Test irrigation wiring with a multimeter

Ask anyone involved with irrigation-system maintenance and they'll probably agree: Troubleshooting an irrigation system's electrical system is one of the most frustrating and time-consuming aspects of the job. As existing systems age, and as the number of installations increases, troubleshooting them becomes more common.

You need a fundamental understanding of how electricity behaves in an irrigation system to easily diagnose a system's ills. For example, being able to track wire with a specialized piece of equipment assumes that you've already established precisely why you need to track the wire. Therefore, the best piece of specialized equipment for troubleshooting is-and always will be-the ability to logically think through a problem.

Electricity basics With irrigation electrical systems, broken and severed wires result in open circuits, with corresponding resistance at high levels (infinity). No current flows across the break, thus, no voltage is necessary to push the current. Small nicks in a wire, or bad wire splices, increase the level of resistance across the conductor, thereby reducing both amperage and voltage. You'll often hear these called partials. Partials are some of the most perplexing problems to diagnose. Nicks in a common wire and a valve's station wire that touch allow current to flow along a shorter circuit than if it traveled through the solenoid coil. This results in a short circuit. Current that follows this shorter path experiences lower levels of resistance due to the decreased length of wire. Therefore, both current and voltage increase. This often is the case in the valve solenoid's coil of fine, copper wire.

In short, open circuits result in infinite levels of resistance with no current or voltage passing across the break. Partials increase the level of resistance across the conductor. They decrease the normal level of amps and volts passing across the "bad bridge." Short circuits reduce the level of resistance through a circuit and increase amps and voltage through the shortened circuit. Understanding these basic principles is a must before working on equipment.

Multimeter applications Every irrigation technician should have a multimeter. They are simpler to use than the standard analog style (which, however, will work) and clearly express all measured values on a digital display. Not only are they less confusing to use, they are more accurate. Multimeters allow you to measure amperage, voltage and resistance through a circuit. (See boxed information, "5 Steps to...," at right.)

It's fine to be able to make the measurement, but you must understand what it means. With solenoids, for example, you must have a reference point with which to compare the resistance readings. Levels of resistance through solenoids differ due to size, age and manufacturer. Generally, most solenoids measure between 20 and 60 ohms of resistance. If an irrigation system has 10 solenoid valves-each the same size and make-they probably will measure about the same resistance.

Six steps to diagnose a non-functioning station * Keep accurate records. Keeping accurate records of solenoid-resistance readings for valves on different systems is important. This knowledge lets you recognize when changes occur. Perhaps, for example, you can manually activate a valve by bleeding water at the top of the diaphragm but the valve fails to operate when the controller supplies power. Determining whether the problem is in the controller, the field wiring or the valve is tricky.

* Use an actuator. A good place to begin is at the clock, with a handy piece of equipment called a portable solenoid actuator. Using a portable battery, these devices provide 25 to 28 volts. Therefore, you can test suspect stations by disconnecting the common and station wires in question and reconnecting them to the actuator. This provides surrogate power to the valve. The problem probably is in the clock if the station works properly when the actuator is installed.

* Check the terminal strip voltage. Check the clock to verify that the proper 25 to 28 volts is coming off the terminal strip for the station in question. If it is, then it's time to look at the valve and field wiring.

* Check resistance of the circuit. While you're at the clock, check the resistance of the problem circuit. If this system has valves that typically measure about 30 ohms of resistance, and the one in question measures only 8, suspect a short circuit. To verify this, go to the valve box and disconnect the field wires from the valve. Check the resistance directly at the valve. Confirm whether the short is in the solenoid. If so, replace the solenoid. If not, as I mentioned, it's time to look closer at the field wiring.

* Inspect splices. If, however, you measured 100 ohms of resistance, the wire may have a bad wire splice or nick that is increasing the circuit's resistance. Again, go to the valve box. Inspect the wire splices from valve to field wiring. Bad splices account for most irrigation problems. Don't just look at the splices, however; remove the wires from their waterproof connectors and make sure the splices are sound. Of course, you may find the problem right there. Were waterproof connectors used? You might find that someone previously used wire nuts-small, plastic "capsules" with a threaded metallic insert into which you insert two or more wires and twist to ensure a connection. Wire nuts alone-even with a foot of electrical tape-will not cut it. Those splices are not waterproof. (King Technology of St. Louis makes direct-bury, gel-filled wire nuts commonly called King Connectors.) Neglecting to use waterproof connectors for the sake of saving $1.50 per valve simply does not make sense. One callback to honor a warranty because of bad splicing techniques will not make anybody any money.

If direct-bury splice tubes were used, remove the wires from the tube and remove the wire nut. You may have to remove some electrical tape from the wiring. Using electrical tape here isn't a bad idea because it helps protect wires from pulling free from the nut as you push and shove splices within the valve box.

Check the condition of the twisted-wire union. It's possible someone used only the nut to twist them together-not lineman's pliers. It's also possible someone carelessly stripped the insulation from the copper and scored the wires. This results in the wire snapping during twisting. Waterproof connectors similar to those used with low-voltage lighting are available for irrigation systems. These connectors don't require you to strip off the insulation. The wires insert into the connector and the insulation is pierced. With the wires still free from the splices, twist the common and station wires together to get a good contact.

* Check field-wiring resistance. Return to the controller and check the resistance through the circuit. With the solenoid removed from the loop, the resistance is only that through the field wiring-probably just a couple of ohms. If the resistance measures more than this, it's time to reach for some of the high-tech tools that now are available to locate such problems.

George Crosby is an associate professor of plant science at the State University of New York at Cobleskill, N.Y. He teaches courses in irrigation-system design, installation and maintenance in the college's Turfgrass Management and Landscape Development program.

Measuring voltage Using a digital multimeter to measure voltage output along a controller's terminal strip is a relatively simple task.

* With the clock operating, cycle through each station. * Set the multimeter to AC voltage. * At each station, touch the black meter lead to the common wire. * Then touch the red lead to the station location on the strip. * Proper output voltage will range from 25 to 28 volts and will vary depending on the clock.

Measuring circuit continuity and resistance To measure circuit continuity and resistance, follow these steps:

* Turn off the controller. * Disconnect the common wire and the station wire you want to check from the terminal strip. * Set the multimeter to the resistance scale. * Connect the black meter lead to the common wire. * Connect the red lead to the station wire.

Essentially, you're testing the resistance to current flow through the circuit. The meter sends a small amount of current from its own internal power source. The resistance is expressed in ohms. Most of the resistance in this case occurs through the many feet of the valve solenoid's fine, tightly wound copper wire. Additional resistance occurs through the field wiring-about 0.25 ohm through each 100 feet of 14-gauge wire.

Identifying a single wire in a bundle You can use resistance to identify a single wire from a bundle. For example, from a dozen red wires and one white common wire, you easily can find an individual red wire.

* Set the multimeter to measure resistance. * Connect the black meter lead to the common. * Connect the red lead to a wire you want to identify.

Wires at the other end of the bundle-which are not in contact to complete a circuit-will register an infinite level of resistance. Thus, they are open circuits. Your digital multimeter will read these as OLs or overloads.

* Have a coworker touch the unknown red wires to the white common wire one at a time. * Look for a small, but measurable, level of resistance on your multimeter. You'll find this when you finally touch the correct one to the common, thus completing the circuit.

A tone generator and inductive amplifier can make the job of identifying wires even simpler.

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