One track slower than the other?

You point the machine dead straight and it wanders off to one side like it's had a couple beers. Or one track just flat refuses to keep up with the other. It's easy to shrug off for a few days, tell yourself you'll deal with it later, and honestly I get it, you've got a job to finish. But a final drive that's starting to go is on a clock, and what you do in that first week usually decides whether this is a seal-and-gear-oil afternoon or a brand-new drive motor and a very quiet ride home.

What the final drive does

Each track has its own final drive: a hydraulic travel motor bolted to a set of planetary gears that turns that motor's speed into the torque that moves the machine. There's one on each side, running in its own bath of gear oil. When one side wears out or loses its oil, that track slows down, gets noisy, or quits, while the other side keeps working fine, which is exactly why the machine crabs to one side.

The warning signs

What usually causes it

Sometimes it isn't the drive at all, it's a hydraulic problem feeding it, a control valve or a travel motor issue. That's worth knowing before you drop serious money on a final drive you might not even need, because a proper check tells the difference.

Check the gear oil, it's the cheapest tell

Before anything else, pull the check plug on the slow side's final drive and actually look at the oil. Is it low? Is there metal on the plug or glitter in the oil? Is it milky, like coffee with too much creamer, from water getting in? Each of those tells a story, and none of the stories end with "it's fine." Low or metal-laced oil means the drive is already eating itself, and topping it off buys a little time but fixes nothing. Catching it here, while it's still just a seal and a jug of fresh oil, costs a fraction of what comes next.

"A whining final drive still moves the machine, which is exactly the trap. It lets you keep pretending it's fine."

Why you shouldn't keep running it

This is the part that costs owners the most money, so listen up. A final drive that's whining or low on oil still moves the machine, which is exactly the trap, it lets you keep pretending it's fine. But every hour running on dry or metal-filled gears grinds them down further. What could have been a seal and a gear-oil service quietly becomes a full planetary rebuild, or a whole new final drive, once the gears and the motor chew each other up. Catch a leak or a whine? The cheapest thing you'll ever do for that machine is shut it down and get eyes on it while the damage is still small and boring.