CAT E70B Hydraulic Troubleshooting and System Recovery

[MikePhua]

The E70B and Its Role in Compact Excavation
The Caterpillar E70B hydraulic excavator was introduced in the late 1980s as part of CAT’s compact lineup, designed for urban excavation, utility trenching, and light demolition. With an operating weight of around 7,000 kg and powered by a four-cylinder Mitsubishi diesel engine, the E70B offered a balance of maneuverability and digging power. Its hydraulic system, though mechanically simple compared to modern machines, was engineered for reliability and ease of service.
The E70B uses an open-center hydraulic system with gear-type pumps and mechanical pilot controls. While robust, this system is sensitive to contamination, wear, and pressure loss. When hydraulic issues arise—such as slow response, weak lifting, or erratic movement—diagnosing the root cause requires a methodical approach.
Terminology Notes

• Open-Center System: A hydraulic design where fluid flows continuously through the control valves until a function is activated.
  
• Pilot Control: A low-pressure hydraulic signal used to actuate main control valves.
  
• Hydraulic Drift: Unintended movement of cylinders due to internal leakage.
  
• Relief Valve: A safety valve that limits system pressure to prevent damage.
  
• Pump Cavitation: A condition where air bubbles form in the pump due to low fluid or suction restriction.
  

Common Hydraulic Symptoms in the E70B
Operators may encounter:

• Boom or stick movement slower than normal
  
• Bucket curl lacking force or stalling under load
  
• Swing function hesitating or jerking
  
• Hydraulic fluid foaming or overheating
  
• Audible whining or growling from the pump
  

In one trenching job in Arizona, an E70B began to lose lifting power mid-shift. The operator noticed the fluid level was fine, but the machine struggled to raise the boom. After inspection, a clogged suction screen was found restricting flow to the pump.
Diagnostic Strategy and Pressure Testing
To isolate the issue:

• Check hydraulic fluid level and condition (should be clear, amber, and free of foam)
  
• Inspect suction screen and return filters for debris
  
• Use a pressure gauge to test pump output at multiple ports
  
• Compare readings to factory specs (main pump ~2,500 PSI, pilot ~400 PSI)
  
• Inspect pilot lines for leaks or kinks
  
• Test relief valve function by deadheading a cylinder and observing pressure spike
  

In one fleet in British Columbia, technicians used a flow meter to confirm that the main pump was delivering only 60% of rated flow. The pump was rebuilt with new gears and seals, restoring full performance.
Component-Specific Troubleshooting

• Hydraulic Pump
  • Symptoms: Weak functions across all circuits
    
  • Test: Flow and pressure at full throttle
    
  • Solution: Rebuild or replace pump, inspect drive coupling
    
• Control Valves
  • Symptoms: One or two functions weak or unresponsive
    
  • Test: Swap pilot lines to isolate valve body
    
  • Solution: Clean spool, replace seals, check spring tension
    
• Pilot System
  • Symptoms: Controls feel soft or delayed
    
  • Test: Pilot pressure at joystick base
    
  • Solution: Replace pilot pump or clean pilot filter
    
• Relief Valve
  

• Symptoms: Functions stall under load
  
• Test: Pressure fails to reach spec under deadhead
  
• Solution: Adjust or replace relief valve
  

In one restoration project in Texas, a retired contractor rebuilt his E70B’s control valve using a seal kit and emery cloth to polish the spool. After reassembly, the machine regained smooth control and digging force.
Preventive Maintenance and Fluid Care
To avoid future issues:

• Change hydraulic fluid every 1,000 hours or annually
  
• Replace return filter every 500 hours
  
• Clean suction screen during each fluid change
  
• Use ISO 46 hydraulic oil with anti-foam additives
  
• Inspect hoses quarterly for abrasion and leaks
  

In one municipal fleet in Ohio, switching to synthetic hydraulic fluid improved cold-weather performance and reduced pump wear during winter operations.
Upgrade Options and System Enhancements
While the E70B is mechanically simple, some upgrades can improve reliability:

• Install magnetic drain plugs to catch early metal wear
  
• Add pilot pressure gauge to monitor joystick response
  
• Retrofit with quick couplers for faster hose replacement
  
• Use external hydraulic filter with visual clog indicator
  
• Add inline temperature sensor to monitor fluid heat
  

In one demolition crew in Florida, adding a pilot pressure gauge helped diagnose joystick lag during hot weather. The pilot pump was replaced, and control response returned to normal.
Final Thoughts
The CAT E70B remains a dependable compact excavator, but its hydraulic system demands clean fluid, healthy pump output, and responsive pilot controls to perform at its best. When problems arise, the solution is rarely electronic—it’s mechanical, tactile, and rooted in pressure and flow. With careful diagnostics and disciplined maintenance, the E70B can dig, lift, and swing with the confidence of a machine twice its size. In the rhythm of excavation, hydraulics aren’t just power—they’re precision under pressure.