I've read up on three way switches, and have replaced a few over the years. I recently installed a bathroom fan, and so had to replace two three way switches. Very simple setup - two switches, one fixture with four lights. As soon as I looked at the old wiring, I got confused. After removing the switches, I did some testing. Each switch box has a hot line coming in. By that, I mean that with no switches installed, each box has a hot pair into it. The only thing that I can figure is that it was a Carter system, as that is the only diagram that I have seen with hot/main into both switches. A little disturbing, as this house was build in 1970, and Carter hasn't been code for ages.
Is my assumption correct? Is Carter the only one with main/hot to both switches? If so, what do I do from here? I'd rather not wire up another Carter setup.
Additional information: I've attached a picture with what I've discovered. My next task (tomorrow) is to open up the fixture and see how it is connected. It seems like 3 way switch in box 2 was wired in series with the rest of the circuit.
I verified A is connected to B by turning off main, then checking continuity with a multimeter. Verified both are hot with main on with a multimeter.
I assumed C by turning off power, then checking continuity with a multimeter. I don't know if there is a junction or anything else along this, but it does connect through.
I have one line going to the fixture (D), but I haven't verified it yet. It could be some combination of C, D and E for all I know.
I had to admit defeat on this one. I wired up the new switches in exactly the same way as the old. It works, but I still don't understand it. At some point I'll revisit and rewire. Thanks to everyone for their help!