 at 4:19

2 Answers 2

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The C terminal connection in your furnace is denoted by the part of the wiring diagram that I have highlighted in blue as shown below. In fact the furnace diagram shows that these wires in the furnace itself are blue (BLU) colored as well.

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You will have to remove that taped up wire bundle to ascertain if there is one wire in the cable from the thermostat that is not used. If so then the it can be re-assigned to be the C function to the thermostat. At the furnace end that re-assigned wire will have to be connected in with the wires as highlighted in blue as above. The connection may entail pig tailing into the existing furnace's blue wires. For example at terminal #3 of the SOLID STATE TIME DELAY module.

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It is possible to purchase push on terminals that adapt a male terminal out to two terminals. (See the adjacent connections on terminal #2 of the module). Using an adapter like this would allow you to make your C connection without having to cut into the furnace wires.

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Picture Source

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  • Thank you so much. I went and bought a piggy back connection this morning and found an unused wire under the tape and used that to make my C wire and it worked. This was extremely helpful
    – Krg07
    Commented Jul 2, 2020 at 16:08
  • @Krg07 - Glad it worked out. Thanks here is usually done by voting and answer selection.
    – Michael Karas ♦
    Commented Jul 2, 2020 at 21:15
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What I did was disconnect all the wires at the thermostat and at the furnace, then picked a color to use for each terminal and made sure I matched them at the thermostat and the furnace. As long as you match the terminal designations at each end, the wire color is not important.

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