Telemetry: Hacking the ELM327 Scanner

The first thing that I decided to do for this project was to hack the ELM327 scanner to get serial transmission directly from the board and not the USB that was attached to the scanner. I know I wanted to eventually use the HC-05 Bluetooth modules which talk over UART, so I needed a way to access the data signals from the ELM327. I ended up opening the ELM327 scanner by removing the 4 screws that are WELL hidden underneath the main label where the LEDs are located.
 photo OpeningElm327_1.jpg

I forgot to take a picture at this point right after opening the scanner, so I borrowed these pictures from google. After opening the ELM327 you will see a chip layout similar to below. My ELM scanner is actually a clone and uses the PIC18f258 chip as you can see in the middle.
 photo OpeningElm327_3-2.jpg

To access the data lines from the ELM327, I looked at the schematic for the ELM327 chip and got the following (color references are from the picture above)
Pin 17 (Purple) —> RS232 Tx
Pin 18 (Yellow) —> RS232 Rx
Pin 19 (Black) —> GND
Pin 20 (Red) —> Vcc (5v)
 photo ELM_CHIP.jpg

I soldered 4 wires to the 4 pins above and cut off the USB cable. I am going to add header pins to the location where the USB cables were connected, that way I can always reconnect the USB if need be. I then connected the Tx and Rx lines to the Rx and Tx respectively of the USB to UART module to test communications.
 photo ConnectTxRx.jpg

I went out to the car, plugged the OBDII into the car, and connected the USB to UART into my laptop. I connected to the serial COM port on my laptop and was able to send AT commands to the OBDII interface through the serial lines I soldered. This was a huge step because now I know that you can communicate through these lines. I can then focus on getting a Bluetooth Connection working. You could possibly buy an ELM scanner that has Bluetooth with it, but I wanted to pair two Bluetooth modules instead of hoping that I could pair to the given module on the ELM327. This is why I did not buy another scanner and decided to hack mine. It did not cost me anything to add the two wires to the scanner and enable serial transmission. There is a product out there that they already soldered the wires for you, but I think it is around $40. Considering the scanner is about $10 from ebay, I think it’s worthwhile to add the wires yourself. I learned a lot about the ELM MCU in the datasheet, and it seems like it will be invaluable when I go to program how to communicate with the OBDII later on.

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