Question
Jeff,

Our community has a 20 year old Rain Bird system utilizing solenoid controlled control valves. We have 24 zones with 2-3 control valves to each zone. One control valve was manually activated by turning the solenoid. Now it runs continuously but can be shut off by manually turning the valve stem in the center of the valve. The solenoid has been change.  

Answer
Could be a couple of things.  First, check the electrical connections to the valve (the wire nuts or whatever was used.  If this doesnt help, you probably need to rebuild the valve.  Turn off the water supply for the entire system, and take the valve apart.  There will be parts inside the valve that can be replaced, and I would recommend replacement.