How do you diagnose and repair a bad 3-way switch?
The 3-way switches are found in almost every home but more commonly in two-story houses. Their purpose is to turn on a light from one switch and then turn it off from another and vise-versa. If you have a two-story house, you turn on the stairwell light at the bottom of the stairs, climb the stairs, and turn the stairwell light off when you reach the second floor. Another example is when you enter your house, you flip a switch to turn on the entrance light. Then, you walk into your house and flip another switch to turn the entrance light off. In both examples, these are 3-way switches.
Free the switch from the box. Three-way switches are always installed in pairs. To correct a problem in three-way switches, follow the following procedure for one of the switches and if it’s not faulty, repeat the procedure for the other. Start by turning off the power to the switch at the service panel. Remove the screws from the wall plate and remove it. Loosen the screws on the mounting strap and pull the switch out of the box to expose the wires. Whether one or two cables enter the box, there are always three wires connected to the switch. The wire that feeds in from the power source or leads to the fixture is attached to the darker screw terminal. The two wires that run between the switches are connected to the silver or brass terminals.
Test for voltage. Use a voltage tester to confirm that the power is off by touching one of the tester probes to the grounded metal box and the other probe to one of the three switch terminals. For plastic boxes, touch one of the probes to a switch terminal and the other probe to the green grounding terminal on the switch. Repeat the procedure for the other two switch terminals. If the tester shows power, go to the service panel and find the correct circuit breaker or fuse. If you can’t, turn the main circuit breaker off.
Mark the wires. All three wires connected to the switch are hot. A black wire is attached to the darker switch terminal. To help yourself when reconnecting the switch, mark this wire with a piece of masking tape. The other two wires are connected to the silver or brass terminals. One is red, and the other is black or recoded white. If the white wire is not recoded to indicate that it is hot, mark it with black electrical tape or paint.
Working on the connections. Check the connections at the terminals. If a wire is loose, rehook it around the terminal and tighten the screw. If the connections appear dirty, clean the terminal and the wire ends with sandpaper. It may be necessary to snip the wire ends and strip the insulation, curl the wire ends and hook each wire clockwise around its terminal. If there are two cables in the box, check the connections under the wire caps. Screw the switch back into the box, turn the power on, and flip the switch on to see if there is power to the fixture. If the switch doesn’t work turn off the power again and remove the switch from the box.
Remove the old switch. Loosen the screw terminals. Disconnect the tagged wire from the darker terminal. Disconnect the other two wires from their terminals and remove the switch.
Test the switch. Use a continuity tester for this task. Attach one of the probes to the darker terminal. Touch the other tester probe to one of the other terminals. Flip the switch on, then off. The tester should show continuity in one position and not in the other. Leave the switch in the position that shows continuity and touch the other terminal with the probe. The tester should show an open circuit in this position of the switch and closed when toggled in the other position. If the switch fails, replace it. If the switch passes the tests, reconnect it, install it back in the box, and check the other three-way switch.
Connecting the switch. Hook the tagged wire to the black terminal and tighten the screw. Connect the other two wires to their terminals in the same way. If there is a grounding terminal on the switch, ground it.
Setting the switch into the box. Grasp the switch by the mounting strap and push it into the box carefully folding the wires to make them fit in the box. Install the wall plate, turn the power on, and toggle the switches one by one to check if the fixture works correctly.