One embodiment is a system comprising an engine structured to output torque to an accessory drive, a rotary load structured to be selectably driven by the accessory drive, and an electronic control system. The electronic control system is operable to selectably engage and disengage the rotary load effective to vary the load on the engine, monitor engine load values in coordination with engagement of the rotary load, store a data set comprising the monitored engine load values in association with values of one or more associated system conditions in a non-transitory memory medium, update a mathematical model of the system stored in the non-transitory memory medium in response to the engine load values to converge one or more model parameters, diagnose or prognosticate a failure state of the rotary load in response to a change in the one or more model parameters, and output a perceptible diagnostic indication of the failure state in response to the diagnosis or prognostication.