Solenoids activate the diaphragm valves. They act like a switch, but control air. When the solenoid is powered by the control board the valve opens and passes air which in turn opens the diaphragm. Solenoids are the most troublesome part of a cleaning system. It is usually the coil which fails, not the stem. Coils will fail from electrical surges. Lightning storms are one of the biggest culprits when a solenoid or control board fails. The second biggest reason is that the retaining clip falls off the stem and the coil works its way out of the stem. If there is no stem or piece of metal in the center of the coil when it activates they will burn out instantly.