Hydraulic control system for automatic transmission

A hydraulic control system for an automatic transmission, in which hydraulic pressure generated in an oil pump is controlled to a predetermined level by a regulator valve, and part of this pressure is supplied as operational pressure to a damper clutch and to a reducing valve, and another part of this pressure is supplied to a hydraulic pressure control section for a selective supply of operational pressure to friction elements that are engaged in each shift range and speed, the hydraulic pressure control section including a shifting control assembly, a hydraulic pressure control assembly, a switching assembly, a fail-safe assembly, and an N-R control assembly,

• wherein the shifting control assembly includes a manual valve, which is indexed with a driver-manipulated select lever to realize line conversion;
• wherein the hydraulic pressure control assembly includes a first pressure control valve and a first solenoid valve controlling N-D range pressure for supply to a friction element operating in third and fourth speeds, a second pressure control valve and a second solenoid valve controlling D range pressure for supply to a friction element operating in second and fourth speeds, and a third pressure control valve and a third solenoid valve controlling D range pressure for control to a friction element operating in first, second, and third speeds;
• wherein the switching assembly includes a switch valve effecting port conversion by line pressure supplied to both ends thereof and by hydraulic pressure supplied to a fourth solenoid valve, and selectively supplying hydraulic pressure supplied to the first pressure control valve to a friction element operating as a fixing element in neutral N, park P, and reverse R ranges, and to a friction element operating as an input element in third and fourth speeds;
• wherein the fail-safe assembly includes a first fail-safe valve controlled by line pressure, operational pressure of a friction element operating as an input element in third and fourth speeds, and operational pressure of a friction element operating as a fixed element in second and fourth speeds, and selectively supplying hydraulic pressure supplied to the switch valve and reverse pressure supplied from the manual valve to a friction element operating as a fixed element in the neutral N, park P, and reverse R ranges, and includes a second fail-safe valve controlled by N-D range pressure, R range pressure, operational pressure of a friction element operating as an input element in first, second, and third speeds, and operational pressure of a friction element operating as an input element in third and fourth speeds, and selectively supplying control pressure of the second pressure control valve to a friction element operating as a fixed element in second and fourth speeds;
• and
• wherein N-R control assembly includes an N-R control valve controlled by control pressure supplied from the second solenoid valve in the reverse R range, and supplying hydraulic pressure supplied from the manual valve to a friction element operating as an input element in the reverse R range.