Transmission control system with protection against excessively high speed downshifting

Claim 1

For an automatic transmission for an engine driven vehicle, comprising a gear transmission mechanism comprising a plurality of fluid pressure actuated friction engaging mechanisms, selective supply of fluid pressure to said friction engaging mechanisms selectively providing a plurality of speed stages from said gear transmission mechanism: a hydraulic fluid pressure control system, comprising: (a) a source of hydraulic fluid pressure; (b) a line fluid pressure regulation valve, which is supplied with the fluid pressure from said source of hydraulic fluid pressure and a booster pressure to produce a controlled line fluid pressure which increases as said booster pressure increases; (c) a manually operated selector valve, which is selectable between a plurality of positions denoting speed ranges including a "D" range and at least one other forward speed range in which the highest gear speed stage available is lower than the highest gear speed stage available in said "D" range, which receives a supply of said line fluid pressure, which produces as output a lower range fluid pressure substantially equal in pressure value to said line fluid pressure when said other forward speed range is selected thereon, and which does not produce as output said lower range fluid pressure when said "D" range is selected thereon; (d) a throttle fluid pressure control valve, which receives a supply of said line fluid pressure, and which produces therefrom a throttle fluid pressure whose value varies according to a quantity representative of engine load; (e) a governor fluid pressure control valve, which receives a supply of said line fluid pressure, and which produces therefrom a governor fluid pressure whose value varies according to road speed; (f) a modulator fluid pressure control valve which receives a supply, when available, of said lower range fluid pressure from said manually operated selector valve, and which produces therefrom a modulator fluid pressure by regulating said line fluid pressure to a fixed pressure level; and (g) a shift valve, comprising a valve element, which receives supply of said throttle fluid pressure and said modulator fluid pressure, if available, which act on said valve element so as to bias it in a first direction, which further receives supply of said governor fluid pressure which acts on said valve element so as to bias it in a direction opposite to said first direction, and in which said valve element is switched to and fro in said first and opposite directions according to a balance relationship between said throttle fluid pressure and said governor fluid pressure, said shift valve controlling supply of actuating fluid pressure to some of said fluid pressure actuated friction engaging mechanisms so as to provide the highest speed stage available in said other forward speed range when said valve element is in its extreme position in said first direction; (h) a shuttle valve which supplies either of said throttle fluid pressure and said modulator fluid pressure, if available, whichever is higher than the other, as said booster pressure to said line fluid pressure regulation valve.