Safety circuit for electronically controlled automatic vehicle

Abstract

A comparator circuit compares an engine speed signal with an engine load signal, the latter being furnished to the comparator through transistors controlled by switching signals representative of the condition of the gear range selection lever, the kickdown switch and the magnetic valve that operates, when actuated, to shift from high gear to a lower gear. The circuit is operative only when the kickdown switch is unoperated and the gear range selection lever is in the 'drive' position that permits operation in high gear. The magnetic valve position indication is coupled to the circuit through a differentiating network, so that the circuit comes into play only for a short interval following the switching over of the magnetic valve from the high gear position. If at that time the ratio of engine speed to engine load, instead of being consistent with the automatic transmission shift program, is in the range where a downshift would involve risk to the safety of the vehicle, the circuit operates to cut off the current supply to the magnetic valve, forcing it to return into the high gear condition. Operation of the magnetic valve as the result of a failure of a component in the electronic control of the transmission or some other accidental disturbance is thereby prevented from endangering the vehicle.