The paper presents the principles of construction and the operational features of a vibration type viscosity sensor with an extended measuring range. A measuring transducer (MT) is a two-freedom degree oscillatory system consisting of a vibrator and a piezoelectric transformer (PET) interacting with each other. The vibrator here is a sensitive element (SE), while PET is used to excite mechanical vibrations in the system and generate an electrical output signal of the sensor. The paper provides the results of mathematical description and simulations of two possible modes of the sensor operations. Strongly coupled oscillations excited in the system at normal synchronization frequencies (NFS) can be used to measure low-viscosity media. The weak coupling mode at the partial synchronization frequency (PSF) enables the wide range viscosity measurement. Installation of an additional PET on the vibrator helps reduce the influence of destabilizing factors and improve the viscosity measurement accuracy due to the measurements of the amplitude ratio of coupled oscillations as the output signal.