Hello !
I'm new here.
I ever wanted to build a monitor display for my car and luckily i found this website !! nice !!
Maybe you know this chip : ELM323, made by www.elmelectronics.com
This appear to be a chip which connect directly to the diagnostic link connector and offer an rs232 interface on the other site, for communication with pc.
The datasheet say :
"The ELM323 is a 14 pin integradet circuit that, with only a few external components, is able to convert between the OBD data format and the standard RS232 serial data format. This allows virtually any personal computer or PDA to communicate with a vehicle using only a standard serial port and a terminal program. If desired, hobbysts can even create their own custom scan tool by adding an interface program."
I don't know now if this is maybe what you searching for, but as i want only a monitoring tool for temperatures, speed, rpm, or else, i think this chip is what i need for my purposes.
I got the service manual for my car, where you can find the significate of vary PID codes an modes, this is a good start.
Now i will order a pair of this chips, build the simple circuit which comes with the datasheet and then i post my results for who is interested.
Sorry for my poor english, i speak italian
David