Summary of AVR Based Car Diagnostic Tools using ATmega169
This project enables diagnostics for GM pre-1996 vehicles using the OBDI/ALDL standard. It utilizes an Atmel Butterfly board (ATmega169) to display and log powertrain data, communicating with the PCM via a USB adapter. A voltage level shifter using transistors and resistors bridges the 3V system with 5V ALDL interfaces. The system was successfully tested on a 1994 Chevy Caprice to identify engine abnormalities by plotting downloaded data on a PC.
Parts used in the AVR Based Car Diagnostic Tools:
- GMC pre-1996 car or light truck
- Atmel Butterfly evaluation board (ATmega169-based)
- Integrated USB adapter
- Transistors
- Resistors
- PC software for plotting results
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.
For more detail: AVR Based Car Diagnostic Tools using ATmega169
- What vehicle systems does this project support?
The project supports GM pre-1996 cars and light trucks using OBDI or ALDL standards. - How is power train information displayed?
Data is displayed or logged using the Atmel Butterfly system evaluation board. - What hardware is needed to shift voltage levels?
A simple configuration of transistors and resistors is used to handle the voltage difference between the Butterfly and ALDL interfaces. - Can I download diagnostic data to a computer?
Yes, users can download data from the PCM to a PC to plot results and find abnormalities. - Which specific vehicle model was used to test the project?
The project was tested on a 1994 Chevy Caprice. - What baud rate does the GM-specific ALDL standard use?
The standard operates at 8192-baud starting from the 1986 model year through 1995. - Is the USB adapter commercially available?
Yes, the integrated USB adapter is inexpensive and available off-the-shelf. - Does the ATmega169 board operate at 5 volts?
No, the Butterfly board operates at +3V while the ALDL interface operates at +5V.
