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A couple months ago my girlfriend picked up this ‘96 ccsb 7.3 stick for her new daily. The P.O. said the odometer would work some times, but mostly it just reads scrambled. The other issue we’re having is the fuel gauge reads pegged full on both tanks (tanks and switch both work) when you switch the tanks the gauge doesn’t budge. The speedo also bounces around by about 25mph when driving.
The oil pressure, temp, volts and tach all work fine in the cluster, with the issues only being with the speedo,odometer and fuel gauge. Odometer seems to still be counting miles even while scrambled.

Anyone have any ideas? Would be much appreciated!

Typically a speedo jumping issue is the speed pickup sensor in the rear end. Sometimes they just have a build up of filings on them, or sometimes they just fail. Not sure how that's related or not to the fuel gauge, haven't had that happen to mine yet. Since all the wiring for both goes back to rear of the truck, maybe there is an issue with the wiring going back there?

Not waiting long enough on my '99 causes the fuel gauge to avoid working. I think is might have to do with low voltage to the electrical stuff?
Agree with 300 - replace the diff sensor and see what happens. A common issue that screws up the speedo and ABS if your 96 has that.