Hydraulic control circuit for automatic transmission of vehicle

A hydraulic control circuit for an automatic transmission of a vehicle, the automatic transmission including a plurality of hydraulically operated coupling devices at least one of which is selectively operated to establish a corresponding one of a plurality of speed steps, the hydraulic control circuit comprising:

• a plurality of electromagnetic valve devices which output respective hydraulic pressures to operate the hydraulically operated coupling devices, respectively,
• the plurality of electromagnetic valve devices comprising a first electromagnetic valve device which outputs the hydraulic pressure to establish at least one low or intermediate speed step of the plurality of speed steps that includes a lowest speed step of the plurality of speed steps, and a second electromagnetic valve device which outputs the hydraulic pressure to establish at least one high speed step of the plurality of speed steps that includes a highest speed step of the plurality of speed steps;
• a first switching valve which is switched, based on the hydraulic pressure supplied from the first electromagnetic valve device, to a first operation state to output a source hydraulic pressure to a first hydraulic passage, and is switched, based on the hydraulic pressure supplied from the second electromagnetic valve device, to a second operation state to output the source hydraulic pressure to a second hydraulic passage, and which is kept in one of the first and second operation states that is an operation state thereof at a time when the first and second electromagnetic valve devices become unable to output, because of respective fails thereof, the respective hydraulic pressures;
• and
• a second switching valve which detects an all-fail state of the electromagnetic valve devices in which none of the electromagnetic valve devices output the respective hydraulic pressures, by detecting a state in which none of (a) one of the first and second electromagnetic valve devices and (b) other electromagnetic valve devices of the plurality of electromagnetic valve devices than the first and second electromagnetic valve devices output the respective hydraulic pressures, and which is switched, upon detection of the all-fail state, to an operation state to supply the source hydraulic pressure to a first hydraulically operated coupling device of the plurality of hydraulically operated coupling devices, and supply the hydraulic pressure supplied from one of the first and second hydraulic passages that corresponds to said one of the first and second operation states in which the first switching valve is kept, to a corresponding one of a second and a third hydraulically operated coupling device of the plurality of hydraulically operated coupling devices.