here is a video (there are several on youtube, this one is pretty easy to follow)
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The truck is newer than yours, but the process is exactly the same. I used a bent paperclip to short my a/b terminals together.
Here are the codes that a square could push, it is a partial list of the GM OBD1 codeset:
12 - indicator code, start of code sequence
13 - O2 (oxygen sensor) circuit (sensor open, no activity)
14 - ECT/CTS (Engine Coolant Temperature/Coolant Temperature Sensor) circuit resistance low or shorted (high temperature)
15 - ECT/CTS (Engine Coolant Temperature/Coolant Temperature Sensor) circuit resistance high or open (low temperature)
16 - VSS (Vehicle Speed Sensor) buffer fault or shorted
21 - TPS (Throttle Position Sensor) circuit high (signal voltage high), higher than 5v DC (highest voltage WOT)
22 - TPS (Throttle Position Sensor) circuit low (signal voltage low), lower than .2v DC (lowest voltage idle)
24 - VSS (Vehicle Speed Sensor) circuit low/circuit fault
28 - TR (Transmission Range) pressure switch assembly / Quad Driver Module (QDM A) error, circuit #2 fault
32 - EGR (Exhaust Gas Recirculation) valve error
33 - MAP (Manifold Absolute Pressure) sensor circuit high, signal out of range, high resistance (low vacuum)
34 - MAP (Manifold Absolute Pressure) sensor circuit low, signal out of range, low resistance (high vacuum)
35 - IAC (Idle Air Control) error
36 - Idle speed control actuator error (when equipped) / Ignition system circuit error
42 - EST (Electronic Spark Timing) circuit error, grounded shorted
43 - KS (Knock Sensor) error
44 - Lean exhaust (oxygen sensor indicates lean condition)
45 - Rich exhaust (oxygen sensor indicates rich condition)
51 - PROM (Programmable memory) error (chip), calibration error
53 - System voltage high (supply to ECM/PCM)
54 - Fuel pump relay circuit low voltage
55 - ECM/PCM error
Here is a good page with details.
Specialized repair and troubleshooting
www.tradecraftspecialties.com