There are 3 possible reasons that backing off the coil helped. 1. The coil's plunger has a rubber piece on the bottom that has become deformed. Sometimes the rubber swells out and closes the gap between the plunger and the hole it covers when the coil is not engaged. Take a very sharp razor and trim the swelled out part. Be sure to trim it very level or it may fail but this time in the open position. 2. The diaphragm is bad. When the diaphragm goes bad it begins to allow more (or in some cases less) water to pass by it than the coil can handle. Small micro holes in the diaphragm, plugged holes in the diaphragm or the metering hole in the diaphragm are reasons this may happen. When I said before your diaphragms were bad this was a broad statement. Taking apart one valve and checking the diaphragm may have fixed your problem by removing sediment or dirt that was causing too little water to bleed past the diaphragm. 3. This one is rare and hard to fix but in rare cases (happened to me the other day for the first time in 2+years) the hole the plunger covers has moved up. This hole (in most valves) is a plastic or stainless insert that the plunger closes off. For some reason this insert moves up just a tiny bit. Now when the plunger gets pulled up by the coil there isn't enough clearance between the hole and plunger and the valve will not open. My guess is your diaphragm(s) will fail on you again sometime soon, one day to 2 years, and will need to be replaced. The job is very simple just be careful to not let any dirt get in the valve or screw holes. Good luck, Tom

you need to check the continuity of the wires first it will help. If you do have continuity it will be easier to locate your valve. You can use what we call a chatter box, basically what it does is it send a signal that makes the plunger in the solenoid open and close very fast. You walk around listening for the sound. You might be able to rent a 521 valve locater from your local rental yard. Hope this helped. Here is a link that might be useful: valve wires

FWIW, I did eventually identify this valve. The bypass appeared identical to the Toro models and I have all Toro sprinklers. I found that it appeared to match Toro 53380 (264-06-03). This was confirmed by a HD Fowler who actually had a replacement in stock. I replaced the guts and everything works fine now. It appears that the reason for the failure was that there must have been a pin on the bleed port internal to the valve. When I cracked the bleed screw, that pin must have been washed away/sheared off. In any case, the bleed screw was no longer capable of sealing the valve for whatever reason. Oh, and I located the last 3 valves by determining approximately where the would likely be and having my wife turn them on and off to actuate the solenoid. After a few clicks, I was able to hear and locate them.

Turbo, The pressure loss in an automatic solenoid valve is the energy source used by the valve to open and close the valve. If the valve doesn't have enough pressure loss it will not have the energy needed to close by itself. Most electric irrigation valve are "pilot operated". The design is made so that water pressure and a spring work together to keep the valve closed. The solenoid opens a small "pilot" passage to relieve the water pressure in an area that is keeping the valve closed allowing the spring to open the valve. There for the cracked top is acting like the pilot valve and relieving the static pressure that keeps the valve closed. Replace the cracked portion or the whole valve(less problematic). Also check your pressure. The reason for the crack may be water-hammer from having too high a pressure in your system. Do you have your irrigation off a faucet(house pressure side) or Teed from your house water mainline before the house water pressure regulator(high pressure side)? Aloha