That then is your input.
It is either GND, else (probably) +5V or +12V when "open" and being pulled up by whatever it connects to (an ECU or ignitor).
Hence sample it thru a resistor (maybe 1k?) and a reverse-biased diode to the Arduino's +ve supply (+5V) to prevent voltages exceeding +5V (plus the diode drop => ~5.6V) to a suitable Arduino input.
It hence pulses between 0V (GND) and +5V.