Method of determining the closing time of the needle valve in a solenoid controlled fuel injector including a solenoid controlled actuator, said actuator adapted to control a needle valve by the movement of a needle to and from a valve seat of said needle valve, said method of determining the closing time of the needle valve when the valve needle contacts the valve seat during an operational cycle of said fuel injector, and subsequently controlling said injector based on said closing time comprising the steps of a) providing a reference curve/signal, Ref , wherein said reference curve/signal, Ref is indicative of the voltage across the solenoid against time of said injector or a reference injector during an operational cycle where injection does not occur; b) activating said injector by sending an activation pulse to said solenoid, and from a timepoint after the end of the activation pulse, measuring the voltage signal across the terminals of said solenoid to provide a solenoid voltage signal, Raw signal, c) determining a difference signal, Delta signal based on the difference between the signal of a step a) and that determined in step b); d) differentiating said signal of step c) to provide a first differential signal Diff1 e) from step d) providing a signal Diff1_Normalised of values, whose values decrease the further away from zero the values of Diff1 are; f) differentiating the plot/signal of step d) to provide a second derivative Diff2 of said Delta_signal g) multiplying the values of plots/signals from step f), step e) and c) to provide a further plot, Mix h) determining the maximum peak value of the plot from step g) , or a filtered version thereof, and determining the valve closing time from the time point of said determined maximum peak; i) subsequently controlling said fuel injector based on the result of step h) . a) providing a reference curve/signal, Ref , wherein said reference curve/signal, Ref is indicative of the voltage across the solenoid against time of said injector or a reference injector during an operational cycle where injection does not occur; b) activating said injector by sending an activation pulse to said solenoid, and from a timepoint after the end of the activation pulse, measuring the voltage signal across the terminals of said solenoid to provide a solenoid voltage signal, Raw signal, c) determining a difference signal, Delta signal based on the difference between the signal of a step a) and that determined in step b); d) differentiating said signal of step c) to provide a first differential signal Diff1 e) from step d) providing a signal Diff1_Normalised of values, whose values decrease the further away from zero the values of Diff1 are; f) differentiating the plot/signal of step d) to provide a second derivative Diff2 of said Delta_signal g) multiplying the values of plots/signals from step f), step e) and c) to provide a further plot, Mix h) determining the maximum peak value of the plot from step g) , or a filtered version thereof, and determining the valve closing time from the time point of said determined maximum peak; i) subsequently controlling said fuel injector based on the result of step h) . <EOS>