Three Way Switch Installation

DEAR MIKE: At either end of my staircase I have a light switch that turns the staircase light on and off. The problem is that the switches don't work correctly anymore. What can I do to fix this? -- Michael L.

DEAR MICHAEL: You are referring to a three-way switch. They are installed in pairs and neither has an on or off position marked on the switch. You should be able to turn the light fixture on or off from either end of the staircase.

Three-way switches are common at staircases, hallways or large rooms with multiple doorways.

Most of the problems we see result from a miswired switch. In many cases, a homeowner will try to replace a three-way switch and not wire it correctly. It ends up where only one of the switches will work and only if the other switch is in a particular position.

In your case, since the switches used to work properly but now do not, I'm guessing you have a broken switch.

Turn off the power to the circuit at the main panel and test to make sure there is no power at the switches. Remove the cover plates and mounting screws and gently pull the switches out of the boxes.

Inspect the wiring to see if there are obvious problems, like maybe a wire that has come loose from a terminal. Although the terminals are metal, switches are made of plastic and they break rather easily. I would start at the switch that is used most often and inspect it for damage.

Label the wire that connects to the common terminal (it will be darker than the other two terminals or will have the word "common" or "com" stamped on it). The two lighter colored terminals will be the traveler screw terminals. The wires connected to them are interchangeable, so you don't need to label them.

Inspect the switch for damage, such as an obvious crack in the plastic.

Use a continuity tester (a $10 multimeter is a great tool to have) and test for continuity. Touch one of the tester probes to the common terminal and then touch one of the traveler terminals. Then flip the switch lever back and forth. The tester should read positive when the switch is in one position, but not both.

Test the other traveler terminal in the same manner. If the switch is good, the tester will read positive when the switch lever is in the opposite position from the positive reading on the other terminal. If the switch tests good, go upstairs and repeat the procedure with the other switch.

Had your problem been that the light turns on only when one of the switches is in a certain position, you would have started checking the wiring. If the switch is in the middle of the circuit, you will see two cables entering the box.

This gets tricky, so pay attention: One of the cables will have two wires plus a ground, and the other cable will have three wires plus a ground. The black wire from the two-wire cable should be connected to the common screw terminal. The red and black wires from the three-wire cable get connected to the traveler terminals. The white neutral wires should be connected with a wire nut, and the grounding wires should be connected with a wire nut and grounded to the box.

If the switch is at the end of the circuit, there will only be one cable entering the box. It will have three wires and a ground. Again the black wire gets connected to the common terminal. The red and white wires get connected to the two traveler terminals, and the ground wire gets connected to the metal box.

If all this sounds too confusing, stop. I'm sure there is an electrician out there who would be happy to collect a healthy check from you for spending 10 minutes in your home.

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