Hydraulic System of the 1985 CAT IT28: Architecture, Functionality, and Field Insights

[MikePhua]

Introduction: A Loader Built on Simplicity and Strength
The 1985 Caterpillar IT28 wheel loader represents a transitional era in hydraulic design—where mechanical reliability met evolving control sophistication. Its hydraulic system, though relatively simple by modern standards, incorporates features that reflect Caterpillar’s commitment to durability and operator responsiveness. This article explores the system’s architecture, terminology, and practical insights from field experience.
Key Terminology Explained

• Fixed Displacement Pump: A hydraulic pump that delivers a constant volume of fluid per rotation, regardless of system demand.
  
• Closed Center Valve: A control valve that blocks flow when in neutral, allowing pressure to build until needed.
  
• Load Sensing (LS): A system that adjusts pump output based on demand, improving efficiency and reducing heat.
  
• Dump Valve: A bypass mechanism that redirects flow to the tank when no implement movement is required.
  
• Control Valve (CV): The assembly that directs hydraulic flow to specific functions like lift, tilt, or steering.
  

System Architecture and Function

• The IT28 uses two fixed displacement pumps:
  • One dedicated to steering.
    
  • One dedicated to front-end loader (FEL) functions.
    
• The system is closed center, meaning:
  • When the control valve is in neutral, pump flow is bypassed to the tank.
    
  • This prevents unnecessary pressure buildup and reduces energy loss.
    
• It incorporates a load sensing constant flow design, where:
  

• A dump valve bypasses flow when no implement movement is requested.
  
• The system maintains pressure readiness without continuous flow through the valve.
  

Field Insight: Serial Number Confirmation
A technician working on a 1986 IT28 confirmed the serial number as 8JB00809, verifying the system’s configuration through CAT service documentation. This highlights the importance of serial-specific data when diagnosing or sourcing parts.
Anecdote: Diagnosing a Lazy Loader
In Port Angeles, Washington, an operator noticed sluggish lift response on his IT28. After confirming the system was closed center with a dump valve, he traced the issue to a partially stuck spool in the control valve. Cleaning and resealing the valve restored full function—underscoring how even simple systems require precise maintenance.
Historical Context: The Shift from Open to Closed Center
During the early 1980s, many loaders transitioned from open center systems—where fluid constantly circulated—to closed center systems that conserved energy and reduced heat. The IT28’s adoption of a closed center, load-sensing design placed it ahead of its time, offering smoother control and better fuel efficiency.
Best Practices for Maintenance and Troubleshooting

• Identify Pump Functionality
  Confirm which pump serves which function before diagnosing flow issues.
  
• Inspect Dump Valve Operation
  A stuck or misadjusted dump valve can cause poor implement response.
  
• Check Control Valve Neutral Position
  Ensure the valve fully returns to neutral to allow proper bypass.
  
• Use Serial Number for Accurate Schematics
  Variations exist between production years; always reference the correct documentation.
  
• Monitor for Heat and Noise
  Excessive heat or growling may indicate flow restriction or valve malfunction.
  

Case Study: Swedish Technician’s Confirmation
A senior technician in Sweden verified the system’s configuration using CAT schematics, confirming it as a load-sensing, closed center system with a dump valve. His experience emphasized the value of international collaboration and manufacturer support in resolving hydraulic mysteries.
Conclusion: A System That Rewards Understanding
The 1985 CAT IT28 hydraulic system may appear straightforward, but its closed center, load-sensing design demands respect. With proper diagnostics, clean fluid, and attention to valve behavior, this loader continues to deliver reliable performance decades after its debut. For mechanics and operators alike, understanding its hydraulic heartbeat is key to keeping the iron moving.