Control system for power transmission device for vehicle

A control system for a power transmission device for a vehicle, the power transmission device including a fluid coupling, the fluid coupling including a lockup clutch, an input member and an output member, the fluid coupling being configured such that the input member and the output member are directly coupled to each other when the lockup clutch is engaged, the control system comprising:

• an electronic control unit configured to
• i) execute engagement control for controlling an engaging pressure of the lockup clutch such that the lockup clutch is half-engaged,
• ii) calculate an integrated amount of heat that is generated in the lockup clutch during the engagement control,
• iii) when the integrated amount of heat generated becomes larger than or equal to a predetermined determination value, estimate a first amount of heat that is generated in the lockup clutch in a period from when the integrated amount of heat generated becomes larger than or equal to the predetermined determination value after a start of the engagement control to when the lockup clutch is completely engaged,
• iv) when the integrated amount of heat generated becomes larger than or equal to the predetermined determination value, estimate a second amount of heat that is generated in the lockup clutch in a period from when the integrated amount of heat generated becomes larger than or equal to the predetermined determination value after the start of the engagement control to when the lockup clutch is released,
• v) when the first amount of heat generated is smaller than or equal to the second amount of heat generated, execute first control for completely engaging the lockup clutch from a state of the engagement control, and
• vi) when the second amount of heat generated is smaller than the first amount of heat generated, execute second control for releasing the lockup clutch from the state of the engagement control.