Hi Chris, what type of sensor is the crank sensor?
Passive or active?
I think it will be Passive(inductive), if so disconnect the sensor from the loom when the vehicle is in a non start condition and carry out a resistance test across the sensor and to earth, check the reading against specs, if good then scope just the sensor whilst cranking the engine. you should get an AC waveform, if so check the peak to peak voltage of the signal. HTH

Regards Mark