Take on two-wire

Learn the basics of two-wire system troubleshooting.

A two-wire irrigation system can provide a lot of flexibility to contractors, running two separate wires from the central controller to each valve through a decoder. Compared to a conventional wire setup where each valve connects directly to a controller on its own wire, that might be an attractive approach in some situations, especially where more valves might be added later.

With added flexibility comes added complexity, and troubleshooting a system with more wires can seem much more challenging. There are a few points that tend to be good starting places for trouble, according to Craig Borland, CIC, CID, CIT, CLIA, CLWM, sales and marketing at The Toro Company and an Irrigation Association landscape instructor.

“First and foremost, two-wire is unique in that it runs an alternating current path out to the field on two wires that interconnect all the zones, which is different from conventional irrigation systems where each station has its own primary (hot) wire, and each station has a common that’s tied to them all and brought back,” he says. “So for a 12-station clock, you’re going to have 14 or maybe 13 wires out there, depending on the number of common wires.”

For a conventional 12-clock system you’re looking at 14 or fewer wire connections. For a two-wire system with 12 stations, you’ve got 26 wire connections or 27 depending on grounding. That means two connections per decoder, two connections at that decoder and then two more connections from that decoder that go to the solenoid and possibly a third one for the ground, he says.

“Second, the quality of connection is paramount,” says Borland. “You’re dealing with AC power to the solenoids to get them to run, but you also have an embedded direct current signal or communication line on the same two-wire path. Now you’re dealing with AC power to the field and a DC signal going out on the same wire path. Now I don’t just have an AC to worry about. I’ve got an AC and a DC signal going out that I have to make certain that the wire connection for each is really kept clean and dry.”

Third, those connections aren’t made in a dry connecting box, he says. “No, they’re buried in the ground where there are things that go bump in the night. You’ve got moisture. You’ve got creatures. You’ve got everything crawling around. They’re going to mess with those connections. So they’ve got to be waterproof to some extent.”

While it’s tough to say that anything is completely waterproof, Borland tends to believe that the larger the protector that the wire is in, the longer it will take before water will seep in.

Wire connections are such an integral part of making certain that a two-wire system works correctly throughout, but it’s easy to forget how much they matter. “In all the troubleshooting I’ve done over the years, it’s just silly how poorly wire connections are made,” Borland says. “It’s the absolute most inexpensive part of an irrigation system, the cheapest part of an irrigation system. And it’s often the one we take shortcuts on.”

A critical part of two-wire troubleshooting is having an understanding of the state of the wiring connections made in the field.

“They last six months to 20 years, and it just depends on the quality of connection you make,” he says. “We want it to be there until the next generation has to deal with it. With that in mind, put the wire connections in a valve box so you can find them again. And then put the two-wire path in some form of conduit.” The contractor can use flex tube, PVC or any other choice of material. That way, if there’s ever a problem, it can be reached for maintenance or replacement and is easier to work with.

Getting started

A good starting place is checking the specifications of the system you’re working with either with the manufacturer or on the internet, says Borland.

A tool that is a must before doing any troubleshooting for two-wire systems is a voltmeter, he says. It needs to be a “leakage current tester” clamp meter that is able to read milliamps.

“Make certain to spend the $200 or $300 to get that,” he says. “Don’t even go out to the property without one.”

While most manufacturers have a test box that can be used, keep in mind that it’s only approved to work with that specific manufacturer’s system. A clamp meter will be a versatile tool that can be used across multiple projects.

When troubleshooting, it’s important to isolate the problem.

“We need to look at the problem and say ‘Is this the symptom? Is this the cause?’” says Borland. When you’re brought out to a client’s property because the system isn’t working, start by checking to see what symptoms are in play and diagnose from there. For example, if the clock display is blank, no power is being supplied. That could mean there’s a breaker that’s been tripped.

After checking the breaker, it’s time to move on to other possibilities. If the clock isn’t working correctly, check the timing mechanism where you program in the start time. It’s possible that a rain sensor is keeping the clock from running, or there’s an issue with the motherboard or daughterboard for the two-wire path. Some systems have up to three boards, so it’s a matter of going step by step to see where the problem is. If you need to, remove a board and try again. If it blows a fuse, remove a board and try again until you find the one that’s causing the problem.

