Q:

Would it be feasible to build an arduino that takes 12v on/off signals from a vehicle diagnostic port (1984 pre-OBD)?

The vehicle is an AMC eagle, they come with a diagnostic port 
I have made a diagnostic tool that lights led lights when signals turn on and off, but it has to be wired in and is bulky. I am wondering if it would be possible to have an arduino plugged in and running off the 12v battery supply (with step-down to 5v) and have it catch the signals and send them over Bluetooth to a computer. Does this sound like it is within the scope of an arduino or am I asking too much? Or is there a better and easier way to do this? 

A:

You can step down 12V to 5V with resistor dividers, or drive transistor base with 12V and let the collector pulldown an input with pullup resistor to signal an active signal (like an optocoupler, but without the slow optical part).
Arduino reading the inputs and sending via bluetooth to something else is pretty straightforward, perhaps send only when a change from 0 to 1, and 1 to 0, on any input is sensed. Check Adafruit.com for bluetooth modules that are easy to use.

