Two Zones, Two Unrelated Faults, One Controller

Zone 4 running with the controller unplugged and the battery out is decisive: no electric power is available to hold the solenoid open, so the fault is hydraulic or mechanical, not a controller or wiring problem.

Ranked causes:

1. Debris on the diaphragm seat or a torn/stretched diaphragm - grit from the main holds the valve open; the solenoid only pilots it.
2. Manual bleed screw or flow-control knob left open - often a homeowner or the previous tech bled the valve to run a zone.
3. Loose diaphragm cap or cracked valve body - usually with water weeping at the cap.
4. Solenoid plunger stuck open (rare alone; it still needs the pilot port open).

Safest first step: close the zone ball valve or the master valve before opening anything, then check the bleed screw and flow control - a ten-second check that resolves a surprising share of stuck-zone calls. If that is not it, disassemble, clean the diaphragm and seat, flush the body, and replace the diaphragm if it is nicked, stretched or has grit embedded. Verify by drying the bleeder and cap and confirming the zone stays off with the controller unplugged.

Read the numbers together:

• 24 VAC at the controller terminal - controller and transformer are fine.
• 52 ohms at the solenoid - the coil is in spec (20-60 ohms), so the solenoid is not the problem.
• 3 VAC at the solenoid - about 21 volts are being lost between the controller and the coil.

That is a wiring-path fault, not a valve or controller fault. Some drop is expected over 180 ft of 18 AWG, but the whole circuit at roughly 0.35 A should lose only a few volts, not 21. The corroded wire-nutted splices in the box are the prime suspect - the valve box is the wettest point in the circuit and corrosion plus a loose connector adds resistance fast.

Confirming tests:

1. Disconnect the station wire at the controller and measure resistance of the pair: 52 ohms plus splice resistance. Well above about 60 ohms indicts the wire or splices.
2. Back-probe voltage at the common splice and at the station splice, walking toward the valve, to find where the drop occurs.
3. Remake splices with waterproof (silicone- or grease-filled) connectors and re-measure at the solenoid - expect 22-24 VAC.
4. Check the common as well: several valves share it, and a bad common splice produces odd multi-zone symptoms.

If voltage is still low after the splices are remade, pull a new conductor or use the spare; on a 180 ft run, 14 AWG is the correct gauge and 18 AWG is a design weak point even once repaired.

Do not do it. A 2 A transformer doubles the available current; it does not raise voltage at the far end of a resistive splice.

• The controller terminal already reads a healthy 24 VAC, so the transformer is not the limiting component. Every lost volt is downstream in the conductor and splices.
• A larger transformer behind a corroded splice still delivers about 3 VAC at the solenoid, plus more heat at the bad connection - in a garage wall, that is a fire risk.
• It also raises the available fault current. If a wire is ever cut or a solenoid shorts, a bigger transformer does considerably more damage and creates a bigger shock hazard.
• If a low-resistance (shorted) solenoid had been the cause, a bigger transformer would hold voltage up while cooking the coil. The right response to a short is a new coil, not more current.

Fix the circuit, not the power supply.