I've removed our previous 20 year old garage door opener and am installing a new one. I'm now at the step of wiring the new control panel to the wall. The garage is finished, and the old control had wiring behind the drywall. There is a brown cable and inside it is a black and white wire, one end going to the control panel on the wall, and the other end coming out of the middle of the ceiling in the garage (to go to the old garage door opener).

I'm using a contactless voltage detector and it's detecting voltage on both of those black and white wires, even though there is no garage door opener (let alone one that is plugged in), and the voltage is detected at both ends of both of these wires. Why would this be? The control panel on the wall is just above a light switch, is it perhaps detecting some "phantom" voltage from that or something? I dont think that would explain why it is detected at the ceiling, where there is no light switch (there is a light, but a few feet away).

My new garage door opener came with a new control panel cable, so I can use that. But I wanted to reuse this wire if possible so that I dont need to put unsightly wires stapled to the drywall. However, I dont want to damage the new garage door opener with improper wiring.

1 Answer 1


Contactless testers are notoriously unreliable.

The best thing to do is to locate the other end of the wire, which should have been connected to your old opener, and check continuity between the BLACK on each end and the WHITE on each end. There should also be no continuity between the BLACK and the WHITE wires.

These are typically just a simple momentary pushbutton switch that shorts the BLACK and WHITE and that triggers the opener to cycle.

Unlike furnace control wires and doorbell wires, garage door opener controls do not have external power supplies.

  • 2
    And then invest in an inexpensive proper multimeter. Every homeowner should have one.
    – isherwood
    Sep 25, 2020 at 15:46
  • @isherwood -- keep in mind that "proper" puts you around $70-$100, BTW -- you don't need something super-expensive, but Cheese-pipeline DMMs do not fare well vs mains surges and such Sep 26, 2020 at 0:04
  • I'm not sure what you're saying. My $30 model does everything I need it to do as a homeowner.
    – isherwood
    Sep 27, 2020 at 1:58

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