We have a three-way switch setup to control our living room light. Our staircase is on the far wall of our living room and one switch is downstairs while the other is at the top of the stairs. The last owner of the home 'completely rewired' the house as it used to have knob-and-tube wiring. However, we've found out since that he only did this in easily accessible areas and anywhere else that he opened the boxes at, he simply pigtailed off of the old wires to feed the new devices.
The problem with the switches was that they both seemed to act as single pole switches in series.. the upstairs switch had to be on for the downstairs switch to work. The switch upstairs was a new three-way switch connected up in that pigtail fashion. The switch downstairs, which we didn't notice until we took the cover off, was a really old three-way switch that was still connected with the original wires (appears it was never modified.) After taking it off of the lines, we were able to see that it must have been fried internally as a continuity test showed that when it was flipped one way, there would be no power to either traveler. However, if it was flipped the other, both travelers would become powered. We figured this was the problem (even though I'm not sure why it would function as it did), added a new switch in it's place, and reconnected the wires on both switches.
The switch that was upstairs was put in place downstairs and we added a new three-way upstairs (not sure entirely why, we just happened to carry it down with us) and so I'm wondering if it could have been fried as well on the inside. As I still don't get why the circuit was acting like two single pole switches. The power fed in through the downstairs switch, went up the walls to the second floor switch, and then went to the light back downstairs and finally fed it's way back to the panel (the neutrals weren't run together originally). I'm not sure where the homeowner tied the old wires to the new ones, but there are no old wires ran into the panel, so he's had to of done it somewhere.
So now, our light works as desired - both switches can cut it on or off in any layout. I gave the above information in case it helps explain any of what I'm wondering about. While we were testing it, I used a non-contact voltage tester at the upstairs switch to see which were hot to insure that we weren't having the same problem as last time. Although the switches work perfectly by visual of the living room light, every other flip of the downstairs switch was showing both travelers hot. Flip it one way and we'd have power to say the left, but flip it the other and both sides were getting powered.. as far as the voltage tester showed.
The downstairs switch would work by diverting the power from traveler to traveler, while the upstairs switch would work by picking which side to accept power from. So if both travelers are getting power, then why would either switch still work at cutting the light off? Possibly just a small amount of power bleeding through?
If it was wired with knob and tube originally, it's possible that the 3 way switch was old enough to be wired as "Carter 3-way":
Image from Popular Mechanics, October, 1971
In this setup, the load is switched between H-N (on), H-H (off), N-H (on), and N-N (off). Since the 3 ways have the load on the common terminal, the "travelers" can both test hot.
Another old 3-way setup that used to be common with knob and tube was a travelling bus or Calfiornia 3 way:
I've run into this a couple times, although in all the times I've seen it the neutral was being switched. It's possible to do the same thing with the hot being switched, in which case you can also have 2 live "travelers".
Note that the top layout hasn't met code for a long time (or the bottom with a switched neutral), but at the same time you can't assume that you won't run into them (or various other 3-way wiring patterns) in an old house. The best way to test for non-conventional wiring like this is with a multimeter at the fixture. Test (what should be) the hot and neutral with different combinations of switch positions and you should be able to sort out how it was wired.
I found this on another site and it helped me get the three way switches working correctly:
There is always a chance that one or both switches are worn out...and must be replaced..
Assuming that the switches worked previously, and that only the switch terminal connections are wired wrong, I advise the following procedure. If you are comfortable using a voltmeter or a neon tester. Follow these easy steps (for 2 -3 way switches).
1) Turn the power OFF and physically remove both switches, and take off the wires from the switches only. You do NOT have to remove the GROUND WIRE. This will leave 6 exposed wires (3 in each box).
2) Turn the power ON and identify the HOT wire (it will be the only one of six). Measure between each wire and the metal boxes or GROUND WIRE until you find it.(remember this wire) Then turn power back OFF. Mark this wire
3) The HOT wire found in step 2 is the COMMON wire, and goes on the DARKEST screw of the switch. The other 2 wires can go on either of the two remaining screws. Wire this switch (#1) back up and put it back into the box. This switch (#1) is now correct and you can focus on the other switch. (#2).
4) Turn the power back ON and identify a HOT wire at switch box (#2). Then toggle the (#1) switch and identify another HOT wire at box (#2). The COMMON wire at this switch (#2) box is the wire that was NOT HOT . (remember this wire) Then turn power back OFF. Mark this wire
5) With the power OFF, wire up the second switch, The COMMON, wire goes on the DARKEST screw of the switch. Then wire up the two HOT wires to the two other screws.
Hope this helps
Very good definition, I would only add, if you have the power off you could also ring out the travelers so the same wire was on the same screw at both switch locations. Doing this would insure of having the wiring correct. Other then that very good.