When the path lights along the front walk go dark one evening while the uplights in the trees keep glowing, the first guess is that a batch of bulbs has failed or that the transformer is failing. The split itself argues against both. A transformer that has lost its power darkens every fixture it feeds, and a whole row of lamps quitting on the same night would be an odd coincidence. A system that is half lit and half dark is telling you that power is leaving the transformer and stopping somewhere: at a splice that has opened, at one output that has shut itself off, at a control that runs only that group, or at a spot where a blade or a shovel went through the cable.
Each cause leaves its own pattern, and the checks that sort them out all happen on the low-voltage side.
How Power Reaches Each Fixture on a Low-Voltage Run
The transformer plugs into a 120-volt outdoor outlet and steps that power down to roughly 12 volts at its output terminals. From those terminals, two-conductor direct-burial cable runs out into the yard. Each fixture connects to that cable with its own short lead, either through a pierce-style connector that bites through the insulation or through a spliced joint, so every fixture on the run draws from the same pair of conductors, side by side, in parallel.
That layout explains the half-dark pattern. On a run fed from one end, power travels outward from the transformer, so a break in either conductor stops it at that point. Every fixture between the transformer and the break stays lit, and every fixture past it goes dark. A fixture that fails on its own leaves its neighbors lit, because each one taps the shared cable independently.
Larger systems divide the load. A transformer can carry two or more output circuits, each feeding its own run, and some installations send one heavy feed cable out to a hub where several branches split off toward different groups of fixtures. Some yards have two transformers, one for the front and one for the back. The number of cable ends that land on the transformer's terminals indicates how many runs leave it.
Reading Which Half Went Dark
Walk the yard after dusk and note which fixtures are out. The table below matches the pattern to the starting point of the search.
| What you see | Where to start |
|---|---|
| Fixtures near the transformer lit, the ones past a certain point on the same run dark | The cable or a splice between the last lit fixture and the first dark one |
| Every fixture on one run dark, other runs normal | That run's terminal, breaker, or fuse at the transformer, or the cable close to the transformer |
| A whole branch dark beyond a hub | The connections inside the hub, or the cable feeding that branch |
| Front yard dark and back yard lit (or the reverse), each on its own transformer | The dark transformer's outlet, its control and its breaker |
The dividing line between lit and dark is the most useful clue you have. If the last lit fixture and the first dark one sit a few steps apart on the same cable, the fault lies in that stretch or at the first dark fixture's own joint. Then recall any yard work since the lights last worked: aeration, new edging, planting, a fence post, a sprinkler repair. A fault that shows up the week after digging tends to sit where the digging happened.
Checks at the Transformer and Its Controls
Outlet and GFCI: If the dark group runs from its own transformer, confirm the outlet it plugs into has power. Where that outlet is GFCI-protected, a trip shows as a popped reset button on the outlet face or on another GFCI receptacle upstream in a garage or bathroom. Press it once.
If the GFCI will not hold a reset, or the transformer smells burnt or its case is hot to the touch, leave it unplugged and call an electrician. The outlet, the house circuit, and the transformer's 120-volt side are their work.
The transformer's breaker or fuse: Transformers carry overload protection on the low-voltage side, a push-button or toggle breaker on some models and a fuse on others, and on some multi-circuit units each output has its own. A tripped output darkens only its run. Reset it once. If it holds and the lights return, watch it for a few nights. If it trips again the moment it is reset, a short sits somewhere on that run, and resetting it over and over does nothing for the cable.
Terminal screws: A cable end that has pulled loose or corroded under its terminal screw kills a run as completely as a cut. With the transformer unplugged, check that each conductor is clamped tightly and that the copper is bright, free of green or white crust.
Photocell and timer: A photocell, mechanical timer, or smart controller switches the transformer's output, so a failed control darkens everything that transformer feeds. It becomes a suspect for a half-dark yard when the dark group has its own transformer or the controller runs separate zones. Check that the photocell's eye is clean and unshaded, that a mechanical timer shows the right time with its pins in place, and that a smart controller's schedule still includes the dark zone. On units with an ON/AUTO switch, setting it to ON is a quick test: if the dark group lights, the control is at fault, and if it stays dark, the problem lies farther out on the cable.
A Cut or Corroded Splice Along the Run
Every place a fixture lead joins the main cable, and every place two lengths of cable were joined to reach farther, is a joint living in damp soil, sprinkler water, and summer heat. Twist-on wire caps intended for indoor junction boxes allow moisture to reach the copper. Once water gets in, the strands oxidize; the oxide resists current; the joint warms under load; and corrosion speeds up until the connection opens. A failed joint shows as green or white crust on the strands, copper that crumbles when bent, or insulation that has swollen or split at the cap.
