Abstract: A system for controlling the torque to be transmitted by a fluid-operated friction coupling (clutch or brake) in an automatic speed changer comprises a processor which receives readings of the rotary speeds of an input member and an output member to be interengaged by that coupling and calculates the instantaneous relative angular acceleration .alpha.=-d.vertline..DELTA.n.vertline./dt from these data, the value .alpha. being supplied to a comparator. A computer receiving the same readings determines the optimum acceleration and feeds the result to the comparator whose output actuates a governor controlling the fluid pressure supplied to the coupling.