The valves which control water distribution for residential irrigation systems are the most critical component of your in-ground sprinkler system. A valve that sticks open can quickly turn your lawn into a swamp and kill all of the grass in that area. Although many homeowners prefer to call their local irrigation maintenance company, fixing a valve that is stuck open is a task that anyone can accomplish.
Locate the flow control on the valve (a plastic screw which sticks up from the center of the valve) and use your fingers to turn it to the right until it stops, and then turn it four full turns to the left. This action flushes air pockets out of the valve in case they are preventing the valve from shutting off, and opening it back up by only four turns ensures adequate water flow, but prevents future air pockets.
Locate the valve's solenoid (a cylinder-shaped plastic piece with two wires sticking out of it) and turn it all the way to the right. This action ensures that the solenoid is in the correct position to shut the valve off.
Locate the main water supply for the irrigation system. It is usually near the outlet from your home or basement. Shut it off.
Remove the cover from the valve by loosening and removing all of the top screws with the screwdriver. Remove the rubber diaphragm, but be careful not to loosen the large spring which sits on top of it. Check and clear the area underneath the diaphragm of any debris. This ensures that there are no obstructions which prevent the valve's diaphragm from cutting the flow of water.
Check the diaphragm for decay or deterioration. Replace the diaphragm with another one if there are any signs of inflexibility or corrosion. Remember to line up the diaphragm with the screw holes, and put the spring back on top of it before replacing and screwing down the valve cover with the screwdriver.
Turn the water supply to your sprinkler system back on. The valve is fixed and will turn off unless your controller is currently giving it a supply of power.