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# Reverse Engineer 3-way Switch

#1
12-23-00, 08:42 AM
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Reverse Engineer 3-way Switch?

Using my simple Continuity Detector, how can I reverse engineer the 3-way light switches used for my dining room light?

I ask because I'm confused by the results I'm getting on one of the switches. I touch one wire to the white or black wire and the other to the ground wire on the switch. The light glows in both cases (white to ground OR black to ground). This didn't occur on the other switch.

Thanks for the help.

Ed

#2
12-23-00, 09:37 AM
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Posts: 18,497
We have at least one terminology problem, and maybe more. I believe that you are NOT using a continuity tester (i.e., an ohmmeter), but instead a voltage tester. Furthermore, since you don't mention the red wire, I'm not sure you have a 3-way switch.

Assuming you really have a three-way switch (i.e., it has three screws in addition to the grounding screw), you can reverse engineer it in one of two ways:

(1) With a voltage tester and the switch still in place in the circuit: Test for voltage between each of the three screws and ground. Then flip the switch to the other position and test again. The screw that lights your tester in both positions of the switch is the "common". The other two screws are the switched legs. The switched leg that lights your tester, in a given position of the switch, is the one that is connected to the common in whatever position your switch is in.

(2) With a real continuity tester (i.e., a ohmmeter), you should test the switch with the switch disconnected from all wires.

Let me know if your true question is different than the one I answered.

#3
12-23-00, 10:02 AM
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Yes, it is a true 3-way switch complete with a Red wire and I was referring to a voltage tester as you state. Still learning here...thanks for clarifying.

And, yes, you answered my question. I'll go and perform the test now.

Ed

#4
12-23-00, 10:36 AM
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John,

Got some strange results with the voltage tester. Here they are:

Switch #1 (Foyer)
=======================
Pos #1 - (dining room light is ON)
Pos #1 - Red & Ground = voltage light DIM
Pos #1 - White & Ground = voltage light ON
Pos #1 - Black & Ground = voltage light ON

Pos #2 - (dining room light is OFF)
Pos #2 - Red & Ground = voltage light ON
Pos #2 - White & Ground = voltage light OFF
Pos #2 - Black & Ground = voltage light ON

Switch #2 (Dining Room)
==========================
Pos #1 - (dining room light is ON)
Pos #1 - Red & Ground = voltage light DIM
Pos #1 - White & Ground = voltage light ON
Pos #1 - Black & Ground = voltage light ON

Pos #2 - (dining room light is OFF)
Pos #2 - Red & Ground = voltage light OFF
Pos #2 - White & Ground = voltage light ON
Pos #2 - Black & Ground = voltage light OFF

Based on your instructions, I'm still can't determine the "common". Is this incorrectly wired? Could the "common" have been run to the light fixture first? Just guessing here...

#5
12-23-00, 04:02 PM
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Join Date: Sep 2000
Location: United States
Posts: 18,497
Ed,

If all of our posters presented such clear information, we'd be in good shape. My complements to your 6th grade science teacher.
-----------------
Well, I'm a bit concerned about DIM. Yes, it could indicate some miswiring -- perhaps lights wired in series rather than parallel. People make this mistake all the time.
-----------------
But your switches are not a mystery. In both switches, the screw that the black wire is on is the "common". Hence your original question is now fully answered.
--------------------
But it appears that you have some problem that you have not yet told us about. What is it? Here are a list of questions:
(1) How many fixtures are controlled by these two switches? I suspect the answer is more than one (if so, this is likely the cause of your problem -- any chance you increased the number recently?).
(2) In each of the two switch boxes, how many cables are coming into each box, and what colors are the wires in those cables? I suspect the answer is exactly one cable in each, with three wires (black,red,white).
(3) At each of the light fixtures, how many cables come into each box and what are the colors of those?
----------------------

#6
12-23-00, 05:05 PM
Wgoodrich
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I think what John is looking for is the key anwer to your puzzle.

With all wires disonncted try to find the power cable. Should be a two wire cable with a bare grounding conductor.

He is asking how many wires in each box in search of where power is coming in at. Light fixture box or the first three way switch or the second three way switch.

If you will tell us the number of conductors is in each cable that is entering each box [both switch boxes and light fixture box] and also how many cables in each box, then John can probably be able to sort out you translation of what you have.

Get back to us with this info. Then we may be able to help you.

Wg

#7
12-25-00, 04:46 PM
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John,

>>(1) How many fixtures are controlled by these two switches? I suspect the answer is more than one (if so, this is likely the cause of your problem -- any chance you increased the number recently?).<<

Only the Dining room light is controlled by the two switches in question.

>>(2) In each of the two switch boxes, how many cables are coming into each box, and what colors are the wires in those cables? I suspect the answer is exactly one cable in each, with three wires (black,red,white).<<

One cable is coming into each box. Each cable contains a black, red, white and ground wire.

>>(3) At each of the light fixtures, how many cables come into each box and what are the colors of those?<<

I need to take down the existing light to give you an answer on this one. I reply tomorrow Tue 12/26.

#8
12-25-00, 05:57 PM
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Location: United States
Posts: 18,497
Thanks Ed. But please also tell us what problem you are trying to solve?

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