Any car newer than 1995 has a fairly sophisticated computer used to monitor various systems ranging from accelerator pedal position to catalyst efficiency. This is a brief introduction to one example of the hardware used to diagnose automotive systems.
All autos in the US (and other countries) since 1996 are required to be equipped with an OBD2 interface. what exactly does this standard interface allow someone to do? And what barriers does it create for someone who is handy with a wrench in their own driveway?
The most affordable diagnostic tools for the average person repairing their own cars, run from around $50 to $300. For about $150 you can get a unit like the one pictured above, however this unit lacks the ability to check the abs computer, body control computer etc. What it will allow you to do is retrieve the error codes, clear diagnostic trouble codes, watch live sensor data, and record sensor data on fault codes. why pay a garage to take 10 minutes and $100 to tell you that you need a new O2 sensor.
So what makes it tick? lets take a look inside.
First off, it looks like is a lot going on inside, well start by looking at the micro controller, at the heart there is an 89c51cc01, this is an mcs51 variant with can(controller area network) and uart, an 32k x 8 Sram, a 16Mbit flash, and an ftdi ft232rl. Having only 1.2kbytes of internal ram, the 8051 is not suited to for complex applications, however with the external bus connected to sram and flash, there is no need to insert different cartridges for different makes. also havig standard codes across all manufactures allows this not to need a gigabyte of storage.
The data that passes back and forth between the scan tool and computer is arranged as small packages. The tool sends a command and some operands then the computer sends back a packet with a header and data and some form of acknowledgment. the tool then interpret the data depending on the mode the tool is set to.
If you are interested in learning more about obd we suggest the flowing pages.
Environmental protection agency page for repair technicians, vehicle owners, and manufacturers