Connecting To An Automotive Diagnostic Computer?

An Anonymous Coward writes: "I was wondering if anyone has tried to hook up a PC to the diagnostic computer on an automobile. Each time the little idiot light on my Blazer comes on I have to cough up $20 - $40 just to find out what the problem is. This seems to me like an amusing and only moderately difficult hack - I can't believe that I am the first person to think of it. However, I can't seem to find any information on the topic at all."

Comments

Diagnostics with a simple multi-meter

[dead_penguin]

Quite some time ago I was searching around the net for exactly the same thing, just out of curiosity. Unfortunately I didn't keep any of my bookmarks on this (and they've probably 404'd anyways), but many manufacturers encode the error "codes" as a series of pulses. Using a multi-meter or a simple 12V bulb, you can count them and get the error code. They're not even encoded in any way (BCD or whatever), but rather the number of pulses is directly equal to the code. Building a simple circuit to record the error codes and later dump them in a serial port (or parallel if that's your thing) shouldn't be too difficult.

A Google search turned up some examples of codes, so they're out there; you just have to look!
Ford Probe

Eagle Talon

Most of the better shop manuals you can buy also have most of these codes. Take a look through some of the aftermarket books like those from Haynes or Chiltons or whatever you can find.