Restoring the Hitachi EX120-2 Excavator to Reliable Operation

[MikePhua]

The EX120-2 and Its Place in Hitachi’s Legacy
The Hitachi EX120-2 hydraulic excavator was part of the second-generation EX series introduced in the early 1990s, a time when Hitachi was expanding its global footprint in construction equipment. With an operating weight of approximately 26,000 pounds and powered by the reliable Isuzu 4BG1 engine, the EX120-2 was designed for versatility in mid-sized excavation tasks. Its popularity surged in North America and Southeast Asia, where contractors valued its balance of power, fuel efficiency, and compact footprint.
Hitachi Construction Machinery, founded in 1970 as a division of Hitachi Ltd., quickly became a global leader in hydraulic excavator technology. By the time the EX120-2 was phased out in favor of newer models, tens of thousands had been sold worldwide. The machine’s modular design and relatively simple hydraulic architecture made it a favorite among independent operators and small contractors.
Initial Purchase and Diagnostic Challenges
One owner acquired a used EX120-2 for $4,000—a price that reflected both opportunity and risk. The machine started easily, and basic functions like idle-up worked, but hydraulic operations caused engine bogging. A leak from the center of the undercarriage hinted at deeper issues. After replacing fuel filters and cleaning banjo fittings, performance remained unchanged.
Terminology:

• Banjo Fitting: A type of fluid connector with a hollow bolt and spherical union, commonly used in fuel and hydraulic systems.
  
• Center Joint: Also known as the swivel joint, it allows hydraulic fluid to pass between the upper and lower structures of the excavator while rotating.
  

The owner rebuilt the center joint for about $100 and replaced the hydraulic oil cooler and several hoses. These repairs stopped the oil leaks but did not resolve the bogging issue. A mechanic later diagnosed a lack of power to the PVC (Pump Control Valve), traced to a blown 1-amp fuse. Replacing the fuse improved performance slightly, but the machine still bogged under heavy hydraulic loads or when cylinders reached full stroke.
Electrical System Overhaul and Persistent Issues
The EX120-2’s electrical system had suffered from prior tampering. The inner harness showed signs of “butchering,” prompting the owner to replace both inner and outer wiring harnesses. Despite this, the bogging persisted. Additional replacements included:

• PVC (Pump Control Valve)
  
• Pump solenoids
  
• Angle sensor
  
• Plans to replace the DP (Discharge Pressure) sensor
  

Terminology:

• Pump Solenoid: An electrically actuated valve that controls hydraulic pump output.
  
• Angle Sensor: Measures the swash plate angle in variable displacement pumps, affecting flow rate.
  
• DP Sensor: Monitors pressure differential across hydraulic circuits to optimize pump control.
  

The mechanic advised halting further replacements until a full diagnostic scan could be performed. However, delays in technician availability led the owner to consider bypassing the electronics entirely using a conversion kit.
Conversion Kits and Mechanical Alternatives
Conversion kits for the EX120-2 aim to eliminate the electronic control system, replacing it with manual or simplified hydraulic controls. One operator in Ireland reported success with a conversion kit installed on an EX120-3, noting improved reliability and ease of use. The kit cost approximately €650 and was sourced from China. The machine retained good speed and responsiveness, though the operator had to manually shut off the ignition to prevent battery drain.
Suggested mechanical upgrades:

• Install a manual throttle to replace the unreliable stepper motor.
  
• Use analog pressure gauges to monitor hydraulic performance.
  
• Retrofit mechanical linkages for boom and bucket control if electronic feedback is disabled.
  

Terminology:

• Stepper Motor (EC Motor): Used in electronic throttle systems to adjust engine RPM based on operator input.
  
• Manual Throttle: A cable-operated lever that directly controls fuel delivery to the engine.
  

In Alaska, the original owner experienced erratic throttle behavior—sometimes the engine would idle down and stall when throttle-up buttons were pressed. Grinding noises from the stepper motor raised concerns about internal wear. Switching to a manual throttle was considered to reduce failure points.
Lessons from the Field and Practical Advice
Across multiple regions, EX120-2 owners have shared similar frustrations with aging electronics. While the hydraulic system remains robust, the electronic controls—especially the PVC, sensors, and throttle motor—often become unreliable after decades of use. Conversion kits offer a viable path forward, especially for operators in remote areas where diagnostic support is limited.
Preventive maintenance recommendations:

• Inspect and clean all electrical connectors quarterly.
  
• Replace fuses with OEM-rated components to avoid voltage irregularities.
  
• Monitor hydraulic fluid temperature and pressure during peak operation.
  
• Use dielectric grease on harness terminals to prevent corrosion.
  

In one case, a forestry operator in British Columbia retrofitted his EX120-2 with a manual throttle and bypassed the PVC entirely. The machine lost some fine control but gained consistency and uptime—critical for seasonal logging operations.
Conclusion
The Hitachi EX120-2 remains a capable excavator, but its aging electronic systems can undermine performance. Owners facing persistent bogging, throttle issues, or sensor failures may find relief in mechanical conversion kits or targeted electrical repairs. With proper upgrades and preventive care, the EX120-2 can continue serving reliably in excavation, forestry, and utility work. Its legacy as a durable, mid-sized machine lives on in the hands of those willing to adapt and innovate.