Engine light will not flash any code???
 

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.

Re: Engine light will not flash any code???
 

Start with an ignition tune up. At least look at the cap and rotor but suggest replacing it and wires as well.

Re: Engine light will not flash any code???
 

That sounds like the ECM is defective and it is in limp home mode.

My 91 had an ECM circuit board come loose inside the case and it would go from running normal to limp home mode randomly after hitting a bump. I figured it out when I pulled the ECM and heard the noise inside.

I put my chip into a spare ECM I had sitting around and it fixed the problem.

Re: Engine light will not flash any code???
 

I agree it sounds like an ECM failure.