Lock-up control system for a torque converter for an automatic

Abstract

A lock-up mechanism for a torque converter for an automatic transmission is arranged to be selectively engaged to directly connect the output shaft and the input shaft of the torque converter when the operating condition of the engine is in a lock-up zone. The operating condition of the engine is represented by a rotational speed signal representing the rpm of the input shaft or the output shaft of the torque converter, and an engine load signal, and the lock-up zone is defined by first to third relations, the first relation being preset from a relation which the rotational speed and the engine load are to have with each other when the torque ratio of the engine output torque to the torque converter output torque can be considered to be 1, the second relation being preset from a relation which the rotational speed and the engine load are to have with each other when the engine output may be considered to be zero, and the third relation being preset from a lower limit of the rotational speed at which lock-up can be permitted without possibility of engine stall. The lower limit is shifted toward the lower speed side of the rotational speed as the transmission stage of the transmission gear mechanism is shifted toward the higher speed stage, and the third relation is changed corresponding to the change in the lower limit of the rotational speed.