(Nov 2010, FYI This page is quite obsolete and links are not being maintained. For some reason it's still popular. Maybe it's taking search hits it doesn't deserve or it's still of use. If you have any useful feedback let me know and I will add it.)
A great freeware PC scan tool is now available and obsoletes the software available here: http://winaldl.joby.se/
Back in '97 I was unable to find any free scan tool so I whipped up a nasty basic program to decode the datastream using brute force. Not much has really happened since then but I keep the page here to assist people looking for information.
My car is (was :) a 1986 Buick Skylark with the GM C3 ECM. I also have some DIY_EFI archive info the 8192 bps (P4?) ECM right here.
The above picture depicts a snapshot of data from the ALDL connector. The top purple trace is on a 5 milisecond grid. The bottom green trace is shown 2x faster, at a 10 msec grid. The bits are 6.25 miliseconds apart. A falling edge marks the start of a bit. For the puprle trace, the first bits are 0's, and the last bits are 1's.
Dead links you might want to track down: ni.umd.edu/gnttype/www/ecmpins.html ECM Pinout Map www.geocities.com/MotorCity/Speedway/5036/ecm-codes.html Another ECM Trouble Codes & ALDL Pinout ni.umd.edu/gnttype/www/malf.html ECM Trouble Codes & ALDL Pinout www.access.digex.net/~miked/codes.html Monte List: ECM code reading www.telepath.com/dbyrer/aldl.html ALDL Pinout efi332.eng.ohio-state.edu/diy_efi/ Do-It-Yourself Electronic Fuel Injection Mailing List www.franken.de/users/dagobah/luke/scantool.html Oliver Scholz Scantool Home Page