Hi everyone!
I've decided to take a break from the OBDI reader for my Toyota Camry and come up with a similar concept. This time, I'm focusing on the Camry's all-original Aisin A140E transmission...
If you have a pre-OBDII Toyota with an automatic transmission (sorry, no manual transmission compatibility...yet... 
), chances are that you have a pin on the DLC1 and/or 2 box marked "Tt." Guess what? This is the transmission terminal! It spits out voltage ranges based on what gear the transmission is in!
In taking a look on the TurboNinjas Camry repair manual page under the Automatic Transaxle manual (located here: http://www.turboninjas.com/camry/ax1, for the A140E transmission, /ax2 for the A540E transmissions in the V6 models), it lists out the exact voltage ranges where shifting and lock-ups occur.
The maximum voltage output of this is 8VDC. The Arduino Nano can only handle up to 5V. To overcome this, PLEASE use a voltage divider. If not, you'll release the magic smoke demons, and it's a sad day when that happens.
This is a great tool for diagnosing my transmission, getting real-time data and gear positions. Again, if your car is Toyota OBDI compliant and has a Tt terminal, this will work. It's easy to build and code, and who doesn't like blinky LED lights in their car???
Let me know what you think about this! Also, if you have any mods or add-ons to this creation of mine, let me know! I know it's a very simplistic and basic concept, but I believe it's best to start simple. I'm always looking for areas to improve upon as I continue to learn about my car's pre-OBDII ECU system and Arduino, and as I continue on, I find out a lot more every day!
As of right now, my code is up on GitHub. This is just a starting point and it is currently in beta testing. You can check it out here!