A problem excavator owners run into from time to time is a final drive that spins backwards. When this happens the initial thought is often that there's a problem with the final drive. Instead, the high pressure hydraulic lines are hooked up in reverse.
Final drives have two large hydraulic ports that connect to two high pressure lines. One port is for flow and the other is for return. When the lines are hooked up in reverse, your drive will run in the wrong direction.
The fix is as simple as the problem, flip the lines. You can easily prevent this problem. When you remove a final drive, mark the lines with colored zip ties on the hydraulic fittings and the hoses.
If you end up with this problem, don't worry, we get calls all the time from excavator owners in a panic because they think something is wrong with their final drive or machine.