Double solenoid, 2-position, no springs. Spool shifts and remains shifted when one solenoid or the other is momentarily or continuously energized. Standard models are assembled for “Internal Pilot” operation. This operator style will operate reliably on line pressures from 150 PSI down to 25 PSI. If the line pressure is 150 PSI to 250 PSI or less than 25 PSI to 28″ Hg, then the solenoid operator must be configured to use an “External Pilot” source.