Question
i recently checked a large irrigation system with multiple zones, manually
turning each zone on to make sure all sprinkler heads work and spray
properly. i got to one zone that i turned on checked and when i tried to turn
it off the water did not stop running.  tighten everything on the valve that i
loosened to start it and it still remains on.  do you have any suggestions as to
why this is? thank you

Answer
Luke,
There's a good chance the solenoid plunger is stuck in the up position. You can try turn the zone on multiple times or just remove the solenoid and clean or replace it.
It's possible the diaphragm is bad but I would work on the solenoid first. It's the easiest.
Good luck.

Matt