3-second answer: 92-96 trucks have an electronic speedometer driven by the PSOM module (programmable speedometer/odometer module, behind the cluster). Common failures: the odometer gears strip (odometer stops, speedo keeps working), the VSS sensor on the rear diff or transfer case dies (speedo drops to zero), or the PSOM itself loses its mind. Diagnose before buying parts.
The failure modes
- Odometer stuck, speedo fine → stripped odometer gears (the famous nylon gear). Replacement gear sets are ~$10 and the cluster comes apart in an hour.
- Speedo dead/zero, odometer also dead → VSS (vehicle speed sensor) or its wiring. The sensor lives on the rear axle (8.8) or the transfer case.
- Speedo bounces or reads wrong → PSOM calibration or the PSOM itself; some rebuilds are as simple as resoldering the module's pins.
PSOM calibration
The PSOM stores the calibration for your tire size/gear ratio. If you changed tire size or gears, the speedo reads wrong — the PSOM can be recalibrated (the procedure is well documented: press the cluster buttons in the right sequence and enter the constant). This is the #1 reason a speedo reads wrong after a lift.
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