Pumped storage power plants are key components for the development of new renewable CO2-free primary energies and the security enhancement of electricity supply. They also offer interesting business opportunities in nowadays liberalized electricity market. However, the fast and frequent switching between pumping and generating modes as well as extended operations at off- design conditions poses technical challenges related to large unsteady hydrodynamic forces. In the present study, a reduced scale model of a low specific speed radial pump- turbine is investigated to identify the onset and development of flow instabilities. The focus is put on the generating mode at off-design conditions involving runaway and “S-shape”. Wall pressure measurements in the stator are performed with the help of 30 miniature piezoresistive sensors. When starting from the best efficiency point and increasing the runner speed, a significant increase of the pressure fluctuation is observed mainly in the channels between wicket gates. The spectral analysis shows a rise of a low frequency component (about 70% of the runner rotational frequency) at runaway, which further increases as the zero discharge condition is approached. Phase analysis reveals that several instability sources, at least three, rotate with runner at sub synchronous speed. Although the nature of the rotating cells could not be described, it is thought that back flow may develop in the gap between runner and distributor with an alternate switch between generating and pumping modes of runner channels. Nevertheless, rotating flow separation may also develop in the runner channels leading to their blockage.