If the speedo works, but the odometer doesn't . The cluster needs replaced.
the little gear that turns the numbers is broke and the only way to fix is replace the whole speedometer.
The odometer has failed.
If the speedometer and odometer both quit I would blame the speed sensor. If only the speedometer quit I would blame the instrument cluster.
i have a 1990 Isuzu trooper as well and the speedometer and odometer also quit working, they both come off of a mechanical cable that is attached to the drive train. that cable breaks, and you lose both speed and odometer indications. they are extremely difficult to find, so if you do, write me back...
If you open the odometer, there are two small plastic gears. One is connected to the speedometer, the other to the odometer. The speedometer gear drives the odometer gear. In all likelihood, one of these gears is missing a few teeth. I have the same problem with my Ramcharger, same year.
check your fuses
Replace the vehicle speed sensor, this should show up as code PO500
it usually is the gear in the transmission Could be the vehicle speed sensor.
check the fuse panel
not quit positive
check the vss located on the tailshaft of the transmission for the speedometer issue the odometer is figured out based on the speedometer also so it will quit as well. this sounds like either the sensor is bad or a short in the wire from it The cord for the speedometer came undone from the transmission or the gauge cluster is bad
If both the speedometer and odometer stopped working it is probably the speed sensor in the transmission gone bad. If only the speedometer quit you may have to replace the dash cluster with a reconditioned unit.