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摘要

A system for controlling the selection of the operative ratio of a vehicle powershift transmission (P) driven from an engine (E) via a main clutch (C) is described. The system includes a selector means (15), a clutch status sensor (18) for sensing engaged and disengaged conditions of the main clutch (C) and control means (CS) which include a ratio-change modulating function. The ratio-change modulating function operates essentially in two modes. When the main clutch is engaged and a change is selected to a non-adjacent ratio it invokes a serial routine. This ensures that each intermediate ratio between the current and selected ratios is momentarily engaged to ensure a smooth substantially jerk-free progression to the selected ratio. When the main clutch is disengaged and a change is selected to a non-adjacent ratio it invokes a direct routine. This allows a change from the current to the selected ratio without engagement of any intermediate ratio.