Here is how to read GM ABS trouble codes on an OBD1 system. This applies to Chevrolet and GMC vehicles from the 1990s. Put a jumper across the pins and then turn the ignition on. The ABS light will flash the trouble codes 3 times each and then repeat them. Here is a list of the GM trouble codes, and what they mean.
Codes 12 and 14 aren't trouble codes, but may appear to indicate normal operation.
21 - right front speed sensor or circuit open.
22 - missing right front speed signal.
23 - erratic right front speed signal.
25 - left front speed sensor or circuit open.
26 - missing left front speed signal.
27 - erratic left front speed signal.
29 - simultaneous drop-out of front speed sensors.
35 - vehicle speed sensor or open circuit.
36 - missing vehicle speed sensor signal.
37 - erratic vehicle speed sensor signal.
38 - wheel speed error.
41 - 54 control valves
61 - 63 reset switches
65 - 66 open or shorted pump motor relay.
67 - open motor circuit or shorted BPMV output.
68 - locked motor or shorted motor circuit.
71 - 74 memory error.
81 - brake switch circuit shorted or open.
86 - shorted anti-lock indicator lamp.
87 - shorted brake warning lamp.
