GMC Troubleshooting Questions? Ask a Mechanic for Answers
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This most likely caused by a bad VSS (vehicle speed sensor). The VSS wiring may also cause the issue, if the wiring coating is off anywhere on the harness and the wires touch it will cause the Speedometer to not work. Check the wiring near the VSS in the tail housing of the Transmission. The VSS is located at the tail of the transmission. It will be roughly about where the bed meets the cab. Where the driveshaft plugs into the transmission is the "tail" of the transmission. If the wiring is good then replace the VSS.
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