Apparatus for controlling actuating fluid pressure on vehicular friction clutch with compensation for decrease in friction coefficient at elevated temperature

In a motor vehicle having an engine and a fuel supply line for supplying fuel thereto, a throttle valve in the fuel supply line for the vehicular engine, a driving shaft driven from the engine, a clutch of the friction type including a clutch input member mounted on said driving shaft and a clutch output member with friction faces, a driven shaft connected to the clutch output member and a clutch actuating mechanism including a hydraulic cylinder to apply an actuating force to the clutch, a vacuum chamber to selectively communicate with a vacuum source and the atmosphere such that the displacement of a piston of the hydraulic cylinder is governed by the magnitude of vacuum in the vacuum chamber, a control apparatus for automatically controlling the rate of variation in said magnitude of vacuum and modulating the variation rate when the clutch has a high temperature to compensate for a decrease in the coefficient of friction of the clutch output member, the control apparatus comprising:

• a combination of a plurality of electromagnetic valves arranged to provide fluid communication of said vacuum chamber with the atmosphere selectively through a plurality of orifices respectively having different cross sectional areas;
• means defining said orifices;
• means for sensing the degree of opening of the throttle valve and producing at least one electrical signal indicating and representing in which of a plurality of ranges is the degree of opening, said ranges being defined by dividing the difference of the maximum opening from the minimum opening into a plurality of successive ranges;
• means for sensing whether the clutch output member is in contact with the clutch input member and producing an electrical signal representing and corresponding to the sensed condition;
• an electrical control circuit to excite one of said electromagnetic valves such that said vacuum chamber communicates with a first one of said orifices for a predetermined period of time when the clutch output member is brought into contact with the clutch input member and the degree of opening of the throttle valve is in the smallest range, said vacuum chamber communicating with a second one of said orifices when said one electromagnetic valve is unexcited, a difference in cross-sectional area between said first and second orifices being such that said magnitude of said vacuum varies at different rates to cause the clutch actuating force to increase at a higher rate when said vacuum chamber communicates with said first orifice;
• a temperature sensor for sensing whether the temperature of the clutch output member is above a predetermined temperature and producing an electrical signal representing and corresponding to the sensed temperature range, the temperature sensor having a reed relay, a stationary member located close to the clutch input member to which said reed relay is attached, a permanent magnet attached to the clutch input member and heat-sensitive means for varying magnetic flux density through said reed relay such that said reed relay is closed at least once per revolution of the clutch input member when the temperature of the clutch output member exceeds said predetermined temperature, means connecting said temperature sensor to said control circuit to supply an electrical signal representing the state of said reed relay, said control circuit having an r-c circuit to prolong said predetermined period of time by a predetermined length after said reed relay is closed.