Fail safe automatic transmission lock up clutch control system

Claim 1

In an automatic transmission for an engine driven vehicle comprising: (a) a gear transmission mechanism comprising a plurality of hydraulic fluid pressure activated friction engaging mechanisms, including a first friction engaging mechanism which, when engaged, provides a highest speed stage, and a second friction engaging mechanism which, when engaged, provides a next to highest speed stage, provided that the other friction engaging mechanisms are engaged or disengaged according to a particular predetermined pattern of combination of engagement and disengagement; and (b) a torque converter with a lock up clutch, comprising a first and second hydraulic fluid channel communicated with an interior of said torque converter, said lock up clutch being selectively engaged or disengaged according to whether hydraulic fluid pressure is supplied to said first hydraulic fluid channel and is released from said second hydraulic fluid channel or it is supplied to said second hydraulic fluid channel and is released from said first hydraulic fluid channel; a transmission control system comprising: (c) a source of hydraulic fluid pressure; (d) a line hydraulic fluid pressure control valve which produces a line hydraulic fluid pressure from said source; (e) a governor hydraulic fluid pressure control valve which produces a governor hydraulic fluid pressure from said line hydraulic fluid pressure, said governor hydraulic fluid pressure being representative of vehicle speed and increasing with increase of vehicle speed; (f) a converter hydraulic fluid pressure control valve which produces a converter hydraulic fluid pressure from said source; (g) a speed shift valve which switches over a supply of said line hydraulic fluid pressure alternatively to provide a brake hydraulic fluid pressure when said gear transmission is to provide said highest speed stage or to provide a clutch hydraulic fluid pressure when said gear transmission is to provide said next to highest speed stage; (h) a lock up clutch control valve comprising a housing formed with a bore, a valve spool member and a piston member coaxially fitted within said bore so as to abut against one another by their ends, and a spring which biases said valve spool member towards said piston, a first chamber being defined at one end of said piston member towards said piston, a first chamber being defined at one end of said piston member remote from said valve spool member, to which said piston member exposes a first effective pressure receiving surface, a second chamber being defined around meeting ends of said valve spool member and said piston member, to which said valve spool member exposes a second effective pressure receiving surface; and a third chamber being defined at another end of said valve spool member remote from said piston member, to which said valve spool member exposes a third effective pressure receiving surface, said third effective pressure receiving surface being so small as compared with first and second effective pressure receiving surfaces that said valve spool member is shifted toward said third chamber when either said first or second chamber is supplied with said line hydraulic fluid pressure while said third chamber is supplied with said line hydraulic fluid pressure, said lock up clutch control valve being switched over so as to supply said converter hydraulic fluid pressure to said first hydraulic fluid channel while releasing said second hydraulic fluid channel when said valve spool member is shifted toward said third chamber, and to supply said converter hydraulic fluid pressure to said second hydraulic fluid channel while releasing said first hydraulic fluid channel when said valve spool member is shifted toward said second chamber; and (i) a lock up clutch control hydraulic fluid circuit, including a least one switch-over valve which is actuated by said governor hydraulic fluid pressure and conducts said brake hydraulic fluid pressure and said clutch hydraulic fluid pressure separately to one and the other of said first and second chambers of said lock up clutch control valve when said governor hydraulic fluid pressure is higher than a predetermined value, while constantly supplying said line hydraulic fluid pressure to said third chamber of said lock up clutch control valve.