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Discussion Starter · #1 ·
About a year ago I replaced two separate pairs of very old 3-way light switches (each pair controls a separate light). I replaced them with commercial switches by Cooper. All connections are with screws, no back-stabbing. For the last year, both pairs of switches have been working fine. Even now, one of the two pairs continues to work fine (both switches turn their light on or off in either position).

Until last night, on the other pair of switches, either switch would turn its light on or off no matter what position the other switch was in. But last night, on that pair of switches, suddenly the switches won't work completely independently of each other, as follows:

When switch 1 is up, switch 2 only turns on in down (which is fine).
When switch 1 is down, switch 2 doesn't turn on in either up or down.

When switch 2 is down, switch 1 only turn on in up (which is fine).
When switch 2 is up, switch 1 doesn't turn on in either up or down.

I have removed the switch plates, but didn't pull the switches (yet), but from what I can see and feel from poking around, all the connections are solid.

Any ideas why would this suddenly start happening? Bad switches? Thanks. / Rav
 

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Shut off the breaker to that circuit first. Then pull the switch and examine the connections. This would be the first line of troubleshooting. It is very possible that their is a loose connection. Check the wire nuts if any. If that does not do it then possible the switch is bad. If you remove any wires from a switch be sure to mark the wires or even take pictures of the wires on the switch so you can recreate the same setup while replacing with the new switch.
 

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Test the switches
On a 3 -way, the Common screw is the one identified, usually black.
The other two screws are Travelers.
1. Shut off Power
2. Remove one switch and disconnect all wires - record where the wires go so you can reinstall it correctly.
3. Use a continuity tester
4. Test for continuity between the Common screw and the two Travelers
5. You should have continuity between the Common and one Traveler.
No continuity between the Common and the other Traveler.
6. Flip the switch
7. Repeat steps 4 & 5
Test results should flip to the opposite Travelers.
8. No continuity = bad switch
Repeat steps 2 - 7 for switch #2
 

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Discussion Starter · #4 ·
OP here. The problem is resolved. I tested for loose connections as afjes2015 suggested, and tested for continuity as Let it Snow suggested. But there was continuity in both positions so the switch was OK, and the connections seemed tight and looked OK. But I got what I thought was an odd continuity reading between two of the wires (a reading of about 3V). I had the power off on that circuit so I don't know why there was any voltage reading at all, except that I know this circuit is part of a MWBC in the basement and the other circuit involved was still live (none of that circuit comes into this switch box), so perhaps there was some induced voltage. I don't know, and perhaps that had nothing to do with the problem. Anyway, I decided to disconnect a push-in connector I had previously used to extend one of those wires. Both wires had been pushed in all the way, but I noticed that a small bare portion of one of the wires was protruding from the connector, so I snipped off a little of the stripped portion of the wire. I put a new connector on, reconnected the switch, and now it's working correctly from both switches. Thank you both for your help.
 

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You are welcome. Glad we could help.


Meantime, you now have new issue that you revealed in your recent post.


...I had the power off on that circuit so I don't know why there was any voltage reading at all, except that I know this circuit is part of a MWBC in the basement and the other circuit involved was still live (none of that circuit comes into this switch box), so perhaps there was some induced voltage...
If this truly is a MWBC then either both should be on at the same time or both should be off at the same time. The breaker being used should be a two pole breaker or two single pole breakers with an approved/rated tie bar for that panel. If one circuit shorts out and kicks the breaker it should force the other breaker to trip with it if there are two single pole breakers. You don't want to have two single pole breakers not tied together. Also if the two single pole breakers are not nested one on top of the other and both are on the same leg of power you could quite possibly overload the neutral which can be dangerous. Many people don''t realize that you can overload the neutral. There is a lot of information on the Internet about overloading a neutral but here is one article to give you an idea. I would take the time to read it.


Also, receiving voltage reading even with a circuit off may show as what we call a phantom voltage which will happen if you use a digital multi meter. Using an analogue meter will not show this.
 

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Discussion Starter · #6 ·
You are welcome. Glad we could help.

Meantime, you now have new issue that you revealed in your recent post.

If this truly is a MWBC then either both should be on at the same time or both should be off at the same time. The breaker being used should be a two pole breaker or two single pole breakers with an approved/rated tie bar for that panel. If one circuit shorts out and kicks the breaker it should force the other breaker to trip with it if there are two single pole breakers. You don't want to have two single pole breakers not tied together. Also if the two single pole breakers are not nested one on top of the other and both are on the same leg of power you could quite possibly overload the neutral which can be dangerous. Many people don''t realize that you can overload the neutral. There is a lot of information on the Internet about overloading a neutral but here is one article to give you an idea. I would take the time to read it.

Also, receiving voltage reading even with a circuit off may show as what we call a phantom voltage which will happen if you use a digital multi meter. Using an analogue meter will not show this.
Thanks for bringing this up. Perhaps I am mis-understanding what defines a MWBC, and perhaps mine isn't. This involves circuit 2 and 3. On the panel, breaker 2 is on the upper right, and 3 is on the upper left below breaker 1. So they are separate and obviously not tied together. The reason I thought this was a MWBC is because, from my testing, both 2 and 3 share one cable coming out of the panel. I have NOT opened the panel internally to verify this (I am not qualified), but this is in an unfinished basement, and, using a combination of a non-contact tester and seeing what turns on and off when I turn each breaker on and off, I have traced which circuit(s) are in which cables. See the attached picture.

In the picture, the NM cable coming in from the right comes from the panel (but see FYI below). It contains both circuit 2 and 3. It enters that junction box and each cable that exits from that contains just one of those circuits, which then proceed to various parts of the house. FYI, prior to what you see in this picture, the cable containing 2&3 goes through a pull-string light junction box where again, some of the exiting cables are 2, some 3, and one is 2&3 which is what you see coming in from the right in the picture.

I assumed that since both circuits started their run in the same cable that this was an MWBC. Is this a MWBC? Thanks very much.
 

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Discussion Starter · #7 ·
If it helps any, there is a junction box just prior to what's seen in the picture I posted in my previous post. In the attached picture of that junction box, the NM cable on the left marked Outgoing is what's seen coming in from from the right in the picture in my previous post.
 

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