A vehicle powertrain includes an engine driven variable displacement pump and an accumulator. A switch network controls the flow of fluid among the pump, the accumulator, a line pressure circuit of a valve body, and the sump, enabling several operating modes. In a balanced operating mode, no fluid flows into or out of the accumulator. In a discharging mode, fluid flows from the accumulator reducing the power required by the pump to increase output torque or reduce fuel consumption. In a charging mode, fluid flows into the accumulator for later use. Fluid also flow into the accumulator in a high-torque mode in which the pump and switch network are set to intentionally increase the pump torque to assist in deceleration.