For those here that are still interested in tinkering with the GM C3 ECM's, I thought I'd put a link here to a thread I wrote on interfacing an external device to the ECM memory via the factory diag/HUD port on the ECM board:

http://www.montecarloss.com/communit...31#Post1031131

Most importantly, I would like to send out a Big Thanks to BobR/RBob who frequents this forum. I asked him numerous questions as I bumped my head figuring this thing out. He always gave me just enough of a hint to get me unjammed!