Interlock preventive device for automatic power transmission

A hydraulic control system for an automatic power transmission which is shiftable between first, second and third forward drive gear ratios, the transmission including fluid operated first, second and third frictional units for establishing said first, second and third gear ratios, respectively, the hydraulic control system comprising:

• a selector valve;
• a first gear shift valve for shifting between the first and second gear ratios;
• a second gear shift valve for shifting between the second and third gear ratios;
• a first fluid passageway leading from said selector valve to the first frictional unit;
• a second fluid passageway leading from said selector valve to the second frictional unit;
• a third fluid passageway leading from said second gear shift valve to the third frictional unit;
• said second passageway including a first branch leading to said first gear shift valve and a second branch leading to said second gear shift valve, whereby said first and second gear shift valves are fluidly coupled in parallel with respect to said selector valve;
• means for preventing interlock of the transmission due to sticking of said second gear shift valve in a condition establishing the third gear ratio under conditions also tending to establish a gear ratio other than said third gear ratio;
• said interlock preventing means including:
• an interlock valve disposed in said first passageway, said interlock valve being shiftable between a first position allowing a fluid pressure to pass therethrough toward the first frictional unit and a second condition preventing fluid from passing therethrough;
• and
• a control passageway operatively disposed between said interlock valve and said third passageway and effecting communication therebetween, said interlock valve including means for shifting said interlock valve into said second condition thereof in response to a fluid pressure in said control passageway.