Patent ID: 8032274

Claim:
A diagnostic system for an automatic transmission including a plurality of friction devices each of which is switched between an engaged state and a released state in response to a hydraulic pressure of a hydraulic oil supplied from a hydraulic pressure source, and a plurality of solenoid valves capable of regulating the hydraulic pressures of the hydraulic oil supplied to the respective friction devices, said automatic transmission establishing a selected one of gear positions between an input shaft connected to a prime mover and an output shaft connected to driving wheels of a vehicle, by selectively setting the engaged or released state of each of the friction devices through control of energization of a corresponding one of the solenoid valves in accordance with the selected gear position, said diagnostic system being adapted to diagnose the solenoid valves of the automatic transmission so as to determine the presence of faults of the solenoid valves, the diagnostic system comprising: a diagnostic device that energizes a solenoid that constitutes each of the solenoid valves under a condition that torque that can drive the driving wheels of the vehicle is not transmitted to the output shaft of the automatic transmission, while monitoring a current that passes through the solenoid, and that determines that the solenoid valve is at fault when the current monitored does not match a current applied to the solenoid for the energization thereof, wherein the condition that torque that can drive the driving wheels of the vehicle is not transmitted to the output shaft of the automatic transmission is a condition that the hydraulic pressure of the hydraulic oil supplied from the hydraulic pressure source has such a low level that the hydraulic oil cannot actuate the friction devices irrespective of whether the solenoids are energized.