Apparently, for some GM OBD1 truck models there is a pin at Terminal H that can access ABS codes but H is blank on mine as well.
BuickGirlFromMars provided this video link to determining ABS Codes.
It seems to me that the Terminal A to Terminal B does provide all of the ABS codes but that their interpretation requires access to the Service Manuals ABS codes discussion. For example, as I also have a stubborn ABS light issue, the OBD1 flashed Code 23 which means a bad IAT (
Intake Air Temp) on every OBD1 error code list I've seen. However, when one reads the Service Manual ABS Diagnostic Trouble Codes section, this same code 23 also means a bad wheel sensor speed reading. The same proves true of Code 56 which under the Service Manual ABS codes indicates an issue with a short to ground on a specific ABS brake line.
I've been trying to use a program called TunerPro RT with a Moates ALDL (no longer manufactured) to pull codes that require the Tech 1, albeit with limited success. For example, it did signal bad wheel speed sensor on its readout.
Sadly, I haven't had time to use it since 2021 due to fix it or die issues on my vehicle.