A control system for an automatic transmission comprises an engine brake control valve, a downshift timing valve, an electro-hydraulic transducer operable in an engine brake control first mode or a downshift timing control second mode, a shuttle valve fluidly disposed between the electro-hydraulic transducer and the engine brake control valve and the downshift timing valve, and a controller for the electro-hydraulic transducer. The electro-hydraulic transducer is controlled to be maintained in ON state thereof during engine load range where the shuttle valve is designed to shift from one to the other position, thereby to render the engine brake inoperable even though the timing control may be carried out if the shuttle valve assumes the appropriate position.