Pump health monitoring

A method of monitoring the health of a positive displacement hydraulic pump characterised by comprising the steps of: establishing a flow path (24, 21a, 26, 39, 38, 17a, 33, 41, 42, 36) having a known flow number between a high pressure outlet line of the pump (11) and a low pressure line of the system; progressively changing the speed of operation of the pump; and, noting the speed of operation of the pump at a parameter responsive to the output pressure of the pump changes in response to said change in rotational speed of the pump.