CAT E120B Excavator Reliability and Restoration Insights

[MikePhua]

The Development of the E120B Series
The Caterpillar E120B hydraulic excavator was introduced in the late 1980s as part of Caterpillar’s global expansion into mid-size excavators. Built to compete with Japanese and European models in the 12–14 metric ton class, the E120B combined Caterpillar’s rugged frame design with Japanese hydraulic finesse. Powered by the Cat 3114 engine delivering approximately 84 horsepower, the E120B offered a solid balance of digging depth, reach, and lifting capacity for general construction, utility trenching, and light demolition.
Caterpillar, founded in 1925, had already established dominance in dozers and loaders, and the E-series excavators helped solidify its presence in the hydraulic excavator market. The E120B was manufactured in Japan under joint ventures, which contributed to its compact dimensions and smooth hydraulic control. Thousands of units were sold globally, and many remain in service today, especially in developing regions and owner-operator fleets.
Terminology Note

• Swing Motor: A hydraulic motor that rotates the upper structure of the excavator.
  
• Swivel Joint: A rotating hydraulic coupling that allows fluid transfer between upper and lower frames.
  
• Final Drive: The gear assembly that powers the tracks, converting hydraulic pressure into movement.
  
• Case Drain Filter: A fine mesh filter that captures debris from hydraulic fluid returning from motors.
  

Core Specifications and Performance
The E120B features:

• Operating weight: ~27,000 lbs (12,250 kg)
  
• Digging depth: 19.9 ft (6.06 m)
  
• Reach at ground level: 28.6 ft (8.72 m)
  
• Bucket breakout force: ~20,000 lbf
  
• Hydraulic pump flow: 62.4 gpm (236 L/min)
  
• Travel speed: 3.2 mph (5.1 km/h)
  

Its compact footprint and low ground pressure (~6 psi) make it suitable for urban excavation and soft terrain. The machine’s boom and arm geometry allow efficient trenching and loading, while the undercarriage provides stable tracking even on uneven surfaces.
Common Issues and Field Diagnostics
Despite its reliability, the E120B is not immune to age-related problems. Frequent issues include:

• Hydraulic system leaks due to worn seals and hoses
  
• Engine overheating from clogged radiators or faulty thermostats
  
• Electrical faults in control circuits and sensors
  
• Track motor imbalance or directional failure
  
• Swing hesitation or drift due to swivel joint wear
  

One operator reported that the left track would only move in reverse, while the right track functioned normally. After inspecting the hydraulic lines and testing pressure, the issue was traced to a stuck valve and a partially clogged case drain filter behind the final drive. Cleaning the filter and resealing the swivel joint restored full travel control.
Hydraulic System Maintenance and Solutions
To maintain optimal hydraulic performance:

• Inspect and replace hydraulic filters every 500 hours
  
• Check fluid levels and condition monthly
  
• Monitor pilot pressure and actuator response
  
• Clean or replace the case drain filter annually
  
• Reseal the swivel joint if tracking or swing becomes erratic
  

A technician in British Columbia rebuilt the swivel joint on an E120B after noticing slow swing and poor tracking. The internal seals had hardened, and debris had accumulated in the rotary passages. After resealing and flushing the system, the machine regained full responsiveness.
Engine and Cooling System Care
The Cat 3114 engine is robust but sensitive to cooling system neglect. Recommendations include:

• Flush coolant every 1,000 hours
  
• Replace radiator cap and thermostat every two years
  
• Inspect water pump for leaks or bearing noise
  
• Use Cat ELC or equivalent coolant with proper additives
  

In one case, a contractor replaced the radiator and water pump after repeated overheating. The new components, combined with a clean cooling circuit, eliminated the issue and improved fuel efficiency.
Electrical System and Control Reliability
Electrical faults can mimic hydraulic problems. Common culprits:

• Corroded connectors near the cab floor
  
• Stiff foot pedals due to lack of lubrication
  
• Faulty solenoids or relays in the control box
  
• Weak battery or poor ground connections
  

An operator discovered that his E120B’s pedals were too stiff to use, leading him to rely solely on joystick controls. After lubricating the pedal pivots and adjusting the return springs, the machine regained full foot control, improving fine movement during trenching.
Wear Components and Structural Integrity
Key wear points include:

• Bucket teeth and cutting edges
  
• Track chains and sprockets
  
• Boom-to-arm pins and bushings
  
• Swing bearing and gear teeth
  

To extend life:

• Use wear-resistant bucket liners
  
• Monitor track tension and replace worn pads
  
• Grease all pivot points daily
  
• Inspect swing gear backlash annually
  

A fleet in Texas retrofitted their E120B buckets with hardened edges and saw a 40% increase in service life when working in abrasive soils.
Operator Tips and Restoration Advice
For older units:

• Perform pre-operation checks including fluid levels, pedal movement, and track response
  
• Park on clean ground to detect leaks overnight
  
• Use diagnostic tools to monitor pump pressure and valve response
  
• Train operators to recognize early signs of hydraulic imbalance
  

A restoration project in New Zealand involved replacing the entire hydraulic hose set, rebuilding the swing motor, and installing a new seat and control panel. The machine returned to full service and was used for land clearing and drainage work.
Conclusion
The CAT E120B remains a dependable mid-size excavator with a strong mechanical foundation and adaptable hydraulic system. While age introduces challenges, most issues can be resolved with methodical diagnostics and quality parts. From swing motor rebuilds to electrical tune-ups, the E120B continues to prove its worth in the field. With proper care, it can deliver thousands more hours of productive service across construction, agriculture, and land development.