Introduction

If you do not know, an On Board Diagnostics II (OBD II) interface is a test connector fitted to all cars manufactured since 2001, which allows the car to be diagnosed for faults.

Using some shareware software and the simple schematic below, I was able to interface to the computer on my Skoda Fabia, it should work with any Volkswagon Audi Group (VAG) car (this includes Skoda and Seat cars).

What you need

A copy of the Vag Com software from http://www.ross-tech.com/
A computer with a free serial port, I use a laptop,
The simple schematic below.

The circuit

 It is based upon the design available from http://www.planetfall.com/~jeff/obdii/, I simplified it to use just the K-line and added pinouts for the RS232 port. My unit was built on a small piece of veroboard.

Click on the thumbnail below to view the schematic.

Parts list

Part Value Device  Package Description     

C1  1uF  C2,5   C2,5-4            
D2  1N4004 1N4004  DO41-10           
J1  745781 745781  745781  Use a 9 way D-type 
OK2 ILD2  ILD2   DIL08  SIEMENS OPTO COUPLER
PAD1    2,54/0,8 2,54/0,8 THROUGH-HOLE PAD  

PAD2    2,54/0,8 2,54/0,8 THROUGH-HOLE PAD  
PAD3    2,54/0,8 2,54/0,8 THROUGH-HOLE PAD  
R2  1K   0207/7  0207/7            
R3  1.5K  0207/7  0207/7            
R5  1K   0207/7  0207/7            

R6  1K   0207/7  0207/7            
R7  1K   0207/7  0207/7            
R8  140K  0207/7  0207/7            
R9  1K   0207/7  0207/7            

T1  2N3904 2N3904  TO92-            
T2  2N3904 2N3904  TO92-  

For OK2, I used an ILD2 opto-cupler but any available optocoupler with similar specifcations could be used. Any readily available low power NPN transistors can be used. Not shown on the scehamtic but I added an LED and series resistor to indicate when the box was powered, a useful addition.

All of these parts should be readily and cheaply available.

Connecting to the car

It is possible to buy a proper OBD-II connector, they are about 10 UKP. My solution cost a few pennies! I used extra long, 0.65mm² pin strip header, it fits the connector. I soldered wires to the header and added heatshrink header for insulation.

Using this pinout:

I connected the adaptor as follows:
Pin 5, signal ground,
Pin 7, K-line,
Pin 16, +12V.
Note: if pin 5 is missing, you may have to use pin 4, chassis ground.

You will of course need to locate the connector. It should be within 1 metre of the steering wheel, inside of the car. On my Skoda Fabia, it is located on the right of the steering wheel, behind the pull down drink can storage. Look at these forums for more details on locating the connectors and VAG-COM/OBD-II interfaces:
Skoda, http://www.briskoda.net
Volkswagen, http://www.volkswagenforum.com
Seat, http://www.seatenthusiasts.co.uk
Audi, http://www.audiforums.com or http://www.audiworld.com

Using the software.