Whether one bad joint darkens one fixture or half the yard depends on how the run was built. A pierce-style connector clamps onto a continuous cable, so when it works loose (from a mower bumping the fixture or soil settling around its stake), only that fixture loses contact. Runs installed by cutting the cable at each fixture and joining both ends back together with the fixture lead are different: each joint carries power for the rest of the run, and a single failed splice darkens everything beyond it.
Look first at the base of the first dark fixture and the last lit one, then in any valve box or splice box where the cable was joined, and anywhere the cable has worked up to the surface.
Damage From Aeration, Edging and Digging
Cable laid shallow under turf or mulch sits within reach of yard work. A core aerator's hollow tines pull plugs of soil and can catch and tear the jacket; a power edger cuts a slot along the walk where the path-light cable runs; and a spade sunk for a new shrub or a post-hole digger for a fence post can sever the cable outright. Steel staples that pin drip tubing or weed fabric can pierce it, too.
The damage shows up two ways. A clean cut through one or both conductors is an open circuit: the transformer stays on, its breaker stays set, and everything past the cut goes dark. A blade that only nicks the jacket, or a crushed section where the two conductors touch, makes a short. Current takes the shortcut, the overload protection trips, and the whole run goes out, sometimes the whole transformer. A run that died with its breaker still set fits an open, and a breaker that trips again on reset fits a short somewhere on that run.
A line of fresh aeration plugs crossing the run, or a new edging slot along the walk, marks the first place to dig.
Finding the Break Without Tearing Up the Yard
Starting at the last lit fixture, follow the cable toward the first dark one and look for wire showing at the surface, a fresh cut through the turf, a disturbed bed, or a fixture leaning out of the ground with its lead pulled tight.
A multimeter narrows it further. Set it to AC volts and, with the system on, read across the two contacts in the socket of the last lit fixture, then the first dark one. A healthy reading at the lit fixture and zero at the dark one puts the break between them or in the dark fixture's own lead joint. On a long gap, a fixture lead or a joint exposed partway along gives a third reading that halves the search.
Once the stretch is known, expose the cable with a hand trowel in small bites so the tool does not make a second cut.
Push a small marking flag into the soil beside every dark fixture and the last lit one on the first night. Daylight hides which fixtures were out, and the flags show the stretch to dig without another walk after dusk.
When the Repair Calls for a Pro
Some faults go past homeowner-safe checks:
- A short you cannot find: The breaker trips on every reset, and no damage shows at the surface. A buried short along a long run takes a cable tester to locate and sometimes a new length of cable.
- Cable under hardscape: The dark stretch passes beneath a driveway, patio or walkway, and the break cannot be reached without lifting pavers or cutting a sleeve.
- A failing transformer: It hums with no output or its terminals are scorched.
- Repeated corrosion: Joint after joint has failed, a sign every connection was made the same weak way and the system needs reconnecting end to end.
A half-dark yard is a map. The line between lit and dark, the state of the transformer's breaker, and the history of recent digging together shrink the fault to one short stretch of cable or one box, and that stretch is where the repair begins.
Frequently Asked Questions
Bare copper twisted together and wrapped in tape takes on water in damp soil and fails again. A lasting repair joins the ends inside a direct-burial connector filled with silicone gel or grease, or with crimped butt splices sealed under adhesive-lined heat-shrink tubing. Leave a loop of spare cable at the repair so the joint can be lifted out of the ground later without pulling the run tight.
Cover the sensor eye completely with electrical tape or an opaque cap so it reads darkness, with the transformer set to automatic. If the lights come on, the photocell works. A system that tests fine this way yet stays off at night may have its sensor facing a porch light, streetlight or security light that keeps it reading daytime. Remove the cover when you finish.
Water reaching a cracked fixture socket, a split jacket, or a failing joint can let current leak between the conductors while the soil is soaked, enough to trip the transformer's overload protection. Once the ground dries, the lights come back until the next watering. Compare the outage nights with the irrigation controller's schedule; a match places the fault within one spray zone's footprint.
It can. Digital timers that hold their clock and program through an outage do so on a small backup battery, and once that battery is dead, the timer comes back blank or flashing after any outage, leaving the lights off or on at the wrong hours. A Wi-Fi controller may also need to rejoin the home network after the router restarts before schedule changes from the app reach it.
That fixture faults its own: a failed lamp or LED module, a corroded socket, or a loose connector on its lead. Swapping its lamp with one from a working fixture tells you which. A fixture with an integrated LED and no replaceable lamp gets a new module or a new fixture.
The shock risk at about 12 volts is low, but the output can push a heavy current through a dead short: a 300-watt transformer delivers about 25 amps at full load. Cutters or a screwdriver bridging both conductors can spark, pit the tool and heat the wire. Unplug the transformer before cutting, stripping, or joining any cable.
Book a repair visit for the dark run — a crew traces the failed splice or cut cable and brings every fixture past it back on. Specialty Landscape Inc. serves Fresno and the Central Valley. Licensed C-27 Contractor, CSLB #1058363. Call (559) 905-2273.
