Figuring This Out
First of all, there is no ON position on a 3-way switch. Take a look and you will see that a regular (not 3-way) toggle switch (obviously not with Decora style) has markings, typically molded into the toggle part itself, for ON and OFF. But a 3-way switch has no such markings. So if one of the switches actually has an ON marking then that's the problem - it isn't a 3-way switch!
Assuming that these are 3-way switches and by "ON position" you mean UP, there are two ways to figure this out. One is by creating some truth tables based on what does and doesn't work and some extra trouble-shooting besides. The other way is to:
- Remove each switch from all 3 wires (4 wires if it has a ground wire).
- Use a multimeter to check continuity between each pair of screws with the switch UP and the switch DOWN.
On a functioning 3-way switch, there will be one screw that will alternate which other screw it connects to for UP vs. DOWN. Normally the two screws that alternate will be the same color and a different color (e.g., brass vs. silver or black) than the third (common) screw.
If both switches pass this test then you have a bad traveler wire somewhere - e.g., perhaps a bad junction somewhere between the switches. If one switch passes the test and the other one doesn't then you've found the problem - replace the bad switch.
A Note About Wire Colors
Wire colors have little meaning in the US system for residential wiring, except:
- Neutral is always white (well, sometimes gray) but white is NOT always neutral
- Green or bare is always ground
- Black, red, blue, yellow, etc. are always NOT neutral or ground
When you have wires in cables, which is common in most of the US/Canada, the cables are mostly:
- Black/White/bare
- Black/Red/White/bare
Since bare is always ground and neutral can only be white, there aren't many options. Except that since, until recently, switches have not required neutral (dumb switches still don't actually use neutral, but in new installations neutral must be available, which means if you are using cables then white becomes *ONLY neutral except in some very limited circumstances (primarily 240V circuits)).
For a typical 3-way circuit, each switch will connect either to a single black/red/white cable or to *two cables, one of them black/red/white and the other black/white. Since OP referenced two black wires, that means that particular switch must be connecting to two different cables. Which means one of the blacks is a black from a black/red/white cable and is one of the travelers and the other is the black from a black/white cable and is a common wire (either hot or switched hot). Which means that some of the possible problems can be figured out by:
- Checking which black wire is in a black/red/white cable (traveler) and which is in a black/white cable (common).
- Put the black traveler wire on the same color screw as the other traveler (red or white of the same cable).
- Put the black common wire on the other color screw.
Screw colors are pretty much always black, silver or brass, plus sometimes green for ground. The ground screw is usually easy to identify anyway because it is normally on one of the corners.
