Engine light will not flash any code???

[72timemachine]

Friend has a 1990 GMC 2500 5.7 ltr. Th400 which has the engine service light on. A scan tool does not work and when I jump the a and b connections where you plug in the scanner to pull the codes, it just flashes once rather than flashing the 12 code which I believe tells you it is communicating correctly and then will spit any trouble codes thereafter.

I pulled a spark plug because the truck feels like it is skipping (engine still feels very strong) and the plug is black so I it seems like it is in a rich condition.

Anyone have a problem like this or know of a thread that I could read for some ideas where to go?

Thanks for any help.