Transmission downshift swap-shift control
An automatic transmission for a vehicle includes a main box controlled using a series of clutches, and an underdrive assembly controlled using a series of clutches and at least one overrunning clutch. In achieving a double-swap downshift from a first transmission “gear” employing a first main box clutch and a first underdrive assembly clutch, to a second transmission “gear” employing a second main box clutch and a second underdrive assembly clutch, a controller delays release of the first main box clutch until after the second underdrive assembly clutch has been prefilled, and fully applies the second underdrive assembly clutch only after slip has been detected in both the main box and the underdrive assembly. Additionally, the controller fully applies the second main box clutch only after the second underdrive assembly clutch has itself been fully applied.
1. A method for downshifting an automatic transmission from a first transmission speed ration characterized by engagement of one clutch of a first gearset and one clutch of a second gearset, the one clutch of the second gearset being an overrunning clutch, to a second transmission speed ratio characterized by engagement of another clutch of the first gearset and another clutch of the second gearset, the method comprising, in series, the steps of:
(a) partially releasing the one clutch of the gearset until the one clutch of the first gearset carries a slightly greater torque than necessary to couple the input shaft to the output shaft through the selected fears of the first gearset;
(b) further releasing the one clutch of the first gearset until the one clutch of the first gearset begins to slip;
(c) partially applying the other clutch of the second gearset until the one clutch of the second gearset begins to slip;
(d) maintaining the partial application of the other clutch of the second gearset until a target ratio of the second gearset is achieved;
(e) fully applying the other clutch of the second gearset; and
(f) fully applying the other clutch of the first gearset and fully releasing the one clutch of the first gearset.
2. The method of claim 1 , further including partially reapplying the one clutch of the first gearset between steps (e) and (f).
3. The method of claim 1 , further including preapplying the other clutch of the second gearset before step (c).
4. The method of claim 3 , wherein the other clutch of the second gearset is preapplied before step (a).
5. The method of claim 1 , further including preapplying to other clutch of the second gearset before step (f).
6. The method of claim 5 , wherein the other clutch of the second gearset is preapplied before step (b).