1988 GMC C1500 speedometer

First of all check for break code 9, and inoperative speedometer/odometer or ECM code 24 with correctly operating speedometer. If ECM code 24 is set and speedometer operate, check vehicle speed sensor buffer output as follows; disconnect vehicle speed sensor at transmission and check sensor resistance. If resistance is between 900 -- 2000ohms, replace vehicle speed sensor and retest speedometer operation. Raise and support rear of the vehicle. Disconnect instrument cluster connector. Start engine and place transmission in drive. Using a digital volt/ohmmeter check for AC voltage yellow wire ( terminal B13) and Purple wire (terminal B16). If no AC volts is detected. Check wires for open circuit and connectors for tightness. If wiring and connectors are okay, replace vehicle speed sensor. If AC voltage varies with driveshaft rpm vehicle speed sensor is okay, replace instrument cluster. Don't forget to check speedometer fuse before commencing the diagnosis procedure.