CAT E120B Hydraulic Pump Failure and Repair Strategies

[MikePhua]

The CAT E120B and Its Hydraulic Architecture
The Caterpillar E120B excavator was introduced in the early 1990s as part of CAT’s compact-medium class lineup, designed for urban excavation, utility trenching, and light demolition. Powered by a turbocharged Mitsubishi diesel engine, the E120B delivered around 90 horsepower and featured a twin-pump hydraulic system capable of precise multi-function control. Its popularity stemmed from a balance of power, maneuverability, and serviceability, with thousands of units sold across Asia, Europe, and North America.
At the heart of its hydraulic system is a dual piston-type main pump, responsible for powering the boom, stick, bucket, swing, and travel motors. The pump is driven directly by the engine via a rubber-cushioned coupler, which absorbs vibration and torque spikes. Over time, this coupler and the pump itself become wear points—especially in machines exceeding 7,000 hours or operating under high heat and load.
Symptoms of Hydraulic Pump Failure
Operators typically report the following signs when the E120B’s main pump begins to fail:

• Sluggish boom and bucket response
  
• Uneven track speed, especially under load
  
• Vibration or noise from the pump compartment
  
• Hydraulic functions weaken as oil temperature rises
  
• Metallic or brass particles found in filters
  

In one documented case, the right track of an E120B slowed to half speed while the left remained normal. Simultaneously, the boom and stick vibrated during operation. These symptoms pointed to partial pump failure—likely one piston group losing pressure or a bearing degrading.
Coupler Damage and Its Consequences
The rubber drive coupler between the engine and pump is a known weak point. When it fails, the pump may rotate unevenly or not at all. In severe cases, the coupler disintegrates, leaving metal fragments and rubber debris inside the pump housing.
A technician in Colorado discovered that the coupler on an E120B had completely disintegrated, requiring the flywheel to be drilled and tapped to remove embedded cogs. The pump showed brass filings—evidence of internal wear—and the machine had to be shut down immediately to prevent further damage.
Recommendations:

• Inspect coupler every 2,000 hours or during major service
  
• Replace if rubber shows cracking, separation, or deformation
  
• Use OEM or high-quality aftermarket couplers rated for torsional damping
  

Filter Inspection and Early Detection
Hydraulic filter analysis is a critical diagnostic tool. The E120B uses a large return filter and a case drain filter. Cutting open these filters can reveal early signs of pump wear:

• Brass particles suggest bushing or thrust washer failure
  
• Silver flakes indicate gear or piston scoring
  
• Black sludge may point to seal degradation or oil oxidation
  

Operators should:

• Replace filters every 500 hours or sooner in dusty environments
  
• Cut open old filters and inspect with a magnet and flashlight
  
• Monitor oil temperature and pressure during operation
  

In one case from British Columbia, a contractor avoided a $10,000 rebuild by catching brass particles early and replacing the pump before catastrophic failure.
Swivel Joint and Hydraulic Isolation
Some symptoms may mimic pump failure but originate from the swivel joint (rotary manifold) that transfers hydraulic fluid between upper and lower structures. Worn seals in the swivel can cause internal bypass, leading to weak travel power and erratic function.
To isolate the issue:

• Test each pump outlet with pressure gauges
  
• Compare left and right track motor response
  
• Inspect swivel joint seals and replace if bypassing is detected
  
• Mark all hoses and ports before disassembly to avoid misrouting
  

Swivel seal kits typically cost around $300 and require a press or custom puller to reinstall the sleeve. A technician in Texas completed the job in one day, restoring full travel power.
Pump Rebuild or Replacement Options
When the main pump fails, operators face a choice:

• Rebuild the existing pump
  
• Install a remanufactured unit
  
• Purchase a new OEM or aftermarket pump
  

Rebuilds may cost $6,000–$8,000 depending on labor and parts. Remanufactured pumps can be sourced for less, but compatibility and warranty vary. New pumps offer reliability but may exceed $10,000.
Best practices:

• Flush the entire hydraulic system before installing a new pump
  
• Replace all filters and inspect hoses for contamination
  
• Use break-in oil for the first 100 hours after rebuild
  
• Monitor pressure and temperature closely during initial operation
  

In one fleet in Florida, switching to remanufactured pumps with extended warranties saved over $30,000 across five machines in two years.
Preventive Maintenance and Long-Term Reliability
To extend the life of the E120B’s hydraulic system:

• Use high-quality hydraulic oil with anti-wear additives
  
• Maintain oil temperature below 180°F during operation
  
• Inspect pump coupler and mounting bolts annually
  
• Replace seals and gaskets during major service intervals
  
• Avoid simultaneous travel and digging under full load
  

Operators in tropical climates often install auxiliary coolers to reduce oil temperature. In Malaysia, a contractor reported a 20% increase in pump life after adding a fan-cooled heat exchanger.
Conclusion
The CAT E120B’s main hydraulic pump is a robust but vulnerable component. With proper diagnostics, early filter inspection, and attention to coupler integrity, operators can prevent costly failures and extend machine life. Whether trenching in urban zones or clearing land in remote regions, the E120B remains a reliable workhorse—provided its hydraulic heart is kept strong and clean.