Nelson Controller: Zone 2 Runs With Every Other Zone — Wiring or Valve Issue?

Problem: Zone 2 Activates Alongside All Other Zones

After replacing four leaking backyard valves, the system works manually but from the timer, Zone 2 starts whenever any other zone runs (losing pressure but staying on). The controller is an older Nelson with a "P" terminal but no apparent master valve.

Diagnostic Strategy

1. Test the "P" (pump/master) terminal: Disconnect its wire. Run Zone 1 manually. - If only Zone 1 runs: Zone 2 was wired to "P" (which powers whenever any zone runs). Move Zone 2 wire to terminal #2, and disconnect the wire currently on #2. - If Zone 1 doesn't run: A master valve exists — reconnect "P" wire.
2. If master valve confirmed: Zone 2 stays on because either: - Its valve is mechanically stuck open (rebuild/replace diaphragm/solenoid), OR - Terminal #2 is energized whenever other zones run (controller defect).
3. Isolate valve vs. controller: Disconnect wire from terminal #2. Run Zone 1. - Zone 2 runs → valve is stuck (rebuild valve). - Zone 2 stays off → controller defective (replace controller).

Key Insight

The "P" terminal powers a pump start relay or master valve whenever any zone activates. If a zone valve is incorrectly wired there, it will run with every cycle. Systematic testing identifies the root cause without guesswork.