I while ago I asked this question while trying to install a smart switch. What's going on with this switch wiring?

The previous dimmer is a standard dimmer and it works fine. Lights have been incandescent and LED with no issues on that dimmer. New dimmer doesn't require neutral but does require ground. Would work at that location but worked in other spots.

Older home, old armored cable with 2 conductors and ground is through armor.

When I used a multi-meter to check the wires I got 120v between the two switch wires.

When I checked between each wire to ground (box) I got 70V from one switch wire to ground and 40V on the other.

What could be possibly be going on with this light/switch wiring and how can it be fixed?

  • 4
    Possible duplicate of What's going on with this switch wiring? - If you want the earlier question answered, it is usually best to edit the original question and supply information requested in comments there. Commented Mar 18, 2017 at 8:36
  • Don't consider a duplicate. The first question I asked about the switch and the comments suggested the wiring was bad. This question I'm asking about fixing the wiring. Commented Mar 18, 2017 at 15:52
  • Do you mean that the dimmer would not work at that location? I suspect the armor on the cable is losing its ability to provide a ground... Commented Mar 18, 2017 at 16:06
  • @ThreePhaseEel yes just that location switch worked in other locations in the house. I don't think it's a problem with the cable not being grounded. Since the voltage across the two switch wires is 120V and the voltage from each wire to ground adds up to 120v I think that somehow some of the voltage is getting from the hot wire to the other leg of the switch wire. Maybe it has something to do with the ground but I think something is going on at the ceiling box for the light. Need some advice on what else I can check. Commented Mar 18, 2017 at 17:04
  • 1
    When you say that you "got 70V from one switch and 40V on another", do you mean you're getting 70V from the line wire to ground, and 40V from the load wire to ground? Are you taking the measurements with the wires connected to the switch, and if so, is the switch in the on position? If you have a solid ground, you should always measure ~120 volts from "hot" to ground (minus any voltage drop).
    – Tester101
    Commented Mar 20, 2017 at 10:59


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