I've installed low-voltage (12V AC) lighting systems in my front and back yard. They're set up separately, but both use the same model of transformer and lights. I set this up in May 2017, so it's been working fine for about 18 months.
After setting up Halloween decorations last week, including one set of string lights, I saw that the lights in the front yard weren't turning on. The wires are tucked out of the way before they go underground, but it's possible I or my daughter snagged a wire and knocked something loose. But I haven't found any physical problems.
The outdoor GFCI outlet seems fine because the string lights still work, on both plugs.
Since the transformers are the same, I swapped them, and the transformer from the front yard works fine in the back yard. However, the transformer from the back yard doesn't work in the front yard. So it seems the wiring and not the transformer is the problem.
Then I put a multimeter on the transformer leads and found that it was only putting out one volt. Testing further, I found that that's what the transformers do when the power cords to the lights aren't connected. So in the back yard, I get the full 12 volts when everything is connected and 1 volt if I disconnect the lights, while in the front yard I get 1 volt no matter what.
I went around to some of the lights, pulled the bulbs and put the multimeter prongs into the bulb sockets and measured the same 1 volt at each location. So the circuit doesn't seem to be broken -- the power is getting to all the lights. It's just not the full power.
Does anyone know what would cause this behavior?
Based on @Tyson's suggestion, I disconnected the wiring from the transformer and tested the resistance through the circuit. It was about .8 ohms, so neither a short nor an open circuit. I pulled out one of the bulbs and measured the resistance of the bulb itself and it was 1 ohm -- I would expect that to be about the same or a little less, but it's a little more. Perhaps one of the other bulbs is different and has less resistance. To be sure I didn't have a broken circuit combined with a short through moisture in the soil, I stuck the leads of the multimeter into the soil, but the resistance there was about 60 ohms.