This paper presents an averaged model and nonlinear observer for an on/off pneumatic actuator. The actuator is composed of two chambers and four on/off solenoid valves. The proposed averaged model has the advantage of using only one continuous input instead of four binary inputs. Based on this new model, a sliding mode observer is designed using the piston position and the pressure measurements in one of the chambers in order to estimate the piston velocity and the pressure in the other chamber. The observation errors converge towards zero in finite time and the performance of this observer is validated in closed loop on an experimental benchmark.