So flummoxed after changing a light switch…

[MA1982]

I’d be very grateful for any advice on what I have done wrong.

I originally had a 2 gang 2 way dimmer that controlled wall lights in the lounge. One gang controlled 2 lights on one wall, the other gang controlled a single light on the opposite wall. There is a regular 2 gang 2 way switch at the other end of the room. These all worked.

The wall lights are very old, so I replaced them with new ones with none dimmable bulbs. As such, I then replaced the dimmer with a new regular 2 gang 2 way switch.

The dimmer (as per photos) had a yellow wire in L1, blue wire in L2 on both gangs. There was a red wire into common that was bridged to the other common. The first blurry photo was as I removed the dimmer plate, I then disconnected those 3 wires & took the second photo to clearly show the terminal names & wiring of the second gang.

I then replaced the dimmer with the new switch, wiring it the same way the dimmer had been (photo 3). Now only 1 gang works, the gang that works the 2 lights. I cannot get the other gang that should control the single light to work/switch on.

I changed nothing on the secondary switch.

I’ve tried a light fitting from the working side, still doesn’t work. I also replaced the old light fitting, also didn’t work.

I did swap both sets of yellow & blue wires (in case I mixed them up) but all that did was swap the working gang switch over & still controlled the double lights. So I know both gangs of the new switch do work.

I’m so confused as to where I’ve gone wrong. I used a voltage pen & there is power going into the single light when I test the two wires (blue & brown) on the light fitting.

Can anyone suggest where I’ve gone wrong or a solution to fix this?

 

[James]

3 possible options

1 new light is faulty / not connected properly / lamp not screwed in correctley

2 new switch although looks ok, could have pinched the insulation instead of the copper on a screw terminal

3 could you have mixed up either the yellows or the blue's they work as a pair so if you got say the yellows swapped over then it may be possible for one light to work and the other not to work, depending on the switch positions at the other end.

Note: a volt pen is pretty useless for diagnosing this sort of problem and not to be trusted.

 

[Avo Mk8]

I did swap both sets of yellow & blue wires (in case I mixed them up) but all that did was swap the working gang switch over & still controlled the double lights. So I know both gangs of the new switch do work.

I agree that does seem to show both gangs of the new switch work, and that there's a live feed to both 2-way circuits.

I used a voltage pen & there is power going into the single light when I test the two wires (blue & brown) on the light fitting.

If the volt pen lights up on the blue wire, sounds like you've lost the neutral at that single light.

 

[MA1982]

3 possible options

1 new light is faulty / not connected properly / lamp not screwed in correctley

2 new switch although looks ok, could have pinched the insulation instead of the copper on a screw terminal

3 could you have mixed up either the yellows or the blue's they work as a pair so if you got say the yellows swapped over then it may be possible for one light to work and the other not to work, depending on the switch positions at the other end.

Note: a volt pen is pretty useless for diagnosing this sort of problem and not to be trusted.

1 It doesn’t work with a known working light fitting (taken from the working side) or the old light fitting that I put back on to test that theory.

2 I’ve checked this after your suggestion, all is good.

3 I can see the yellow & blue are paired within the socket. I’ve kept them in their pairs. I swapped one pair for the other like I said above, but it just switched the working gang over, still controlling the two lights.

 

[MA1982]

I agree that does seem to show both gangs of the new switch work, and that there's a live feed to both 2-way circuits.

If the volt pen lights up on the blue wire, sounds like you've lost the neutral at that single light.

I checked again, the volt pen does sound on both wires. So if I’ve lost the neutral at the switch for the single light, what should I be looking to do to get it back?

 

[James]

I checked again, the volt pen does sound on both wires. So if I’ve lost the neutral at the switch for the single light, what should I be looking to do to get it back?

the yellow and blue wires at the switch are both Live wires, at any one time, only one should be live.

again I have to point out that a volt pen is not suitable for diagnosing this sort of fault.

 

[Avo Mk8]

I checked again, the volt pen does sound on both wires. So if I’ve lost the neutral at the switch for the single light, what should I be looking to do to get it back?

Not talking about the switch, just the light fitting itself.
From what's been said, and your photo's, the wires at the single light fitting would typically be a neutral 'feed' (eg from the consumer unit), and the switched live (from your second switch).
So if you are talking about a 'neon' pen tester (as James mentions, not ideal for this sort of thing) I nevertheless would expect it to light on only one of the bulb connections. There shouldn't be a voltage on the other bulb connection, and if there is, it points to a lack of continuity back to the CU neutral.
Has a wire broken or become disconnected?

 

[MA1982]

the yellow and blue wires at the switch are both Live wires, at any one time, only one should be live.

again I have to point out that a volt pen is not suitable for diagnosing this sort of fault.

I appreciate that re. the volt pen, it’s just what I had access to at the moment.

 

[pirate]

A volt pen will often pick up a voltage from an adjacent wire...that can be very confusing and lead to false conclusions.Just saying...

 

[littlespark]

What else is in the switch box? I see a couple of reds on a Wago. Is there anything else?

The original blurry photo doesn’t show the red commons tied together…. But on the new switch they are.

I think this isn’t the “live” end of the two way circuits, but the switchwires…. And all the reds have got mixed up.


Can you take off the other switch, photograph the connections and put back? Just to see what’s at that end…. And also a photo of whatever else is in this back box.

 

[Avo Mk8]

The original blurry photo doesn’t show the red commons tied together…. But on the new switch they are.

I interpreted the second photo as the single choc bloc top right feeding the incoming single red (at the top with all the bare copper) to both switch commons (one out of the bottom of the choc bloc to the right switch, the other from the top). But you may be right that's not the case.

 

[littlespark]

I interpreted the second photo as the single choc bloc top right feeding the incoming single red (at the top with all the bare copper) to both switch commons (one out of the bottom of the choc bloc to the right switch, the other from the top). But you may be right that's not the case.

You could be right. I wasn’t noticing the choc block. Which magically changes colour on the “after” photo.

Could still do with knowing what else is in the back box….