Troubleshooting Random Zone Faults on a Rain Bird ISM-6 Controller

Intermittent "ERR" Faults Across Multiple Zones

A 5-zone system (two original Nelson valves, three newer Rain Bird valves) controlled by a Rain Bird ISM-6 began showing random zone faults ("ERR" on the display) about a week after a storm. The faults appeared on various zones—sometimes zone 2, zone 4, or zones 2 and 3 together—and eventually affected all zones.

Extensive Troubleshooting Already Done

• Replaced the controller with a new ISM-6 – no change.
• Replaced the entire control cable with a new continuous run, using waterproof wire nuts – no change.
• Tested each valve individually by connecting only one valve at a time to zone 1 and jumpering all zone terminals. Every valve ran flawlessly through multiple cycles.
• Measured current draw: Nelson valves ~3.4 mA, Rain Bird valves ~2.6 mA. One Rain Bird valve showed a brief 13 mA inrush during switching without faulting.

Key Insight: Combined Load May Overload Controller's Fault Circuit

The older Nelson valves have lower impedance, drawing more current. Rain Bird controllers have sensitive fault detection calibrated for modern solenoids. When all valves are connected, the combined inrush or steady-state current may exceed the controller's threshold, triggering a fault even though each valve alone is fine.

Recommended Steps

1. Run the system with only the three Rain Bird valves connected. Then test with only the two Nelson valves. This isolates which group causes the fault.
2. If the Nelson valves are the issue, replace them with modern Rain Bird valves—the simplest, most reliable fix.
3. Check the controller's 24 VAC supply voltage under load. A brownout (voltage below 24 V) increases current draw and can cause false faults. Measure at the controller's 24 V input terminals while zones are running; a healthy transformer should deliver 25.5–27 VAC.

If the problem persists after valve replacement and voltage verification, consider a controller with a more robust fault circuit, but the valve mismatch is the most likely cause.