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?
Update
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.
Best Answer
The usual cause of this kind of symptom would be a short, likely an internal short in the power supply, but if you're seeing the power supplies both do the same thing and both work on one side of the house, it must be some kind of "feature."
I have not used a power supply that works like this; most low voltage DC power supplies will show full voltage on the load terminals with nothing connected. As mentioned in the comments, maybe it's a safety feature. (If so it's a little surprising to me, since 12V is pretty safe, I could see this making more sense with 120V...)
But going with this presumption that this is a safety feature, where the transformer cuts voltage in a no-load condition, my first guess would be that implies one (or maybe even both) of the conductors between the power supply and the first light are broken. Maybe they got damaged somehow while decorating, maybe someone drove a stake in the ground and hit a conductor.
If the wires were in tact to the first light, you would have a load. If the fault was downstream from the first light, you'd get at least one or some lights working.
The simplest way to test this would be to run a test cable across the yard from the power supply to the first light and see if everything works.