This project focuses on tapping into GM pre-1996 car and light truck diagnostic information. These systems are commonly referred to as OBDI, or ALDL (Assembly Line Diagnostic Link). They are based, in part, on the GM-specific 8192-baud ALDL standard that was used starting in the 1986 model year through 1995. The data stream definitions used by GM are freely available for download and provide necessary information to communicate with a PCM from this era.
Power train information is displayed or logged using Atmel Butterfly, an ATmega169-based system evaluation board, with an intuitive user interface. Since the Butterfly operates at +3V, and the ALDL and USB interfaces operate at +5V, a voltage level shift is the only real external hardware necessary. This is accomplished using a simple configuration of transistors and resistors. The integrated USB adapter is inexpensive and off-the-shelf, available commercially.
“The project was tested on my 1994 Chevy Caprice to help diagnose a rough running engine shortly after startup. When the car engine ran rough, I could download the data on a PC, plot the results, and begin looking for abnormalities in the PCM data,” said Chris Brophy, the project designer.