Diagnostic device for sfhit control actuator

A diagnostic device for a shift control actuator of a continuously variable transmission, comprising:

• a variator for steplessly changing a shift ratio from a predetermined maximum shift ratio to a predetermined minimum shift ratio;
• a chamber for decreasing the shift ratio of the variator by being filled with hydraulic fluid, and increasing the shift ratio of the variator by discharging the hydraulic fluid;
• an upshift control valve that switches between a filling state where the hydraulic fluid supplied from an oil pump is filled into the chamber and a non-filling state where the filling of the hydraulic fluid into the chamber is suspended;
• a downshift control valve that switches between a discharging state where the hydraulic fluid is discharged from the chamber and a non-discharging state where the discharge of the hydraulic fluid from the chamber is suspended;
• a fail-safe valve for suspending the discharge of the hydraulic fluid from the chamber when the upshift control valve is in the filling state regardless of the state of the downshift control valve, and permitting the downshift control valve to discharge the hydraulic fluid when the upshift control valve is in the non-filling state;
• an upshift control actuator for driving the upshift control valve to switch between the filling state and the non-filling state;
• a downshift control actuator for driving the downshift control valve to switch between the discharging state and the non-discharging state;
• an actual shift ratio detector for detecting an actual shift ratio of the variator;
• and
• a shift controller for controlling the upshift control actuator and the downshift control actuator so that the actual shift ratio approaches the target shift ratio,
• wherein, when the shift controller instructs the downshift control actuator so that the downshift control valve becomes in the discharging state, and a state where the actual shift ratio is substantially kept at the minimum shift ratio continues for a predetermined period of time or more, the shift controller determines that the upshift control actuator is kept in the filling state.