So basically I stopped for 30mins, started my Cat, and the gearbox oil temperature started jumping out of nowhere. I think it's the sensor, just I don't know where it's located to change it. If someone would send a manual, it would help out a lot.
It's far more likely to be a wiring/connector fault if the gauge in the cab is jumping. Looking at the Diagnostic Codes might provide some insight. To identify the sensor location and an electrical schematic requires the Serial Number.
If the wiring/connector fault would occur, wouldn't it start jumping from 0 to the normal temperature instead of the maximum?
It could do all sorts of things but without a Serial Number it's still a cr@p shoot. I can't even look for information without it........... When you say "gearbox temperature" I assume you mean transmission temperature.?
AFX? Image item #11? 134-2252
If it is an AFX-prefix machine there is an even chance that the OP will have the benefit of an on-board Click Box facility to access Diagnostics. That would be helpful.