A 4-speed automatic transmission with electronic-hydraulic control is known from two publications in ATZ 85 (1983), 6 pages 393 to 405. From paragraph 4, Transmission Control (page 402) and paragraph 5, Safety Functions, as well as from FIGS. 6 and 7 of these prior art publications, it is known that in a currentless state, the vehicle can be driven purely hydraulically, corresponding to the selection slide valve shift, either forward in the 3rd speed or in reverse, whereby a neutral position (idling setting) is also possible.
In the publication 4 HP 22 E of the ZF GETRIEBE GMBH SAARBRUCKEN, Oil Circuit Diagrams, this electronic-hydraulic transmission is illustrated in detail and completes FIG. 6. This system is possible because it controls an overdrive transmission, wherein the 4th speed is a high-speed gear--overdrive- and the engine does not reach the highest rotational speed. However, such a control is not appropriate for a 4-speed transmission with a direct 4th speed or an overdrive transmission wherein the highest rotational speed is reached in the highest gear, because a shifting down from the 4th to the 3rd speed in case of electric-current supply failure leads to excessively high rotational speeds in the engine and could result in damage to the engine.
USPTO Backgrounds
USPTO Backgrounds