Excavator Tracking to One Side: Is It the Final Drive?

An excavator pulls toward whichever track turns slower. The final drive turns that track, so it takes the blame first. But a new drive cures none of the other causes, and several of them cost far less to find and fix.

The checks run roughly from cheapest to rule out to most expensive. You can make the early ones yourself. Anything that opens a hydraulic line belongs to a technician working from the service manual, because those lines can stay under pressure long after the engine stops.

Why does my excavator pull to one side?

When it pulls, and what comes with it, narrows the cause before anyone opens a line.

What you notice	Likely cause	Who checks it
One track tighter or packed, or a roller worn flat	The undercarriage on that side	You, with the machine shut down
A pedal or lever that stops short	The linkage, or the pilot valve under it	You, then a technician
One side slow in high speed only	That motor’s two-speed changeover	A technician
Pulls only while you work the boom or arm	The straight-travel valve	A technician
Other functions slow as well	The main pump feeding that side	A technician
Blade weak too, on a machine with one	The center swivel joint	A technician
Slow and weak under load, nothing else to see	Wear inside the travel motor, or a dragging brake	A technician
One track will not move at all	A brake that will not release, no oil arriving, or a broken drive	A technician
Noise, metal in the gear oil, a leak or a wobble	The final drive gearbox	A technician

Is the track itself dragging?

Look here first, with the machine shut down. A track that is hard to turn slows its side the way a weak drive does.

• Tension. A track adjusted tighter than its partner takes more effort to turn. Compare the sag on the two sides, then bring both to the figure in the machine’s manual. The figure varies by machine, and a rule of thumb is not a specification. The adjuster holds grease under very high pressure, so adjusting it belongs to the manual’s procedure.
• Packed material. Mud, rock or debris packed into the sprocket, idler or rollers tightens a track as surely as the adjuster does. Mud that freezes overnight can lock rollers solid.
• A seized roller or idler. One that has stopped turning, top roller or bottom, drags the track across it and wears a flat spot into its tread.
• Uneven wear. Undercarriage wear is seldom even. Worn grousers grip less on soft or loose ground, and a hooked sprocket on a worn chain can jump a tooth under load. Either lets that side lose ground.

If one side is clearly tighter, packed or worn, fix that first and see whether the pull goes with it.

Are the controls reaching full stroke?

A travel pedal or lever that stops short sends its track less oil. A floor mat or packed dirt under the pedal is enough. So is a stretched cable or a worn joint, where the pedal works the valve directly.

On most excavators the pedal works a small pilot valve, and the pressure it sends shifts the travel spool in the main control valve. A worn pilot valve, or a restriction in the line to one side, leaves that spool part-open. A technician compares the pilot pressure on the two sides.

On machines with electronic travel controls, a failed sensor or solenoid can slow one side too, and it usually leaves a fault code. Read the codes before any line is opened.

Is one track slow in high speed only?

Compare the two travel speeds on open, level ground, with nobody near the machine. If it tracks straight in low and pulls only in high, look at the two-speed changeover.

In high speed each travel motor shifts to a smaller displacement: it turns faster, with less torque. A pilot signal, switched by a solenoid, tells the motors when to shift. A motor that fails to shift stays in low while the other goes up, and the machine pulls toward the slow one.

On many machines one solenoid switches both motors, so a fault there usually slows both sides. A one-sided fault points at that motor’s changeover spool or piston, or the pilot line to it. That can be a sticking spool rather than a worn-out motor.

Is the pump, valve or swivel at fault?

A motor short of oil behaves like a worn one.

The pump that feeds that side

On many excavators, traveling straight, each track is fed by a different main pump, or a different section of one pump. A weak pump then slows its own track. Circuits vary by machine.

A weak pump seldom slows travel alone: the other functions it feeds are usually slow too. If that is your pattern, read the signs of a failing hydraulic pump before ordering a drive.

The control valve and relief valves

Each track has its own spool in the main control valve. One that is scored, worn or sticking passes less oil to its motor. Each travel circuit also has relief valves, on the control valve, the motor or both. One that leaks or opens early lets oil bypass the motor.

Weakness in one direction only usually points to a valve serving that direction, rather than wear inside the motor.

If the machine tracks straight until you work the boom or arm, suspect the straight-travel valve. On many two-pump machines it gives both tracks one pump whenever the boom, arm or bucket moves. When it sticks, those functions take oil from one track.

The center swivel joint

The swivel joint, also called the rotary or center joint, carries oil down to the travel motors while the upper structure turns above them. Seals inside it keep each passage apart. When they wear, oil crosses from one track’s passage into the next, or to drain, and that track slows.

On a machine with a blade, its oil passes through the swivel too, so a weak blade beside a slow track points here. No new drive cures a leaking swivel.

Is the fault upstream, or in the drive?

One test splits the problem in two. A technician swaps the left and right supply lines below the swivel joint and travels the machine, with nobody near it. The oil that fed the slow side now feeds the other motor.

If the slow side moves across, the fault is upstream: the controls, the pump, the control valve or the swivel. If it stays put, it is in that side’s motor or gearbox, provided the undercarriage was ruled out first.

The test means opening lines that can hold pressure after shutdown, and while the lines are crossed, so are the controls. It is work for a qualified technician with the service manual.

A fault that stays with the drive does not always mean a new drive. On many travel motors the relief valves, the counterbalance valve and the two-speed spool sit in the motor’s own valve cover. A technician checks those before condemning the motor.

Is it the travel motor?

If the fault stayed with the drive, the motor is next. A worn motor leaks oil past its pistons and valve plate into its case, so less of it turns the track. That side goes slow and weak, worse under load, often with nothing to see from outside.

A technician measures the flow from the case drain against the figure in the service manual. That number separates a worn motor from a sound one.

A parking brake that does not release fully is the other motor-side cause. It drags, and after the same work that hub runs noticeably hotter than the other.

Is it the final drive gearbox?

A gearbox failing badly enough to slow its track rarely does it quietly. Expect grinding or whining, metal in the gear oil, oil at the hub, or a sprocket that wobbles. If a slow side is the only symptom, the gearbox is usually not the cause.

If you have any of those, read the signs of a failing final drive before you order. Checking the gear oil means opening a drive that may be hot and under pressure, so leave that to a technician.

If it is the drive, what do you order?

Whether you need a travel motor alone or a complete final drive depends on which half failed, as our guide to telling the two apart explains.

Either way, start with the number on the tag, and give the machine’s make, model and serial number, plus what the technician found. Use the request form, or go straight to a dealer near you. If the tag is missing or worn smooth, send the rest anyway.