Failsafe control system for automatic transmission

A control system for an automatic transmission in a vehicle including a fluid transmission unit for receiving rotation from an engine and a speed change unit with an input clutch which is applied when a forward running range is selected and which has an input side for receipt of rotational input from the fluid transmission unit and an output side for rotational output to the speed change unit, said control system comprising:

• first R.P.M. detection means for detecting R.P.M. of the clutch input side;
• second R.P.M. detection means for detecting R.P.M. of the clutch output side;
• stop state detection means for detecting if the vehicle is at a standstill;
• starting operation detection means for detecting a starting operation by a driver for starting the vehicle in motion;
• a hydraulic control unit for establishing any one of a plurality of gear stages in said speed change unit, said hydraulic control unit including:
• a hydraulic servo for applying/releasing the input clutch;
• a source of oil pressure and a primary valve for regulating the oil pressure from said source to generate a forward running range pressure;
• a control signal oil pressure generating solenoid valve for generating a control signal oil pressure;
• a control valve for regulating the forward running range pressure according to the control signal oil pressure to generate a control oil pressure;
• a change signal oil pressure generating solenoid valve for generating a change signal oil pressure;
• and
• a change-over valve switchable by said change signal oil pressure between a first position wherein said changeover valve feeds the forward running range pressure to said hydraulic servo and a second position wherein said change-over valve feeds the control oil pressure to said hydraulic servo;
• and
• a control unit including:
• shift output generating means for generating a shift output to achieve one of said plurality of gear stages;
• pressure reducing means for outputting, when standstill is detected by said stop state detection means, a change signal to the solenoid of said change signal oil pressure generating solenoid valve, to bring said change-over valve into the second position, and a control signal to the solenoid of said control signal oil pressure generating solenoid valve to reduce the control oil pressure and to thereby bring said input clutch into a predetermined state of release;
• pressure boosting means for outputting, when a starting operation is detected by said starting operation detection means, a control signal to the solenoid of said control signal oil pressure generating solenoid valve to boost the control oil pressure and to thereby bring said input clutch into a predetermined applied state, and for outputting, when said input clutch comes into a predetermined applied state, a change signal to the solenoid of said change signal oil pressure generating solenoid valve to bring said change-over valve into the first position;
• fail decision means for deciding a failure if the clutch input/output R.P.M. difference between the R.P.M. of the input side and the R.P.M. of the output side of said input clutch is over a set value which is set on the basis of the clutch input/output R.P.M. difference when the starting operation is detected by said starting operation detection means;
• and
• fail-safe means for causing said shift output generating means to generate a shift output to a higher speed gear stage, over-riding any control signal from said pressure boosting means, when a failure is decided by said fail decision means, and for outputting a change signal to the solenoid of said control signal oil pressure solenoid valve to thereby bring said change-over valve into the first position when a failure is decided by said fail decision means.