1988 V1500 Suburban SES light dimms when getting codes

I think this would be the best section to post this in. I have a code scanner that blinks the Check Engine light to report codes. When I am getting the codes from my 1988 V1500 Suburban, The code 12 starts to report fine then it start to blink slightly erratic and then dims. AFter the 3rd repitition of the code 12 I can barely make out another code that could be a 24 or 34 but it's hard to tell. If I turn the truck off and try again, it is still too dim or just completely dark from the start. If I let it sit for a bit, It starts out fine again but gets dim the same. When The scanner is not attached, the Check Engine light is bright and fine. Using the same scanner on my 94 Saturn, it works just fine. Does this mean there is something wrnog with the computer in my truck???

-Pat
Not necessarily, compatibility may be an issue or even a weak battery. You don't need a scan tool for this truck to retrieve codes. You can take a paperclip bent into a horseshoe and ground the ALDL connector with the key on engine off. The check engine light will begin to flash out the codes this way.

Steve
Steve has the right idea by jumpering the ALDL connector to see if it’s your reader or whether it’s the computer that has lost diagnostic capabilities.
Connect the paper clip or whatever between A&B terminals and then turn the ignition on. The Check engine light should flash 12 telling you that it’s entered diagnostics and then flash trouble codes if any. It should flash each code three times and then start over again with code 12.
If you were to start the truck with the ALDL jumpered the check engine light will or should flash quickly while the system is in open loop and then begin to flash much slower after the O2 has come on line (closed loop).
Light on is a lean indication, O2 voltage above 450mv; light out rich indication O2 voltage below 450mv.
Here a diagram showing the ALDL and the terminal designations.