Range section protection valve assembly

A valve assembly (310) for use in a control system (300/400) for controlling a pressurized fluid operated shift actuator (220) for an auxiliary transmission section (14) of a compound transmission (10) including a main transmission section (12) having engaged and not-engaged positions connected in series with said auxiliary transmission section, said auxiliary transmission section including a selectable high speed ratio (direct) and a selectable low speed ratio (reduction), each of said ratios engageable by a synchronized jaw clutch assembly (92/128), said actuator (220) comprising a differential area piston (221) having a first piston surface (224) defining a first chamber (224A) and a second piston surface (222) defining a second chamber (222A), said first piston surface area being larger than an area (A.sub.222) defined by said second piston surface, pressurization of said first chamber (224A) causing said actuator to urge said synchronized clutch assembly to engage said high-speed ratio and pressurization of said second chamber (222A) causing said actuator to urge said synchronized clutch assembly to engage said low-speed ratio, said system including means (204, 210) for indicating the current engaged or not engaged condition of said main transmission section, switch means (98/98A/412) for selecting a desired auxiliary section ratio, a common source (234) of pressurized fluid and an exhaust (EX), said valve assembly including a valve body (342) defining an axially extending bore (346) for slidably and sealingly receiving a valve spool (348), said valve body defining four ports opening to said bore, a first port (316) connected to said exhaust, a second port (318) connected to said source, a third port (320) connected to said second chamber and a forth port (322) connected to said first chamber, said valve spool having a first axial position (324) in said bore for establishing fluid communication between said first and third ports and between said second and forth ports, a second axial position (326) in said bore for establishing fluid communication between said first and fourth ports and between said second and third ports, and a third axial position (328) for establishing fluid communication between said second, third and fourth ports and blocking said first port from the other ports.