Our old (circa 1970) house has a post light out front. The switch is in an inconvenient location and is thus never used, so I thought I'd drop in a smart switch - a Belkin Wemo, in this case - to replace the old one and just put it on a sundown timer.

Here's the old switch:

enter image description here

Only one of the switches on there was actually active.

Per usual with this old house, all three of the wires connected to that switch were black so I have no idea which any of the wires were. The only one I marked with blue painter's tape was the the third switch which was screwed on below the other two, which were attached opposite each other up top.

enter image description here

Through the usual trial and error I have found two configurations which will power the smart switch on - basically swapping the "load" and "power" lines on the Wemo switch.

My problem is that neither configuration will light the bulb on the post light up. The LED bulb we had out there, which worked fine on the old switch, flickers on and off faintly. A halogen bulb I swapped in didn't light up at all.

So I'm not really sure how to proceed. I'm familiar with lights that either light up or don't, but a light that is sort of on but not really is a new one for me.

Any thoughts and advice would be greatly appreciated. In case it makes any difference or would be useful, I do have a continuity tester.

  • Is there a 120v on the wire connected to red and the wire connected to black?
    – Kris
    Sep 7, 2019 at 18:36
  • Apologies, not sure I understand the question.
    – sogrady
    Sep 7, 2019 at 18:36
  • What’s happening inside the blob of blue tape?
    – Kris
    Sep 7, 2019 at 18:52
  • 1
    Can you post a photo looking into the back of the switch box please? Sep 7, 2019 at 18:57
  • 1
    Through the usual trial and error Stop doing trial and error on mains electrical. The problem is your "pass" condition is "it works". There are many combinations which work but will kill you. Sep 7, 2019 at 19:15

1 Answer 1


Always use logic to solve electrical problems. Uninformed trial and error will lead you to many combinations which will "work" but create a lethal condition, either immediately or by breaking the redundancy that keeps electrical safe, so a trivial/routine failure elsewhere that shouldn't create a hazard, now does. The fact that you are connecting a switch white to a wall black says you should not be making guesses.

With 3 wires on a dual switch, one wire is "common" to both switches (and is typically supply, so it is always-hot). This is identifyable from the labeling or obvious connecting metal on the old switch.

The other two wires are the switched-hots going to one light or another.

Now, since one of your switches "does nothing", the switched-hot wire associated with that switch can be eliminated. It should be capped off, on the off-chance it is being backfed somewhere, and so it doesn't accidentally come in contact with a hot screw.

That leaves 2 wires. One is always-hot, the other is switched-hot to the lamp.

However, the fact that the smart switch has a white pigtail tells us important things. UL forces them to use certain colors for certain things:

  • Green, yellow-green or bare for ground
  • White or gray for neutral

Since the manufacturer could have easily used any wire color, we may infer the green wire MUST be safety ground, and the white wire MUST be neutral. You must consult the instructions to see if those connections are mandatory.

Just like pesticide use, use of the equipment according to instructions is mandatory by law.

Grounds should be connected first, for perception reasons. In the event of a wiring issue, this makes you focus on the actual wiring problem, instead of erroneously blaming the ground for failing somehow (not likely). All grounds go together.

Neutrals connect only to that same circuit's neutral, not any other circuits'. This is usually straightforward if neutral is present. Neutral will be present here.

Now we have narrowed this down to 2 wires from the wall and 2 wires from the switch. Since all wires are hot, and the job of a switch is to connect them or not, experimentation is not harmful. However a simple voltage tester will tell us the hot wire, and the switch's instructions will tell us the hot wire (coughblackcough).

  • It is most likley that OP has overlooked a bundle in the box where neutrals are tied together perhaps as I recently came across twisted and soldered with electric tape wrapped around them. He will need to pigtail a neutral from there to the smart switch to get it to work.
    – Kris
    Sep 7, 2019 at 20:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.