US Patent 7258212 Oil pressure control device for automatic transmission

In an automatic transmission having multiple oil pressure control valves for controlling friction engagement elements, a first and second pressure modulating valves are provided between an oil pressure generating means (pump) and the oil pressure control valves. A switching valve is further provided between the pressure modulating valves and the oil pressure control valves. In case that one of the pressure modulating valves becomes out of order, the oil pressure of a working oil is supplied to the oil pressure control valves from the other pressure modulating valve, so that the automatic transmission can be continuously operated even in the case that one of the pressure modulating valves becomes out of order.