Multiple gain trim valve for a selectively engageable friction device

A transmission control system having a trim valve for controlling the engagement pressure of a torque transmitting mechanism comprising:

• a pressure source;
• means for distributing a trim control pressure, a boost control pressure and a main pressure from said pressure source;
• a valve body including a main pressure port connected with said main pressure distributing means, a trim pressure port being connected with said trim control pressure distributing means, a boost control pressure port being connected with said boost control pressure distributing means an exhaust port and a feed port connected with the torque transmitting mechanism;
• said trim valve comprising a spool valve slidably disposed in a valve bore in said valve body, a plug valve slidably disposed in said valve bore, said valve bore cooperating with said spool valve to form first responsive area connected with said feed port and second pressure responsive area connected with said trim pressure port, and cooperating with said plug valve to form third pressure responsive area connected with said boost control pressure port and fourth pressure responsive area connected with said trim pressure port, said spool valve being responsive to pressure in said first and fourth pressure responsive areas to control distribution of fluid between said feed port and said main port and said exhaust port when said boost control pressure port is not pressurized, and said spool valve being responsive to pressure in said first pressure responsive area, said second pressure responsive area when said boost control pressure is at a first pressure level and said trim control pressure is at or below a second predetermined level and being responsive to said first pressure responsive area, said third pressure responsive area and said fourth pressure responsive area when said trim pressure is above said second predetermined level and said boost pressure is at said first predetermined level.