3 way switches can be wired many different ways but they all work basically the same way. Understanding how they work can make troubleshooting and repair much easier for you. The key to fixing your 3 way switch problems starts with the proper placement of your common wires. All 3 way switches need 1 common wire and 2 traveler wires to work. They also need a constant hot from the power source and a switched hot up to the light fixture along with the neutral wire. They will also need a 3 conductor cable going from one switch box directly over to the other switch box. This cable will have the 2 traveler wires and 1 common wire in it.
A 3 way switch has 3 lugs for the wires to attach. The common lug on the switch will be a darker color than the 2 traveler lugs. The common lug will not have a lug directly across from it on the switch. This is basically how a 3 way switch works. The common lug on the 3 way switch makes contact with one of the two traveler wires at all times. Say the common lug is A, the first traveler wire is B, and the second traveler wire is C. If the 3 way switch is flipped up then A will make contact with B and when the switch is flipped down then A will make contact with C. The common simply switches back and forth between each traveler wire. Each time the switch is flipped the common switches and makes contact with the other traveler wire.
Determining which wires are the common in your switch boxes is not that difficult of a process but is extremely important and comes first when troubleshooting 3 way switching problems. One of the common wires will be the hot feed and the other common will be the switched hot going up to the light fixture. Finding the constant hot wire in one of the switch boxes is the first order of business. You will need a voltage meter for this.
In most situations you will have either one 2-conductor and one 3-conductor romex in each switch box. Or you may have two 2-conductor and one 3-conductor romex in one switch box and the other switch box will have just one 3-conductor romex in it. The constant hot that you want to find will normally be in one of the 2-conductor cables. Check the black and white from each 2-conductor until you find the constant power.
Once you have located this constant hot feed wire then that will be one of your common wires. Then locating the switched hot wire that goes up to the light fixture is the next task and that will be your other common wire. As long as you have the common wire attached to the common lug on your switch, it does not matter which traveler wire goes to which traveler lug on the switch. But if a traveler wire gets attached to the common lug on the switch then that is why the 3 way switches will not work right. In this case you can turn the light off or on from one end but can't turn it back off or on from the other end. So the easy fix comes from finding the two common wires and getting those two wires hooked to the common lugs on your switches.
Depending on how your particular 3 way system is wired will determine which wires go where on your 3 way switches. Sometimes the hot feed romex and the switch leg romex that goes up to the light fixture will be in the same switch box and sometimes the hot feed will be in one box and the switch leg will be in the box on the other end. Then sometimes the hot feed may go into the light box along with two 3 conductor cables, one going to each switch box. But in every situation there is always a constant hot and a switched black wire going to your light fixture and these two black wires are the two that you must find above all others.
If you have a 3 way switch wiring configuration that you do not understand then please see this article for more information: Learn The Different Ways To Wire 3 Way Switches.
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