A powertrain system includes an electric motor/generator unit. Upon detecting an open high-voltage switch associated with a high-voltage DC electrical bus, low-voltage electrical power is employed to energize the high-voltage DC electrical bus, control parameters are adjusted to operate the electric motor/generator unit in a fault tolerant electric generation mode, an internal combustion engine is operated to spin the electric motor/generator unit, and the electric motor/generator unit is operated in the fault tolerant electric generation mode.