Why An Excavator Travels Slowly On One Side: A Complete Diagnostic Guide

When an excavator travels slowly on one side, the first instinct is to blame the track on that side. Sometimes that is right. Often it is not, because track speed is set upstream by the hydraulic system: the main pump, the travel control valves, the counterbalance valves and the travel motors. If flow to one side is lower, that track turns slower, however good the undercarriage is.

Replacing undercarriage parts without finding the cause is the most common and costly mistake on this fault. It does not fix the problem, and the real cause keeps working against the machine.

This guide walks through the full diagnostic process for one-sided slow travel: what the operator can report, which quick mechanical checks to rule out first, hydraulic pressure and flow tests, and component-level faults. It also covers what the fault means if you are looking at a crawler for sale, how Gulf Coast conditions affect wear, and how fleet managers can catch the problem early.

These tests involve high-pressure hydraulics and moving machinery. Follow the manufacturer’s service manual, and have a qualified technician carry out pressure and stall tests.

Why Replacing Track Parts First Often Fails 

The reasoning is sound: Pulling left means that something is wrong with the left track. Operators test track tension, check for obvious damage, check the sprocket, and if none is found, they assume that the track is worn or the rollers are dragging. Parts are ordered, labor is booked, and tracks are serviced. The machine then returns to work and pulls left once again if the track wasn’t the issue.

Hydraulically powered travel system on the excavator. Every track has its own individual travel motor, hydraulic circuit, and travel valve spool. When the left motor is not getting as much flow as the right motor (due to a worn spool, failing motor, stuck counterbalance valve, or pump imbalance), the left track will turn slower, regardless of the mechanical condition of the left motor.

The undercarriage shows the symptom, but it is often not the cause. Each day the machine runs with a hydraulic flow imbalance, the weaker component works harder than it should and wears out sooner.

How The Excavator Travel System Actually Works

The travel system of an excavator consists of three basic components: the main hydraulic pump, travel control valve, and travel motors.

Most 20-30-ton excavators have a main hydraulic pump that is a tandem axial piston pump (two pump sections powered from a single input shaft by the engine). A hydraulic system is provided in each section, providing one side of the machine. This is usually called the front “pump” and the rear “pump”. This split pump design causes unequal flow to each circuit when one section wears faster than the other (due to age and poorly maintained hydraulic fluid), and the machine starts to run out of step when under load.

The hydraulic flow goes through the main control valve assembly from the pump. The travel spools are special directional valves which control the flow to each of the travel motors according to the pilot signal sent to them from the operator’s travel pedal or lever. If the operator is in full forward, the pilot circuit will apply the same pressure to both the travel spools, which should open equally and provide the same flow to both motors. When one spool sticks, wears, or is given less pilot pressure, it will open less than the other, resulting in one-sided slow travel.

The travel motors are variable displacement axial piston motors that have a two-speed switching function. Every motor is equipped with a counterbalance valve, which is a pressure-holding valve that will not allow the motor to freewheel when climbing uphill, as there will be back pressure on the motor outlet circuit. A misaligned counterbalance valve will have a direct impact on the speed at which the car travels and how it brakes on that side of the car.

Key Variables That Indicate Which Component Has Failed

The symptom pattern includes the diagnostic information prior to performing any test.

• When loaded, the machine tends to pull to one side, but tracks straight when running unloaded on flat ground; the problem is most likely the main pump flow imbalance between the two pump sections. When subjected to load, the weaker section will not be able to sustain a good flow, and the difference is made apparent. Both sections provide sufficient flow for both tracks at light load to maintain similar track speeds.
• If the machine pulls at the same rate at all speeds with or without load, the problem is with the travel control valve spool or with the pilot pressure circuit on the side of the machine. A mechanically restricted spool will restrict the flow, both with and without load.
• If the fault is only present at high speeds and goes away at low speeds, the fault is likely to be caused by the two-speed switching mechanism or variable displacement swashplate in the affected motor. Low speed mode will have a higher displacement and will not be as sensitive to minor flow differences, while high speed mode will accentuate any differences in flow.
• When the machine slows down or goes downhill and pulls to one side more than the other (braking harder on one side), the first thing to check is the counterbalance valve on the motor in question.

Step-By-Step Diagnostic Process

The proper diagnostic procedure is to first observe, then perform non-invasive tests, then perform hydraulic testing, and finally disassemble components.

1. Start with an observation of the operator. Ask the operator if the machine pulls in one direction or the other, when it shows up, when it began, and if it has made any progress. If the onset is gradual and occurs over weeks, it’s a sign of wear; if it occurs suddenly, it’s a sign of a failure event.

