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How should I rewire a miswired 3-way switch?

I just moved into a house and am trying to figure out all the switches. I have now determined there are two switches at opposite ends of a hallway that turn on/off the hall lights, so I'm guessing it's a 3-way switch. However, they don't work quite as expected. Which ever light switch I use to turn off is the only one I can use to turn on the light. Once the light is on, either switch can be used to turn it off. Is this something simple to fix? Will it cost a lot of money?

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You'd need to open the switch and inspect the wiring. If you're comfortable around electricity and know the proper safety procedures, then do this and send us photos of the wiring going to each switch (upload to imgur.com and send a link). –  BMitch Sep 18 '12 at 21:44
    
If you turn the lights off at one switch, then flip the other switch, then go back to the first switch and flip it again, do the lights turn on? If not, then you might have 2-way switches instead of 3-way. –  Niall C. Sep 18 '12 at 21:47
    
sorry to bother you guys, I think this is the answer to my question... at first I couldn't find an answer on the site. diy.stackexchange.com/questions/8240/… not sure if i should delete my question now, but thanks for your help! –  helen Sep 18 '12 at 21:58
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@helen This Blog entry might help you understand the differences between 2- and 3-way switches. –  Tester101 Sep 19 '12 at 10:47
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@helen The only way to wire the switches to get the behaivor you describe is to connect a traveller wire to the common terminal of both switches. If you follow the instructions in the link you found, you will get this issue easily corrected. –  bcworkz Sep 19 '12 at 21:29
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marked as duplicate by Steven, bib, Niall C., Chris Cudmore, ChrisF Sep 20 '12 at 9:24

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2 Answers

Take a look inside

If you look at an image of how the switches work internally, it makes it a bit easier to understand what's going on.

2 3-way switches

This Blog post contains images of the various states of different types of switch combinations, which might help you understand what's going on in the circuit.

The Truth behind the logic

A properly functioning set of 3-way switches, should function like an XNOR gate (or XOR gate depending on how it's hooked up).

XNOR

If both switches are in position 1 or 0, the light is on. If the switches are in opposing positions, the circuit is open and the light is off.

XOR

If both switches are in position 1 or 0, the light is off. If the switches are in opposing positions, the circuit is closed and the light is on.

The first step in troubleshooting multi-way switches, is to set up a truth table. With 3-way switches, the table should look something like one of these.

XNOR Truth Tableenter image description here

The best way to do this is to designate one switch as A and one as B, then say up is 1 and down is 0. Then follow the input values in the table, and log the output. For the output, 1 means the light is on, and 0 means the light is off.

  • Start with both switches in the down position, and log the result.
  • Move switch B to the up position. Log the result.
  • Move switch B back to the down position, and flip switch A to the up position. Log the result.
  • Flip switch B back to the up position, and log the result.

Now that we can clearly see what is happening with the switches, we can start to diagnose the problem.

What are all these screws?

When installing 3-way switches, you'll notice there are 4 screw terminals.

  1. Black(Common)
  2. Brass(Traveler)
  3. Brass(Traveler)
  4. Green (Ground)

Ground

The bare (ground) wire in the circuit should be attached to the green (ground) screw.

Travelers

The travelers are interconnected between the two switches, so it doesn't really matter which traveler is connected to which brass screw. Though it's common practice to be consistent, so if the red wire is connected to the left screw at switch 1, it should be connected to the left screw at switch 2 as well.

Common

Finally, the common screw is used to connect either to the incoming power or the fixture, depending on which switch you're working on. For example. Power (from the breaker) might enter at switch 1, so we'll connect the black (hot) wire from the incoming cable to the common screw on switch 1. In this case, the common screw on switch 2 will connect to the switched wire to the fixture.

Neutrals and grounds

All the neutrals should be tied together, and all the grounds should be connected to all other grounds.

If you are not comfortable working with electricity, never hesitate to contact an Electrician.

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thanks Tester101 for the thorough explanation and all for the responses! –  helen Sep 20 '12 at 7:36
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with 3-ways you have 2 switched lines running between the switches (with exactly one being hot at all times)

as it is it seems that the switches are just single-poles put in series (both needing to be on before the light turns on)

chances are that one of those lines is disconnected/got severed

it can also be that you simply don't have 3-way switches installed or the second switched line is not there (which you'll need to run along side the other switched line)

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