Abstract: A signal transducer for use in association with a fuel injection valve of an internal combustion engine to detect the moment of commencement of fuel injection. The axial position of the fuel valve needle is sensed and a proportional signal is applied to a signal differentiator which generates therefrom an output signal with a substantially steeper initial slope. This signal is easier to evaluate with respect to the onset of fuel injection than the direct transducer signal which has relatively low absolute amplitudes and whose amplitude increases only very slowly at the moment of opening of the injection valve.