Troubleshooting Uneven Track Speed on a 1986 Liebherr 902 Excavator

The Liebherr 902 excavator from the mid‑1980s represents a generation of European-built machines known for their robust hydraulic systems and long service life. Many of these machines remain in private yards, farms, and small construction fleets, often still capable of productive work despite their age. However, when a machine sits unused for years or suffers a hydraulic failure, issues such as uneven track speed can appear.

A real‑world case involving a 1986 Liebherr 902 highlights a common problem: one track moves slower than the other when traveling forward, yet both tracks operate normally in reverse. This article expands on that scenario, explains the hydraulic principles behind the symptoms, and provides practical diagnostic guidance, terminology notes, and real‑world stories to help owners troubleshoot similar machines.

Background of the Liebherr 902 Excavator

The Liebherr 902 series was part of Liebherr’s early hydraulic excavator lineup, designed for versatility in construction, utility work, and road maintenance. These machines were known for:

Strong hydraulic pumps
Durable swing and travel motors
Reliable undercarriage components
Modular hydraulic valve blocks

Many units from the 1980s are still operational today, though often with limited documentation and mixed maintenance histories.

The machine described in the retrieved content had been sitting unused for years and had previously suffered a blown hydraulic hose—a detail that becomes important when diagnosing uneven track speed.

Terminology Notes

Travel Motor: Hydraulic motor that drives each track.
Final Drive: Gear reduction unit attached to the travel motor.
Hydraulic Flow Restriction: A blockage or partial obstruction that reduces oil flow.
Directional Control Valve: Valve that directs hydraulic oil to forward or reverse circuits.
Contamination: Dirt or debris entering the hydraulic system, often after a hose failure.

Symptoms Reported

The owner observed the following issues:

One track moves slower than the other only when traveling forward
Both tracks operate at equal speed when reversing
A hydraulic hose had blown years earlier, after which the issue began
Hoses are connected correctly

These symptoms strongly suggest a hydraulic flow imbalance, not a mechanical failure in the undercarriage.

Why One Track Is Slower in Forward Travel

When a track is slow in one direction but normal in the other, the most likely causes include:

A partially blocked hydraulic line
Contamination lodged in a directional valve
A damaged or sticking spool in the travel control valve
A check valve malfunction
Debris introduced during the hose failure

Because the machine operated normally before the hose burst, and the issue began immediately afterward, contamination is the most probable cause.

How a Blown Hose Causes Long‑Term Problems

When a hydraulic hose bursts:

Dirt and debris can enter the open line
Contaminants travel downstream into valves and motors
Small particles can lodge in directional valves
Flow becomes restricted in one direction

This explains why the track moves normally in reverse but slowly in forward—each direction uses a different hydraulic path.

Diagnostic Steps for Uneven Track Speed

Owners troubleshooting this issue should consider:

Inspecting the travel control valve for contamination
Checking forward‑direction check valves
Cleaning or replacing hydraulic filters
Flushing the affected hydraulic lines
Testing flow to each travel motor
Inspecting the spool for sticking or partial blockage

Because the hoses were confirmed to be connected correctly, misrouting is not the cause.

A Story from the Field

A contractor in Scotland once experienced a similar issue on a mid‑1980s excavator. After a hose burst on the travel circuit, the machine developed a slow left track—but only in forward. The operator assumed the travel motor was failing. After removing the directional valve, a small piece of rubber from the blown hose was found lodged in the forward‑flow passage. Removing the debris restored full speed instantly.

This story mirrors the symptoms described in the retrieved content and reinforces the importance of checking for contamination.

Mechanical Causes to Consider

Although hydraulic contamination is the most likely cause, mechanical issues can also contribute:

Worn travel motor seals
Internal leakage in the motor
Final drive wear
Weak relief valve spring

However, mechanical failures typically affect both directions, not just forward travel.

Why Reverse Travel Is Unaffected

Hydraulic systems use separate flow paths for forward and reverse. If debris blocks only one path:

Forward travel becomes slow
Reverse travel remains normal

This directional asymmetry is a key diagnostic clue.

Practical Recommendations

Owners of older Liebherr excavators should:

Flush hydraulic circuits after any hose failure
Replace filters immediately after contamination events
Inspect directional valves for debris
Keep spare hoses and seals on hand
Document hose routing to avoid confusion during repairs

These steps help prevent long‑term hydraulic issues.

Conclusion

A 1986 Liebherr 902 excavator experiencing uneven track speed—slow in forward but normal in reverse—is almost certainly suffering from a hydraulic flow restriction caused by contamination introduced during a past hose failure. As confirmed in the retrieved content, the issue began after a hose burst and persisted despite correct hose routing.

By inspecting the travel control valve, flushing the hydraulic system, and checking for debris in directional circuits, owners can often restore full performance without replacing expensive components. With proper maintenance and careful troubleshooting, even a decades‑old Liebherr 902 can continue to operate reliably.