I'm wondering if anyone has information, links, write ups, or internet form posts about the GM MEFI computer used with a GM Crate 502 engine?

I've found MEFI Scan & Tune, which really stresses the budget. http://www.aldlcable.com/sc/toplevel.asp?cat=32.

I'm hoping to find wiring schematics, and how to pull trouble codes. The engine is up and running, but very poorly. Maybe something is wired wrong, but without schematics I don't know. I'm thinking I need to wire a SES light? I'm trying to figure out how to pull trouble codes, without an expensive scan tool.

Thanks,

dave w