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?
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.
Yes, it is possible. The 12v signals would have to be conditioned for the Arduino to receive them (0 - 12v or maybe 0 - 12-14.5v?). Then the Bluetooth side would be the same as any other Bluetooth data transmission.
So your question comes down to: Is this a fun project that's worth the effort you put into it for the learning experience? Or is this meant to answer a need, and the most practical means to that end is the best means?
Ex: I wanted to know how long / what kind of driving would it take to get my car's oil to full temp (~100C), starting from dead-cold in the winter? I had a thermo-logger I'd built for another experiment, so I taped a sensor to my oil filter (yeah, not the greatest, but quick and dirty) and lead a cable outside the hood, through the driver's window, to the experimental PC board on my dash-board. It wasn't a miserable failure but nothing to be proud of, either.
The solution I settled on was a USD$12.00 OBD port sender and an iPhone app. The results were much better, and while I didn't have the built-it-myself experience, I got the data and the answers I needed.