First time posting on the DIY of StackExchange. I currently have a staircase with a light on the 2nd floor controlled by 2 3-way switches on both floors. I would like to add an existing light to the currently-covered junction box on the 1st floor. However, when I looked at the existing wires in the first floor junction box, I was unable to find a combination of wires to which I could connect a 120V light that would function properly.
On the right part of the image I have diagrammed the 3 different terminals/twists (?) (each has at least two wires connecting) available. When I flip one of the 3-way switches to turn the 2nd floor light off (as pictured in top right), those are the voltages across the 3 terminals. Ditto for when light is on (pictured bottom right).
I also diagrammed the downstairs 3-way switch in case wiring there may need adjustment (ground not pictured) using a current detector to determine flow depending on state.
Addendum: 3-way switch on 2nd flr is wired in the same fashion. However, when 2nd Flr light is off, wire leading to either 1 OR 2 is hot only (!) (the two will reverse if the 1st floor switch is used to turn on the light). When 2nd Flr light is on, in all cases, none of the wires leading to 1, 2, or 3 are hot.
Can someone help elucidate what my current set-up looks like and what changes may be required?
I am limited in number of links I can share, so I posted on a separate imgur thread: http://imgur.com/a/vEh3P