Diagnosing Track Motor Imbalance and Hydraulic Issues on the Hitachi EX120-1

[MikePhua]

The Hitachi EX120-1 and Its Mechanical Simplicity
The Hitachi EX120-1 hydraulic excavator, introduced in the early 1990s, represents a generation of machines built with mechanical reliability and straightforward hydraulic architecture. Powered by an Isuzu diesel engine and equipped with a dual-pump hydraulic system, the EX120-1 was designed for mid-size excavation, trenching, and site preparation. Hitachi Construction Machinery, founded in 1970, became globally recognized for its EX series, which sold extensively across Europe, Asia, and North America.
The EX120-1 features pilot-operated controls, a swing motor, and two travel motors—each fed by separate hydraulic circuits. Its design favors field serviceability, making it popular among owner-operators and small contractors. However, age-related wear and undocumented modifications can complicate diagnostics, especially when symptoms appear in the travel system.
Identifying Uneven Track Speeds and Motor Behavior
A common issue in aging EX120-1 units is uneven track speed—where one travel motor consistently runs faster than the other, regardless of direction. This imbalance causes the machine to veer during travel and can indicate internal leakage, pump imbalance, or control valve malfunction.
Operators often notice the discrepancy when lifting the tracks off the ground and spinning them at idle. A significant speed difference, such as one motor turning twice as fast as the other, suggests a deeper hydraulic fault rather than simple wear.
Initial checks should include:

• Comparing forward and reverse speeds on both tracks
  
• Observing motor behavior at different engine RPMs
  
• Inspecting the swivel joint for leaks or contamination
  
• Verifying the function of the travel speed selector switch
  

In one case, resealing the swivel joint eliminated external leaks but did not resolve the motor imbalance, pointing to internal hydraulic issues.
Understanding Hydraulic Flow and Case Drain Function
Each travel motor on the EX120-1 is connected via three hydraulic lines:

• Two large hoses for forward and reverse drive flow
  
• One small hose for case drain, which returns internal leakage to the tank
  

The case drain line acts like a crankcase breather in an engine, allowing excess fluid from internal leakage to escape. A high flow rate from the case drain indicates worn motor seals or internal bypassing. A healthy motor should produce only a light dribble during operation.
To test motor condition:

• Disconnect the case drain line
  
• Elevate the track and run the motor at low RPM
  
• Observe the flow from the drain line
  

Excessive flow suggests the motor is bypassing internally and may require rebuild or replacement.
Travel Speed Control and Solenoid Activation
The EX120-1 uses a hare/tortoise switch to toggle travel speed. This system is governed by a microcontroller and a bank of solenoids near the pilot filter. When functioning correctly, the switch adjusts engine RPM and hydraulic flow to the motors.
In some units, disconnected or failed solenoids prevent the system from engaging high-speed travel. Reconnecting the solenoids and verifying voltage at the control leads can restore functionality. The travel speed system does not alter motor displacement directly but modulates engine output and flow rate.
Pump Configuration and Flow Distribution
Contrary to some assumptions, the EX120-1 is equipped with a dual hydraulic pump system. Each pump supplies oil to specific functions:

• One pump feeds the swing motor, dipper arm, and one travel motor
  
• The other pump supplies the boom, bucket, and the opposite travel motor
  

This configuration means that pump imbalance can manifest as uneven track speed. To test pump output:

• Compare cylinder speeds controlled by each joystick
  
• Time the boom lift cycle from ground to full extension
  
• Observe any lag or hesitation during simultaneous operations
  

A healthy boom lift cycle should complete in 4–5 seconds at full throttle. Slower response may indicate pump wear or flow restriction.
Final Drive Leaks and Motor Failure Indicators
In one documented case, the faster motor developed a hydraulic leak from a welded patch on the final drive casing. This area, normally filled with gear oil, should not contain hydraulic fluid. The presence of hydraulic oil in the gear housing suggests a failed motor seal, allowing high-pressure fluid to breach into the gear case.
To address this:

• Split the track and remove the final drive
  
• Mark hose positions for reinstallation
  
• Inspect the motor internals for metal shavings or seal damage
  
• Replace or rebuild the motor as needed
  

Hitachi has historically maintained strong parts support for EX-series machines, but availability may vary by region. Rebuilt motors are often more economical than new units and can restore full performance if properly installed.
Recommendations for Long-Term Reliability
To maintain and troubleshoot the EX120-1 effectively:

• Keep a detailed service log with hose routing, serial numbers, and test results
  
• Regularly inspect swivel joints, case drains, and solenoid connections
  
• Use hydraulic flow meters or pressure gauges for deeper diagnostics
  
• Replace worn bottom rollers and track components to reduce strain on motors
  
• Clean and repaint exposed components to prevent corrosion and improve visibility
  

Operators should also familiarize themselves with the machine’s hydraulic layout and control logic. Understanding how flow is distributed and how each subsystem interacts can dramatically improve repair outcomes.
Conclusion
The Hitachi EX120-1 remains a capable and serviceable excavator, even decades after its release. Diagnosing uneven track speed and hydraulic anomalies requires a blend of mechanical insight and methodical testing. From case drain analysis to solenoid troubleshooting, each step reveals more about the machine’s condition. With patience and precision, even a first-time owner can restore performance and extend the life of this classic workhorse. In the world of earthmoving, knowledge is the most powerful tool—and the EX120-1 still has plenty of dirt to move.