Oil pressure control means for an automatic transmission

An oil pressure control means for an automatic transmission having a gear mechanism and low and high speed friction engaging means and adapted to provide alternatively two speed shift stages by changing over engagement of said two friction engaging means, comprising:

• a source of line pressure;
• a shift valve adapted to be shifted between a first shift position where it provides a passage for connecting said low speed friction engaging means to said source of line pressure while it provides a drain passage for said high speed friction engaging means and a second shift position where it provides a passage for connecting said high speed friction engaging means to said source of line pressure while it provides a drain passage for said low speed friction engaging means;
• low and high speed accumulators connected individually to the passages for supplying the line pressure to said low and high speed friction engaging means;
• a first control circuit means controlling by the oil pressure in said low speed friction engaging means so as to reduce the oil pressure in said high speed friction engaging means down to a level which causes slippage of said high speed friction engaging means while the oil pressure in said low speed friction engaging means rises but is still maintained at a level which permits slippage of said low speed friction engaging means;
• and a second control circuit means controlled by the oil pressure in said high speed friction engaging means so as to increase the oil pressure in said low speed friction engaging means up to a level which causes substantial engagement of said low speed friction engaging means when the oil pressure in said high speed friction engaging means lowers down to a level which causes substantial disengagement of said high speed friction engaging means.