How To Troubleshoot 3-Way Switches

Let’s look at how to troubleshoot and repair your 3 way switches. A  3-way switch that is wired wrong can not work right. 3-way switches allow you to operate one or more light fixtures from two different switching locations. A stairway is a good example of the need for a 3-way switching situation. You turn the light on from the top switch and then you go down the steps and turn the light back off at the bottom of the steps.

The most common problems on 3-way switches are the ability to turn the light on from one end but not let you turn it back off from the other end. Sometimes it will let you turn it back off but then you can’t turn it on again from the other end. This is due to the common wire not being hooked to the common lug on the switch.

A typical 3-way switching setup.

Let’s first examine how a 3-way switch works so you can better understand how to troubleshoot and repair the problem. There are 3 lugs on a 3-way switch, One is the common and the other two are the traveler lugs. Depending on which way the switch is flipped, the common lugs makes contact with either one of the traveler lugs. Let’s call the common lug A, the first traveler lug B and the second traveler lug C. When the 3-way switch is flipped up the common A makes contact with traveler B. If it is flipped down then common A makes contact with traveler C.

The hot wire on the circuit is tied to the common lug on one of the 3-way switches and the load, or the light wire, is tied to the other common on the other 3-way switch. Each one of the traveler wires run between the two switches and directly connect each corresponding  traveler lug to it’s mate on the other end. Depending on which way the switch is flipped, it lets the hot wire that is matched up with one of the travelers from that one end send the hot to the other end on that traveler and it either matches up on the other end with the traveler that is making contact with the light wire through the other common or it matches up with the other traveler that is not making contact with the other common. If it does match with the light common it turns the light on and if not it turns the light off.

Now if you are not totally confused now, we will look at how to fix this problem. First you have to locate the constant hot wire on one of the switches and make sure that it is tied to the common lug on that switch. The common lug on a 3-way switch will not have a lug directly across from it on the other side of the switch and it will usually be a different colored screw. There are two lugs on one side and one lug on the other side. The common will be one of the two lugs on the same side with each other. It will not have a lug across from it.

Once you have made sure that the hot is on the common then go to the other end and look for the two traveler wires. They will be in the same romex together. Then you should have a separate black wire that is coming from a different romex than the two traveler wires. That black will be your common going to the light and should tie to the common lug on that end switch.

Now depending on how this particular 3-way was wired, it is possible that you would only have one 3 conductor romex in each switch box. There would be a black, a white, and a red with a bare ground wire. Which ever colors on the other end were the travelers should be the same on this end for the travelers so that leaves  the other color as being the common on this end.

If you have both commons on the right lugs at the switches the 3-way will work right. That is unless you have a bad switch. You can check the switch with a mulimeter by checking for voltage or by taking the wires off the switch and checking with continuity. If checking for voltage put one meter lead on the ground wire or the white neutral if you have one in the box and touch the other meter lead first to the common lug on the switch and then to each traveler lug. If you have voltage at the common then you should have it on one of the traveler lug also. Flip the switch and check both traveler lugs. If you have no voltage on the traveler then the switch is bad.

Be sure and turn the power to the circuit you are working with off unless you are checking for power. Once you have finished with that check turn the power back off until you need it again or have finished making the repairs and have put the switches back in the boxes and installed the cover plates. Electricity is a dangerous thing and you must respect it or it can seriously injure or even kill you under the right circumstances.
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