Diagnosing Hydraulic Failure in the CAT E120B Excavator

[MikePhua]

The CAT E120B and Its Mechanical Heritage
The Caterpillar E120B hydraulic excavator was introduced in the early 1990s as part of CAT’s compact-medium class lineup. Designed for versatility in urban construction, utility trenching, and light demolition, the E120B featured a robust undercarriage, a swing boom, and a dual-pump hydraulic system. With an operating weight of approximately 27,000 lbs and a bucket breakout force exceeding 20,000 lbs, it became a popular choice for contractors seeking reliability and manageable transport size.
Caterpillar, founded in 1925, had by then become a global leader in earthmoving equipment. The E-series excavators were built to compete with Japanese and European models, offering mechanical simplicity and parts interchangeability. The E120B was powered by a Mitsubishi diesel engine and used a tandem hydraulic pump system to drive all major functions.
Terminology Annotation

• Hydraulic Pump Coupler: A flexible mechanical link between the engine flywheel and the hydraulic pump input shaft, often made of rubber or composite material.
  
• Pilot Pump: A small auxiliary pump that supplies low-pressure hydraulic fluid to control valves and pilot circuits.
  
• Swivel Joint: A rotating hydraulic manifold that allows fluid transfer between the upper and lower structures of the excavator.
  
• Case Drain Filter: A fine filter that captures wear particles from hydraulic motors and pumps before returning fluid to the tank.
  

Symptoms of Hydraulic Failure
Operators reported several symptoms indicating hydraulic distress:

• Vibration at the pump when operating any hydraulic function
  
• Right track moving significantly slower than the left
  
• Reduced digging power and sluggish bucket response
  
• Boom and stick vibrating during movement
  

These symptoms suggest a loss of pressure or flow in one of the main hydraulic circuits, possibly due to pump degradation, coupler failure, or contamination.
Initial Diagnostic Steps
To isolate the problem, technicians recommended:

• Swapping the two main output hoses from the pump to test whether the issue follows the hose or remains with the track
  
• Inspecting the hydraulic filter for metal shavings, especially brass or aluminum
  
• Cutting open the return filter and case drain filter to check for internal pump wear
  
• Monitoring the swing and digging functions to determine if both pumps are affected or only one
  

In one case, brass filings were found in the front pump, indicating internal wear. The coupler between the engine and pump was also completely destroyed, with no rubber remaining and one cog drilled out of the flywheel to remove debris.
Pump Failure and System Contamination
The E120B uses a dual-pump system, with each pump driving separate circuits. If one pump fails, it can affect travel speed, swing torque, and digging force. When pump components degrade, brass and steel particles enter the hydraulic fluid, risking contamination of valves, cylinders, and motors.
To prevent catastrophic failure:

• Shut down the machine immediately upon detecting vibration or performance loss
  
• Flush the hydraulic system thoroughly before installing new components
  
• Replace all filters and fluid, including pilot and case drain filters
  
• Consider a “kidney loop” filtration service after 20 hours of operation to remove residual particles
  

A CAT technician warned that running the machine with a failing pump could lead to a full system teardown. A similar case involving a CAT 330 resulted in an $80,000 repair bill due to pump explosion and widespread contamination.
Repair Costs and Component Replacement
The quoted repair for the E120B included:

• New front and rear hydraulic pumps
  
• New pilot pump
  
• Drive coupler replacement
  
• Full fluid and filter change
  
• 20-hour post-repair filtration service
  

Total cost: approximately $8,000. While high, this price reflects the labor-intensive nature of pump replacement and the need to prevent future damage. Some operators may find rebuilt pumps or aftermarket alternatives at lower cost, but warranty and compatibility must be verified.
Preventive Measures and Long-Term Solutions
To extend the life of hydraulic components:

• Monitor pump noise and vibration regularly
  
• Replace couplers every 3,000–4,000 hours or upon signs of wear
  
• Use OEM-grade hydraulic fluid and filters
  
• Install magnetic drain plugs to capture metal particles
  
• Perform annual fluid analysis to detect early contamination
  

In 2023, a fleet manager in Texas implemented a quarterly hydraulic inspection protocol across 15 excavators. Within a year, pump-related failures dropped by 60%, and average repair costs were reduced by 35%.
Conclusion
Hydraulic failure in the CAT E120B often stems from pump wear, coupler degradation, and fluid contamination. Early detection and decisive action can prevent extensive damage and costly repairs. With proper maintenance, filtration, and component monitoring, the E120B remains a capable and durable excavator for mid-scale operations. In hydraulic systems, silence and smoothness are signs of health—vibration and noise are warnings not to ignore.