Schematics coming soon...
To interface the car's electrical system (12-15 volts) with the parallel
port (~5 volts very low current) I connected a transistor between each car
sensor and parallel port pin. For my fuel injector I used a PNP switching
transistor as I want to measure the amount of time that the injector line
is low. The speed sensor line is open most of the time but it sends
positive pulses which vary with the vehicle's speed. I therefore had
to use an NPN transistor. The type of signal sent will vary from vehicle
to vehicle and this information is only known to be valid for 1991 Jeep Cherokees
(it should be correct for other year cherokees as well).