2. Ensure track tension is checked both ways, as over-tensioned track will cause mechanical restriction, which will simulate hydraulic restriction. If not even, correct and retest before going on. Check for jammed rollers, broken or worn sprocket teeth, or clogged debris on the slow side. These visual checks can eliminate mechanical drag without having to perform a hydraulic test and take minutes.

3. Check the travel brake as well. If the spring-applied brake on one side does not fully release, that track drags and runs slow. A noisy or hot final drive on the slow side is another sign of a mechanical cause. These checks take minutes and can save an hour of hydraulic testing.

4. Test the pilot pressure at the travel valve on both sides while a travel command is applied. The two readings should be within the tolerance in the service manual of each other, and both should match the manual’s pilot pressure. A low reading on the slow side points to a fault upstream: the pilot pump, the pilot relief valve, a blocked pilot filter or a solenoid valve on that circuit.

5. Check the main circuit pressure at each travel motor inlet while traveling. If the pressure is much lower on the slow side, then it is confirmed that there is a restriction of flow upstream of the motor. However, if the pressure on both sides is the same, but the motor is turning slower, the motor is failing; it is not operating efficiently to convert hydraulic power to mechanical output.

6. Swap the counterbalance valves between the two travel motors and retest. If the slow side follows the valve to the other motor, the counterbalance valve is the fault. This is a low-cost test that needs little disassembly. Follow the manual, and make sure the machine is blocked and the system is depressurized before removing any valve.

Component-Level Fault Analysis

Travel Motor Wear

Occurs at the cylinder block, pistons, and valve plate of the axial piston motor. When clearances open, the flow increases inwards, and the volumetric efficiency decreases. The motor gets the right flow, but it is not converted into rotation of the shaft, so travel is slow on that side.

Travel Control Valve Spool

A precision-machined spool slides in its bore to control flow. Dirty hydraulic oil wears the spool and bore, which opens up the clearance and lets flow bypass the motor circuit. A spool stuck by contamination or varnish, or one with a weak centering spring, will not open fully when travel is commanded.

Worn Seat or Damaged Spool 

A worn or damaged counterbalance valve can create excess back-pressure at the motor outlet, which slows travel on that side and raises circuit temperature. A related symptom is motor runaway on grades, which happens when the valve does not hold enough pressure.

Wear in One Pump Section

This causes a reduction in volumetric efficiency in one section of the Pump, but not the other. This can be confirmed by measuring the flow through case drains from each section; if the flow is too high, then it means that the internal clearances of that section have exceeded serviceable limits.

Pilot Pressure Circuit

Pilot pressure is low or unequal, which causes the travel spool to not open completely on command. Faults are due to a pilot pump failure, pilot relief valve out of specification, blocked pilot filter, or a solenoid valve that fails to fully open; these are some of the easiest and least costly failures to diagnose.

Values to Take From the Service Manual

Pressure and speed values vary by model, serial number and software version, so always use the figures in the service manual for the exact machine. 

During a stall test, a reading below the travel circuit relief pressure on one side points to flow restriction or motor inefficiency on that circuit. A pilot pressure reading well below the specification points to a pilot circuit fault.

Technical Specifications: Travel System Reference 

Key Highlights:

• The baseline for travel circuit pressure testing is the relief pressure thresholds. If the reading is below the travel circuit relief value on one side of the circuit in a stall test, then it is a sign of flow restriction or motor inefficiency on that circuit.
• The pressure specification for the pilot circuit is used for measurements in the field when performing spool and pilot pump diagnostics. If the reading is over 3-4 bars lower than the spec, it is a confirmed pilot circuit fault.
• Both machines feature variable displacement travel motors with hydraulically controlled two-speed switching. If the switching signal is faulty, one motor can be locked in high displacement while the other motor operates in low displacement, giving the appearance of a flow or pressure fault, thus causing speed asymmetry.

Maintenance, Wear, And Hidden Costs

The health of the travel system is most greatly affected by the condition of the hydraulic fluid. The metal particles that are normal wear of components circulate in the travel spools, motor valve plates, and pump pistons, causing wear at each clearance surface. The two best practices to keep the travel system components in service longer are to change the fluid per the manufacturer’s recommendations and to use a filter with the correct micron rating.

Travel motor seal wear is a maintenance event that will gradually occur and is seldom addressed until it presents a visual symptom. Case drain flow will increase with internal seal degradation, which can be detected during regular service using a flow meter and short hose, before it becomes serious enough to impact travel speed.

The cost of waiting grows with every operating hour. Catching the fault early, for example by cleaning a spool or adjusting a pilot relief valve, is usually a small labor job. Left to run until the spool bore wears, the damage can spread to the control valve, a travel motor or the main pump, and those repairs cost far more. [

Total Cost Of Ownership And RoI Of Early Diagnosis

One-sided slow travel usually starts as a minor hydraulic efficiency problem, such as a slightly sticky spool, a slightly out-of-balance counterbalance valve, or a loss of volumetric efficiency in a pump section. Repair costs are low at this time, and downtime will be short. The machine corrects at the first symptom and uses a small portion of the repair budget of the machine that runs the fault until the fault component fails.