Look at the number of volts coming out of the power supply to the clock. If it’s not matching what’s listed in the product specifications, check the breaker or ground fault interrupter. If the transformer is warm to the touch, you know there’s power running to it.

“You start out at the very beginning and just work your way down the list,” he says. “It’s a methodical process. Eliminate what it isn’t to discover what it is.” Break the system down into smaller parts to check through them individually without getting overwhelmed by the possibilities. Is the clock the problem? It could be the wiring in the field or potentially the decoder that’s at issue.

Tracking the problem

Once you’ve gotten a display, turn something in the system on, then run the diagnostics in the controller.

“Every manufacturer’s two-wire product has a diagnostic or some way to let the clock tell you what it thinks the problem is,” Borland says. “From there, you start working your way out.”

If the diagnostic clock isn’t communicating with all of the stations, you’re likely not getting power coming out of the clock. Remove the two-wire path from the clock and ohm check it with a multimeter.

“The two-wire path should always be an open line,” he says. “If you’re getting resistance, find the path going out of the clock. There should be a junction box right near the clock. Open it and remove all the wire connectors. Ohm check each of the wires and look for where the short is. Once you find it, that’s the path you’re after.”

Finding where the short is in a two-wire path can seem like it could take forever, but Borland narrows the problem area in the distance down quickly by working in halves. Once you’ve found where the path leads, walk about half the distance to the very end of the line. Find the junction box there, remove the wire connections and check it again. Do not reconnect the wires at the clock, leave those open.

If there’s a short halfway down the line, you can check to see whether it’s still further down or closer to the clock. Heading back toward the clock, Borland says to go halfway again, and do those checks again. Working like that, going halfway each time, you’ll find the problem quickly.

Sometimes, a junction box has three or four pairs of wires connected to it. Borland says to think of it as a major intersection. From there, to continue working in halves, you can look around to see where there are additional valves following those pairs of wires to make a guess as to where the line continues. “Because contractors don’t like going all over the place, a straight line to those valves is pretty much the best direction you can go to continue checking,” he says. “Not always, but often. In parking lots or working around planters, you’ve got to just guess where the line went.”

Checking decoders

If you ohm check it all and don’t find resistance, connect it back to the clock. A unique thing about two-wire that makes it a little easier to troubleshoot is that it’s a single, solitary two-wire path that goes throughout all of the stations. That means that if you turn power on, every decoder on the property should be getting current, says Borland. Hook up your clamp meter, check the red or white wire and then the black wire, and read the current draw. AC decoders draw on average about half to one milliamp per decoder while idle. While there’s a zone turned on, it can be about 150 milliamps to 200 milliamps total. If you’ve got 100 decoders, that’s about 50 milliamps to 100 milliamps, plus 150 to 200 for the solenoid, making 250 to 300 milliamps total.

“The key thing is not what the milliamp read is, per se, but it’s more about the black and red wires reading the same number. You want to make sure the pair of wires have the same number on each wire,” Borland says. “That’s the key. The majority of these systems should read in the range of 300 milliamps or less.”

Do each wire individually. When doing a milliamp test, clamp around the wire.

One common mistake while taking these readings is not giving the meter enough time, Borland says. You want to measure the electron flow path going down that wire, so you put it around one of the wires and you count slowly to three. Then read the meter. “We can be such an impatient bunch of people that we don’t give the meter a chance to settle down. If after three seconds it hasn’t settled down, you’ve got a different problem. That’s likely a decoder that’s failing.”

From there, go out to the first box. Check each wire individually, one by one. Clamp test each one and count to three. After each junction box, once you’ve checked it, there’s 0.5 or 1 milliamp that’s no longer in the system because you’ve gone past it. The number should be getting smaller.

Don’t forget to clamp check the surge device. Many manufacturers of two-wire paths have a surge device that you could connect to a two-wire path and connect it to a ground rod. It should have no current draw on it.

“Just walk the property. Do some deep knee bends, get into the valve boxes, open them up. Clamp on each wire one at a time and look for the high current draw. Any idle decoder that is not running the solenoid that is reading more than 2 milliamps, that’s a bad decoder. If it’s running a zone, 200 milliamps is legit. But if it’s reading a lot on there at a single decoder, that’s a problem,” he says.

Taking each step in turn, you should be able to narrow down the problem area and make replacements as necessary, Borland says.

Kyle Brown is editor-in-chief of Irrigation & Lighting magazine and can be reached via email.

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