I am trying to solve a 3-way switch circuit issue in an old house. The
wiring in the house is old, and it is either knob and tube wiring or
whatever came after that and before "Romex" etc. There are no ground wires
visible anywhere in either circuit, and everything is so dusty and dirty
that I can't tell what is a white wire and what is a black wire etc.

And, to be more precise, there are actually two different 3-way switch
circuits that I am trying to get resolved at the same time.

Downstairs, near the front door entrance, the are two 3-way switches -- one
of the 3-way switches controls the downstairs living room ceiling light.
The other 3-way switch controls the hallway light that is outside of the
upstairs bedrooms.

At the top of the stairs, there are two 3-way switches -- one of those 3-way
switches controls the downstairs living room ceiling light, and the other
3-way switch controls the hallway light that is outside of the upstairs
bedrooms.

I can tell which 3-way switch (upstairs and downstairs) is for the living
room ceiling light, and which one (upstairs and downstairs) is for the
hallway light.

To make things a little more complicated, the whole setup was intentionally
temporarily wired incorrectly a few years back because I couldn't figure out
how to get things working correctly and I needed a way to at least be able
to turn the living room light and the hallway light on and off -- even
though the 3-way switch function was disabled in the process. In other
words, I temporarily have it that only one switch controls each light. It's
a long story why I did that, but that is not important now.

Now, here's the story on the wiring: Coming into each of the 3-way switch
boxes upstairs, and each of the 3-way switch boxes downstairs, are 3 exactly
wires. There are no other wires in each switch box -- no neutrals wired
together etc. -- just 3 wires in each box and no other wires leaving the
boxes etc.

I found the following link to various 3-way switch wiring options:
http://www.easy-do-it-yourself-home-...g-diagram.html .



Based on what I have (just 3 wires in each box) I am assuming that the two
circuits are wired as shown in Option #8 on that link. The power source
appears to go to each light, and from each light, the switch wires run to
each of the switches. I concluded that because I don't see any other wiring
options where there are ONLY 3 wires in each switch box.



And, now for the problem: I need to put new 3-way switches in, both in the
upstairs boxes and in the downstairs boxes. The downstairs boxes have push
button 3-way switches that are broken, so they need to be replaced. I need
to figure out how to rewire the system, using the existing wires, and
connecting those wires to the four new 3-way switches that I bought.



I am thinking that, with the power on, but the switches removed, I can
figure out which wire is the hot "source" wire that goes to one of the
switches for each circuit. I can connect that hot wire to the hot common
for each of those two switches, and connect the other two wires in each of
those boxes to the "traveler" screws on those switches.



But, then, when I go to the other switch at the other end of each circuit,
how can I tell which wire in each of those boxes goes to the hot "common"
screw? Once I know that, I can connect that hot common wire to the black
"common" screw and just connect the other two wires to the traveler screws.
But, how do I know which of the 3 wires goes to the hot "common" screw?