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Home >> GMC >> 1991 >> Forward Control 6.2 J, Automatic >> Repair and Diagnosis >> Accessories & Equipment >> Gauges & Instrument Panels >> Instrument Panel - Except C/K Series Pickups >> Removal & Installation >> Speedometer/Odometer Test >> Speedometer Inoperative Or Inaccurate

Speedometer Inoperative Or Inaccurate

  1. Disconnect the digital ratio adapter, and place the ignition in run. Check for voltage between the brown wire in the harness and a good chassis ground. If the voltage is less than battery voltage, check for an open or short in the brown wire.
  2. Check for voltage between the pink/black wire in the harness and the black/white wire. If voltage is less than battery voltage, check for an open or short in the black/white wire.
  3. Raise and support the vehicle, start the engine, and place the transmission in drive. Check for AC voltage that changes with the engine RPM between the purple/white wire, and the light green/black wire at the digital ratio adapter. If there is not AC voltage at these wires, check for opens in the purple/white wire and the light green wire. If there are not shorts or opens, replace the vehicle speed sensor.
  4. With the vehicle still raised and supported place the transmission in drive. Check for AC voltage that changes with the engine RPM between the light green/black wire and the white wire at the DRAC connector. If no voltage change, replace the digital ratio adapter.
  5. With the vehicle still raised and supported place the transmission in drive. Check for AC voltage that changes with the engine RPM between the yellow and the black/white wires at the digital ratio adapter connector (Connector attached) if the AC voltage changes with RPM, replace the digital ratio adapter.