Orbit rotating sprinkler heads are driven by water pressure, and if the pressure is too low, the sprinkler won't rotate. It's pretty easy to tell if you have low pressure in the zone because all the sprinkler heads will exhibit the same behavior, and if they do, you can probably trace the problem to the zone valve. If more than one zone is affected, there's a problem with the pressure at the main water supply.
When a single sprinkler head won't rotate and all the others are fine, there's no need to check the zone valve or the main valve because the problem is in the sprinkler itself. Fixing the problem may call for a bit of disassembly.
Video of the Day
How a Rotating Sprinkler Head Works
Orbit manufactures two types of rotating sprinkler heads. The type most likely to be installed in a residential setting is the pop-up type, and inside it is a turbine and a set of gears. Water enters through the base of the sprinkler and passes through a filter and the turbine before it exits the nozzle. It rotates the turbine as it passes through, and the turbine, in turn, operates the gears that rotate the sprinkler head.
The other type of sprinkler Orbit manufactures is the impact sprinkler, which is most often used for irrigating crops but may also be useful for irrigating large lawns. It's a simpler device that doesn't use gears. When water exits the nozzle, it strikes a loose-fitting paddle that disperses the water stream. The force generated by the paddle as it flaps is what rotates the head.
Pop-Up Sprinkler Head Not Turning
If you're sure there is sufficient water pressure in the system and a single head won't rotate, the problem could be a clog in the pipe to which it's attached. Unscrew the sprinkler head and remove it and then turn on the water to flush the pipe. Keep the water running until you see a full, steady stream from the pipe.
While you have the sprinkler in hand, disassemble the head, which you can usually do by removing a few screws. Pull out the filter and clean it in a bucket of water. When the filter is clean, replace it, reassemble the sprinkler head, and screw it back onto the pipe. If it still fails to rotate, the turbine or gears are faulty, and the only remaining option is to replace the sprinkler head.
Impact Sprinkler Not Rotating
An impact sprinkler is also affected by low water pressure, so if the stream isn't strong enough to turn it, unscrew the head and flush the pipe. If there is a strong stream but the head won't rotate, chances are it needs to be readjusted.
There are two metal stops on the outside of the sprinkler base, and their separation determines the arc of the sprinkler's sweep. You can adjust each of these by squeezing the wire clamps to which they are attached and rotating them around the base of the sprinkler. If you position them on either side of the sprinkler head, the sprinkler will spray in a 180-degree arc, and the arc increases if you increase the separation. Remove the stops altogether if you want the sprinkler to rotate through a complete circle.