I have recently bought a 1988 K-5 for my son. We were driving around the other day and the 'Service Engine Soon ' came on very dim and after awhile it got brighter and brighter until it was bright. When we shut off the ignition switch the 'light' stayed off for about 5 miles and repeated (dim light to bright).
When we got home I tried to read the trouble codes by jumpering the A and B terminals on the ALDL connecter. It started by blinking code 12 and 24 then the light started to fade away to nothing as the other codes came through. When I tried it again could barely read code 12 before the light dimmed away. I am trying to get a better wiring diagram. I think I have a bad ground someware. Has any one had this problem?

 

Never had that problem, and never heard of it before either. Seems strange to say the least. Keep us posted.