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I’ve encounter very bizarre behavior when replacing a very old three-way switch in my mother’s old house, and I’m trying to understand just what might be going on.

One light is controlled by two three-way switches. One switch wore out, causing the other to also stop functioning.

I replaced only the broken out three-way switch. Light works again! Except…

When the new three-way switch is up, the light is on. If I flip the second switch up, light flickers and remains on. Light only goes off when both switches are down. Same behavior with the second switch — if I turn the light on (switch up), flipping the replaced switch causes it to briefly flicker but still stay on.

Is this a wiring screw-up on my part, or are these three-way switches possibly not wired as three-way switches? I don’t know the behavior prior to the first switch being replaced, but I’m told it functioned “normally” before. Other older three-way switches in the house still behave properly.

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    Take note that this means you have put something back together differently from the way that you found it and were not able to notice that you were doing it incorrectly. With electricity this can get you and/or your family killed. Please be careful and always pay careful attention to what you are doing.
    – J...
    Commented Dec 30, 2021 at 19:09
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    @J... 's point is 100% valid. In defense of OP and for future reference, it is quite possible that OP copied "top left to top left", etc. not realizing that 3-way switch screw positions are not standardized even though so many other things are. Commented Dec 30, 2021 at 21:30
  • I appreciate both of your comments! 100% correct that I should have paid more attention instead of just placing the wires on (seemingly) the same terminals they were on the broken switch! Writing another comment below on this, but I do wonder if the two switches are even properly 3-way as it would normally seem, since I don't know the behavior prior to replacing the broken switch.
    – Monster
    Commented Dec 31, 2021 at 4:30
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    I've encountered this situation many times. It's often caused by different screw configurations between switch brands. You can't rely solely on position. You have to understand the concept of the traveler and be able to identify them.
    – isherwood
    Commented Dec 31, 2021 at 20:18
  • Are you sure that both switches cause a flickering when moved? Commented Jan 1, 2022 at 5:59

2 Answers 2

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All three way switches have two travelers and one common. But the positions vary. Usually the traveler screws will be one color (eg brass) and the common another (eg silver). Figuring out which wires are travelers is sometimes easy (2 wires from one cable and common hot or switched hot on a different cable). Is suspect you've got common and a traveler swapped.

Work through it methodically. Since you only changed one switch, the problem should be in that box. Look at (upload pictures and we may be able to help identify things):

  • How many cables/conduits are coming in. Typically there will be two - one for incoming power (or going out to the light) and the other going between the two switches (travelers and switched hot or neutral, depending on how it was wired). But there might be just one or there may be more (if there are other switches or just stuff passing through).
  • Colors of screws - to identify positions on the switch - and colors of wires - which might help identify functions of wires (and might not).
  • Compare colors of wires to wires connected to the other switch.
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  • Thanks for the comment! I'm not sure it's a simple issue of having swapped a common for a traveler. The behavior of both switches is up = light on and down = light off. This is regardless of the behavior of the other. If either switch is up, the light remains on, and both must be down for off. Both up causes a (brief) flicker when the second switch is flipped up. I'm trying to understand how a wiring with two 3-ways supports that behavior. Thanks!!
    – Monster
    Commented Dec 31, 2021 at 4:31
  • I'm worried that the current setup might be something like... Switch 1 where Traveler 1 = Light, Traveler 2 = Switch 2's Common, Common = HOT. Switch 2 where Traveler 1 = Light, Traveler 2 = Neutral, Common = Switch 1's Traveler 2 That seems the only way I can think to support this behavior?
    – Monster
    Commented Dec 31, 2021 at 4:42
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It is quite easy to miswire a three-way light circuit so that the light is off only if both switches are off.

It is little more difficult to miswire a three-way light circuit so that if one switch is on, flipping the other switch causes the light to flicker but cannot turn it off.

I cannot figure out how to miswire a three-way light circuit to get all the behavior you describe, that is, when both switches are on, flicking either switch causes the light to flicker but cannot turn it off.

However if one of the switches is actually a four-way switch used as a three-way switch, then there is a way to miswire the circuit to get the effect you describe. Neither switch needs to be broken and only two travelers are required.

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