Automatic transmission upshift control apparatus

Abstract

An upshift is effected by releasing an engaged first friction element and engaging a released second friction element. The first fiction element operates on a fluid pressure applied thereto through a first passage and the second friction element operates on a fluid pressure applied thereto through a second passage. An accumulator is provided in the first fluid passage. The accumulator has a back pressure port through which a back pressure is applied thereto by the function of a control valve having a first port connected to the back pressure port, a second port connected to a source of back pressure and a third port connected to a drain. The control valve operates, between a first position connecting the first port to second port and a second position connecting the first port to the third port, on first and second working signal pressures applied thereto. The first working signal pressure is the fluid pressure applied through the second passage to the second friction element and the second working signal pressure corresponds to the engine load. The control valve changes from the first position to the second position when a pressure difference between the first and second working signal pressures reaches a predetermined value. The application of the first working signal pressure to the control valve is inhibited for a predetermined time after an upshift is commanded in the automatic transmission or until the existing gear ratio reaches the gear ratio provided when the upshift is commanded in the automatic transmission.