A sprinkler system running past its programmed time wastes water, increases utility bills, and can damage landscaping through over-saturation or minor flooding. The underlying cause for a system that will not shut off is typically traced to either an issue with the control unit, which acts as the system’s “brain,” or a malfunction in the valve assembly, which functions as the system’s “gate.” Identifying whether the problem is electrical or mechanical is the first step in troubleshooting and finding a solution.
Programming and Controller Errors
Continuous watering can sometimes be traced to simple errors made at the primary control unit. An irrigation controller may be inadvertently set to a “Manual Run” or “Test Cycle” mode, which bypasses the scheduled programming and can run indefinitely until manually stopped. Similarly, some users accidentally program an excessively long run time, such as 999 minutes, which forces the system to water for hours beyond what is necessary.
Controller hardware can also fail, leading to non-stop operation. A power surge can damage the internal circuit board, causing it to continuously send a low-voltage signal to a zone valve. If the controller has a backup battery that has failed, the unit may experience memory loss, defaulting to a continuous run setting upon regaining power. These issues are generally the easiest to diagnose and often require only a simple reprogramming or a complete controller replacement.
Electrical Malfunctions at the Solenoid
If the controller is functioning properly, the problem may be an electrical fault within the solenoid, the component that translates the electrical signal into physical action. The solenoid is essentially an electromagnet that uses the controller’s low-voltage signal, typically 24 volts AC, to lift a small plunger and open the valve. When the watering cycle ends, the electrical signal is cut, allowing the plunger to drop and the valve to close.
The valve can remain open if the solenoid fails to de-energize and release the plunger, keeping the valve open electrically. This can be caused by a short circuit in the wiring harness, which bypasses the controller’s shut-off command and sends a constant signal to the valve. Corroded or damaged wiring connections can also lead to an unintended, persistent low-level voltage that is just enough to keep the solenoid partially engaged.
To confirm an electrical issue, a multimeter can be used to test the solenoid’s resistance (ohms), which should fall within the manufacturer’s specified range (often 20 to 60 ohms). A reading of near zero ohms indicates a short circuit, while an infinite reading suggests a broken coil. If the valve stops running immediately after the main power to the controller is turned off, the problem is confirmed to be electrical.
Mechanical Issues Inside the Zone Valve
When water continues to flow even after the controller is unplugged, the issue is mechanical, residing within the zone valve housing. The valve regulates water flow using a flexible rubber diaphragm that is held closed by water pressure from above. A common issue involves fine debris (sand, silt, or small pebbles) becoming lodged between the diaphragm and the valve seat, preventing a watertight seal.
A torn, warped, or stiff diaphragm can also be the culprit, as damage compromises its ability to move and seal the water port completely, allowing water to bypass the seal. The internal spring, which assists the diaphragm in returning to its closed position when the solenoid is de-energized, may also fail or weaken over time. In this scenario, the pressurized water flow overcomes the closing force, and the valve remains partially open.
Another frequent mechanical problem is the manual bleed screw being accidentally left slightly open after maintenance or inspection. This screw is designed to manually release the water pressure above the diaphragm to open the valve for testing, but if not fully tightened, it continuously vents the pressure needed to keep the valve closed. Fixing these issues requires turning off the main water supply, disassembling the valve, flushing internal components, and inspecting or replacing the diaphragm.
Problems with External Sensors
External sensors, most notably rain sensors, are designed to override the controller’s watering schedule, but a malfunction in these devices can cause the system to run continuously. A common rain sensor uses hygroscopic disks that absorb water and expand, which then triggers a switch to interrupt the electrical current to the controller. If the disks fail to dry out or the sensor’s internal switch gets stuck in the “wet” or “off” position, it may prevent the system from starting, but the opposite can also occur.
If the sensor’s wiring is faulty, or if the sensor’s connection to the controller is damaged, the controller may not receive the proper “stop” signal, or it may receive a constant “run” signal. In some cases, a poorly positioned sensor can get continuously sprayed by a sprinkler head or become clogged with debris, causing it to remain in the “wet” state for an extended period. To test for this, the sensor can be temporarily bypassed at the controller terminals; if the system shuts off immediately, the sensor or its wiring is the definitive source of the problem.