You can read the codes by jumping two pins in the harness under the steering wheel. Jump pins A and B for engine codes and jump A to H for ABS codes.
Here is an exerpt from www.fullsizechevy.com
, Heres the original post.
"This procedure only works on OBDI vehicles up to '95. Do NOT try this procedure on a 96 or newer vehicle!!
You want to jumper pins A and B. That should be the two upper right pins as you look at the plug.
1. With power off, jumper pin A to pin B
2. Turn the ignition to ON but DO NOT start the vehicle.
3. The Service Engine Soon (SES) light should flash code 12 (1 flash, pause, 2 quick flashes)
4. After a longer pause, code 12 should repeat two more times.
5. If there are any codes stored in the ECM they will follow Code 12.
6. If no other codes have been stored then Code 12 will continue to repeat until the jumper is disconnected.Remember all OBDI codes are two-digit."
(use the above procedure with pins A and H to read ABS codes.)
"Here'* the procedure for clearing ABS codes:
1. Turn the ign switch to the "ON" position.
2. Use a jumper wire to ground DLC terminal "H" to "A" for 2 seconds.
3. Remove the jumper wire for 1 second.
4. Repeat the grounding for 2 seconds.
5. An indication of success is when the "ANTILOCK" and "BRAKE" lamps turn on and then turn off.
All of the above is directly from my '95 FSM.
Pin A, the ground, is the topmost right corner pin & pin H is one pin over from the bottom left corner."
Another thing to remember is that not all codes will cause the ABS or SES lights to come on. Some are just recorded in the computer.