A transmission having selectively engaged fluid actuated friction drive establishing devices and an engagement timing control system having a trimmer regulator valve for each drive establishing device, a trimmer bias regulator valve and accumulator. The oncoming trimmer valve has a spring biasing device responsive to engagement pressure to increase oncoming ratio drive engagement pressure at a controlled rate, initially at a high rate up to a minimum slack take up pressure, then to gradually increase the drive pressure from the minimum pressure at a low rate in a predetermined trimming time period for establishing the drive and then to increase the pressure at a very high rate to high line pressure. A trimmer bias system pressure is supplied to a bias chamber of the trimmer valves and acts in the oncoming trimmer valve bias chamber to increase the pressure level of the gradually increasing drive engagement pressure proportional to torque demand. A limited volume flow trimmer bias supply pressure is supplied to the trimmer bias system having an accumulator and the trimmer valve bias chambers which change volume during a shift relative to the limited volume supply flow to control the trimmer bias system pressure and the resulting drive ratio pressure so the engagement pressure is reduced to the minimum at the beginning of the trimmer time period at all torque demand values and the increase with torque demand is delayed and gradual for smooth shifting.
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