Hydraulic control system for automatic transmission

A hydraulic control system for an automatic transmission of an engine driven vehicle, the automatic transmission having at least drive and low speed ranges and reverse drive and the engine having a throttle, the control system comprising:

• first, second and third coaxial valves, said valves being axially displaceable in opposite first and second directions, said second valve axially abutting said first valve and axially spaced from said third valve;
• stop means for limiting axial movement of said third valve in said first direction;
• means for introducing governor fluid pressure to bias said first valve in said second direction, said governor fluid pressure being directly related to the speed of said vehicle;
• first port means for introducing line pressure to bias said third valve in said first direction to said stop means;
• friction engaging means for permitting engine braking of said vehicle;
• second port means for selectively conducting line pressure to actuate said friction engaging means when said low speed range is selected, said second port means including means for applying line pressure to bias said second valve in said first direction;
• first spring means interconnecting said second and third valves for opening said second port means to accuate said friction engaging means with line pressure when force in said first direction caused by said first spring means exceeds governor pressure biasing said first valve in said second direction and for positively moving said first and second valves to their limit in said first direction;
• and
• means responsive to said throttle for supplying a detent pressure of a predetermined level to act on said first valve in a direction opposite to said governor pressure when said throttle means exceeds a predetermined level and for preventing application of said detent pressure to said first valve when engine braking is effective, said detent pressure supply means comprising a regulator valve including:
• a first input port receiving said line pressure;
• an output port generating said detent pressure;
• a first spool having first and second lands defining a first chamber in continuous communication with said output port and with a second chamber by means of an oil path through said first land;
• a drain port;
• a spring acting on said second land in substantially equal opposition to fluid pressure in said first chamber for oscillating said first spool to alternatively connect said first input port and said drain port with said first chamber to generate the predetermined level of detent pressure;
• a second spool coaxial with said first spool and disposed in said second chamber;
• and
• a second input port communicating with said second chamber and receiving said line pressure when said low speed range is selected for forcing said first and second spools against said spring to prevent generation of detent pressure at said outlet port.