You need to know the voltage of the square wave signal that is being produced.
To be safe you can connect it up to a zener diode, this will make sure that the voltage never exceeds the voltage of the zener breakdown voltage. I use 5.1V Zener diodes. I have attached an example circuit that you could use.
The first diode should be a fast switching diode like a 1N4148 to stop any negative voltages. The Zener can be a 5.1V Zener diode of the correct power rating. You might also need a 100nF capacitor to filter out any noise that the engine on the dyno may produce.
Connect the output of your sensor to the 10k resistor, connect the other end to an analog input of the arduino, you can then use the serial plotter to confirm that you are seeing a square wave signal. If you are then you can connect it to an interrupt pin and go from there.
