Downshift timing and engine brake control for automatic transmission

Claim 1

A control system for an automatic transmission for an automotive vehicle having an engine operable with varying load, the automatic transmission having various friction elements including an engine brake effecting friction element, the control system comprising: a fluid pressure source; means communicating with said fluid pressure source for generating a load indicative fluid pressure which is variable with the load; a first valve, when rendered operable, controlling application of hydraulic fluid pressure to the engine brake effecting friction element; a second valve when rendered operable selectively effecting a downshift timing control; means responsive to said load indicative fluid pressure for rendering operable said first valve and said second valve, selectively; said load indicative fluid pressure responsive means including: an electro-hydraulic transducer operable in a first mode or a second mode; a shuttle valve shiftable in response to said load indicative fluid pressure between a first position thereof where it permits said electro-hydraulic transducer to control operation of said first valve and a second position thereof where it permits said electro-hydraulic transducer to control said second valve, said shuttle valve being shiftable between said first position and said second position around a predetermined magnitude of load; means for generating a load indicative signal variable with the load; means responsive to said load indicative signal for controlling said electro-hydraulic transducer such that said electro-hydraulic transducer is held in a predetermined state thereof when said load indicative signal represents the magnitude of the load falling in a predetermined range extending across said predetermined magnitude of the load, said electro-hydraulic transducer operates in said first mode when said load indicative signal represents the magnitude of the load less than any load falling in said predetermined range, and said electrohydraulic transducer operates in said second mode when said load indicative signal represents the magnitude of the load greater than any load falling in said predetermined range, said predetermined state of said electro-hydraulic transducer being such that when said shuttle valve assumes said first position, said first valve is caused to deactivate the engine brake effecting friction element, while when said shuttle valve assumes said second position, said second valve is caused to effect the downshift timing control.