Fail-safe methods for utilizing an over-center pump/motor in a hydraulic hybrid vehicle are disclosed. A high-pressure fluid shutoff valve and an optional electrically or manually operated valve are additionally provided as means to ensure disconnection of the high pressure source in the event of a failure. Displacement stroke position and pressure differentials across the pump/motor are continually monitored. On detection of various modes of failure or irregularity in control of displacement, actions are taken including any of: the high pressure and low pressure accumulators are shut off automatically or manually, a check valve between the high and low pressure ports of the pump/motor is activated, and a small amount of pressurized fluid is released from the high pressure circuit to depressurize the captive fluid. Safe startup and shutdown procedures are also specified. The system provides for safe operation of a hydraulic hybrid vehicle that includes an engine pump and drive motor, at least one of which operates over-center. The system optionally provides a displacement control valve that is controlled by feedback from an actuator or displacement position sensor and that on loss of electric power to the displacement control valve will cause the controlled pump/motor to stroke to a zero displacement position.