In series of my experiments, here goes again:
I am trying to interface a series of pulses that come from car's ignition systems, and because my tachometer electrics are broken (all that is intact is the coil that runs the needle and accepts current) I would like arduino to convert the pulses to current.
Can this be done?
probably, but you will have to do calibration.....
perhaps use interrupts on the arduino....
pwm a ssr or transistor connected to the 12V....
just shooting from the hip here..... 
You don't need an arduino for this all you need is a monostable. Just use the pulses to trigger a fixed width pulse, then smooth that pulse with an R & C, then apply that voltage to your current indicating device. You might need an amplifier of some sort but you would need that with the arduino. You can make a monostable from an NE555 chip or a 74LS121 or 74LS123.