In this paper the results of the experimental measurements at Berlin electron storage ring for synchrotron radiation (BESSY-II) are presented. Grow/damp measurements of the transverse coupled-bunch instabilities were performed using a multibunch feedback system and a synchronized time-domain bunch motion recorder. The results of these experiments are described and explained by the beam-ion instability. Simplified simulations and revised theory of the instability extended to a nonlinear regime are used for the interpretation of the results.