A travel fault also wastes energy. When the hydraulic system loses efficiency, the engine has to supply more power for the same track output, and the lost power turns into heat. Over thousands of operating hours, that can add up in fuel cost even though it never appears on a repair order.

Buying Guide: Detecting Travel Faults On A Used Excavator

Any used track excavator pre-purchase inspection should involve an evaluation of the travel system. These are the tests that are required before the 20-30 ton class is purchased:

• Perform a full-speed travel test on flat ground both forwards and backwards. The machine should run straight, both ways, without the operator making any corrections.
• Test each type of travel (fast and slow) separately. Fault only at high speed in the two-speed switching system or motor displacement control.
• Ask for a sample of hydraulic oil to be sent to the lab for testing. High levels of iron and copper particles can be a sign of wear and tear in motors, pumps, or valve parts even before symptoms are apparent.
• If possible, check the flow of case drain on both travel motors. If case drain flow is much higher on one travel motor than the other, that is a known sign of motor wear.
• Check the service history for previous trips involving repairs to the travel system, replacement of hydraulic components, or contamination.

When inspecting, if a travel fault is identified, include the cost of the repair in the offer price. There is no need to make a spool/counterbalance valve issue a deal breaker. A high-hour machine, with no service history, and an unknown pump or motor condition, is a more serious risk and will either need to be offered at a price that will allow for a worst-case scenario repair, or it will need to be rejected.

Local Market Conditions: Texas And Gulf Coast Factors

Conditions in the Houston area and along the Gulf Coast can speed up travel system wear. Soft, clay-heavy soils and high humidity can raise the risk of hydraulic contamination, through seal ingestion and moisture entering through the breathers.

Clay-packed tracks can create an imbalance in the undercarriage and exacerbate a slight hydraulic imbalance to become an obvious travel fault. Clay-heavy material operators should monitor track tension more often than normal, especially following wet conditions. The oil change interval for hydraulic systems should be based on actual oil analysis results, not on a fixed oil change schedule. Oil in high-contamination applications can be changed more often, or oil can be filtered in the oil kidney loop between oil changes. Contractors sourcing used heavy equipment for sale in Texas should factor these conditions into any pre-purchase inspection.

Operator Training And Fleet Manager Responsibilities

While operators are the initial point of detection for travel faults, they need to be aware of what they should be looking for and reporting. Most will discover the pull to one side problem well before it becomes serious, but with no system of reporting, that information will remain in the cab and not be passed to the maintenance team.

Fleet managers need to educate operators to be able to tell the difference between a travel fault and a grade effect. A machine that crosses a uniform side-slope will naturally be subject to the force of gravity and will, therefore, tend to move downhill; it is not a fault. A machine is moving on flat and level ground in both forward and reverse directions exhibiting a fault symptom. The straight-line travel test on flat ground should be short and form part of the pre-shift checks. The steering correction applied by the operator is reported at the end of the shift if the operator is applying the same steering correction to keep the path straight all the time, not silently until the fault worsens.

Final Verdict: Hydraulic First, Mechanical Second

If an excavator runs slowly on one side, start with the quick mechanical checks: track tension, debris, rollers and brake release. If those are clear, test the hydraulics before you order any undercarriage part: check pilot pressure, test main circuit pressure at each travel motor, swap the counterbalance valves and check pump section output. Many one-sided travel faults are found at this level, and fixing them costs far less than replacing tracks, rollers or motors that never caused the fault.

If you are looking at a crawler for sale, MY Equipment lists used excavators with specifications, and you can book an inspection and appraisal to have the travel system checked before you buy. Browse used track excavators, or ask about an inspection.

FAQs

Why does my excavator pull to one side when traveling? 

Unequal hydraulic flow to the travel motors is a common cause, but check track tension, debris and travel brake release first.

How do I know if the fault is hydraulic or mechanical? 

Start with the quick mechanical checks: track tension, debris, rollers and brake release. If they are clear, compare pilot and travel pressures on both sides and check case drain flow.

Can a counterbalance valve cause one-sided slow travel? 

Yes, a faulty valve can restrict hydraulic flow to one travel motor.

What does case drain flow tell you about a travel motor? 

High case drain flow can indicate internal wear inside the motor.

Should I replace undercarriage components if the machine pulls to one side? 

Not until you have ruled out the cause. Check tension and brake release first, then test the hydraulics. Replace parts only when the tests point to them.

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