3-Way Switch Troubleshooting

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
When a 3-way switch circuit has been working and now only works if one of the switches is in a certain position, we know (usually) that one of the switches is bad. What I've been trying to come up with is a way to know which switch is bad without taking both switches out and testing. I wired a couple of 3-ways up on my work bench, , one being bad, to see if I could come up with something but could not as it depends on the switch positions.
What I usually do is just change both switches since they are most likely the same age, or try to find out which switch is used the most and check it first.
Anyone have a better method?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Jumper the travelers together at either one of the switches. The good switch won't be able to turn the light OFF when the bad switch is set so that the light in ON. But because the bad switch won't make contact in one of its positions, it can still turn the light OFF.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Jumper the travelers together at either one of the switches. The good switch won't be able to turn the light OFF when the bad switch is set so that the light in ON. But because the bad switch won't make contact in one of its positions, it can still turn the light OFF.
Thanks, but I could test with a meter faster than I could connect jumpers. I was trying to figure out which is bad by just flipping each switch without taking the switches out. In other words, flip the switches and just go to the bad one to replace it. So I suppose, time wise, its just better to replace them both.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Not possible:

1) Both switches working:

Up/Down OFF
Up/Up ON
Down/Down ON
Down/Up OFF

2) First switch open circuit when Up:

Up/Down OFF
Up/Up OFF
Down/Down ON
Down/Down OFF

3) Second switch open circuit when Up:

Up/Down OFF
Up/Up OFF
Down/Down ON
Down/Down OFF

Patterns 2 and 3 are identical, so you can't distinguish by just flipping the switches.

Cheers, Wayne
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
hmm... I wonder if you could have some kind of indicator light, like those fuses with tiny led's in them. You would need to sense when power is one position but not the other. it would have to something between the two travelers to sense that voltage is present, because there will always be voltage on one leg. the problem is knowing when there is a fault vs when the controlled load is off.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
If the line side 3-way is defective then neither of the travelers will have 120V when it's in the bad switch position. Therefore the load side 3-way switch will not be getting 120V on any of its terminals in this case. This could be detected with a non-contact tester. Of course if the cover plate is metal it might have to be removed, and a multigang switch box could make it difficult to verify this way.
If the line side 3-way is good, then the non-contact tester should light up at both of the 3-way switch locations no matter what switch positions are selected.
Of course, using a non-contact tester is not foolproof but it could increase your odds that you change out the right switch the first time.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I would start by testing for power on each traveler at either switch.

If you can energize both travelers, then the load-end switch is bad.

If you can only energize one traveler, then it's the feed-end switch.

In my opinion, replacing parts is a poor method of troubleshooting.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
I would start by testing for power on each traveler at either switch.

If you can energize both travelers, then the load-end switch is bad.

If you can only energize one traveler, then it's the feed-end switch.

In my opinion, replacing parts is a poor method of troubleshooting.

If after you find out from the customer that the circuit has been working and you've tried the switches and the light only works with a certain combination of the switches, then not much troubleshooting is needed. I don't go throwing parts at problems, but this is different.
I simply wanted to know if there was a way to determine which switch was bad before taking them both out.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
You are assuming that the problem is not in one of the travelers.Which is good bet unless any sort of carpentry or electrical work has been done recently.
 

readydave8

re member
Location
Clarkesville, Georgia
Occupation
electrician
Many times HO says that.if they turn off one particular swotch.,light won't work until turned back on

I don't know why, but that usually turns out to be the bad one
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
Many times HO says that.if they turn off one particular swotch.,light won't work until turned back on

I don't know why, but that usually turns out to be the bad one
That's probably the one they use the most, and they don't realize that both switches have to be in a particular position to make it work.

It's always like.....this one has to be down for the other to work.

But it can easily be said the other one has to be specifically up or down to make this one work
 

Terminator5047

Senior Member
Location
Saint Louis
Occupation
Electrician
To test a light switch with a multimeter:

  1. Turn off the circuit breaker that delivers power to the light switch being tested.
  2. Uninstall the light switch.
  3. Turn on the multimeter and put it on the Continuity or Ohms setting (depending on the model you have).
  4. Determine whether you have a single-pole switch or a three-way switch. Single-pole switches have two side terminal screws and may have a ground screw near one of the ends. A three-way switch will have a dark-colored screw (the “common” terminal) and two normal side terminals (the “travelers”).
    1. To test a single pole light switch, touch one multimeter lead to each of the side terminals. When you flip the switch on, the multimeter should show a reading of 1.
    2. To test a three-way switch, hold one multimeter lead to the common screw, and touch the other lead to one of the travelers. When you flip the switch on, the multimeter should show a reading of 1. Then touch the second lead to the other traveler to test it as well.
 
Top