A vehicle includes a transmission having a fluid pump, a high pressure fluid circuit, a low pressure fluid circuit, and an accumulator. The vehicle also includes a controller that is programmed to, in response fluid demand exceeding a capacity of the pump, isolate the low pressure circuit from the pump. The controller is also programmed to, in response to vehicle speed exceeding a threshold while the low the pressure circuit is isolated from the pump, discharge the accumulator into the high pressure circuit.