Why Is My CAT 973 Moving So Slowly

[MikePhua]

The CAT 973 and Its Hydrostatic Drive System
The Caterpillar 973 track loader was introduced in the early 1980s as part of CAT’s push to modernize its crawler loader lineup. Built with a hydrostatic transmission, the 973 offered smoother directional control, variable speed operation, and improved fuel efficiency compared to its torque converter predecessors. With an operating weight of over 30 tons and a bucket capacity exceeding 3.5 cubic yards, the 973 was designed for heavy-duty applications in mining, demolition, and bulk earthmoving.
Caterpillar’s hydrostatic system relies on two variable displacement pumps driving two hydraulic motors—one for each track. This setup allows for infinite speed variation and precise maneuvering. However, it also introduces sensitivity to oil viscosity, temperature, and component wear.
Symptoms of Slow Travel Without Power Loss
Operators have reported that their 973 moves sluggishly in both forward and reverse, despite maintaining full engine power and pushing capability. The machine behaves normally when cold but slows down significantly after reaching operating temperature. This pattern suggests a hydraulic issue rather than a mechanical failure.
Key symptoms include:

• Reduced ground speed after warm-up
  
• No change in speed when throttle is reduced
  
• Normal pushing force and breakout power
  
• No fault codes or warning lights
  

These signs point to a possible mismatch between hydraulic oil viscosity and system requirements, or a malfunction in the underspeed override system.
Terminology and System Components
- Underspeed Override Valve: A hydraulic control that limits travel speed unless the engine is at wide open throttle (WOT). Designed to prevent overloading the drive system at low RPM.
- Charge Pump: A small auxiliary pump that supplies pilot pressure and maintains fluid volume in the hydrostatic loop. Critical for maintaining drive motor responsiveness.
- Pump Drive Box: A gearbox that splits engine power to drive the two hydrostatic pumps. Requires its own oil reservoir, typically around one gallon.
- ISO Viscosity Grades: International standards for hydraulic oil thickness. Common grades include ISO 32, ISO 46, and ISO 68.
Impact of Oil Viscosity and Brand Choice
One operator discovered that after replacing a ruptured drive hose, he refilled the hydraulic system with 303 universal oil—a low-cost fluid not designed for high-pressure hydrostatic systems. The machine became noticeably slower, especially after warming up. After switching to ISO 46 hydraulic oil, performance dramatically improved, restoring the loader’s responsiveness to near skid steer levels.
Using incorrect oil can cause:

• Increased internal leakage at operating temperature
  
• Reduced pump efficiency
  
• Delayed response from control valves
  
• Excessive heat buildup in the hydrostatic loop
  

CAT recommends HYDO Advanced 10 or equivalent ISO 32 hydraulic oil for optimal performance. In warmer climates or high-hour machines, ISO 46 may be acceptable, but ISO 68 is generally too thick and can cause sluggish behavior.
Testing and Adjustment Strategies
To diagnose slow travel issues:

• Check hydraulic oil grade and brand
  
• Inspect and replace filters, cutting them open to check for metal debris
  
• Test the underspeed override valve by disconnecting its linkage and observing speed changes
  
• Monitor charge pump pressure and flow
  
• Verify pump drive oil level and condition
  

If the machine speeds up when cold but slows down when hot, oil viscosity is likely the culprit. If speed remains constant regardless of temperature, mechanical wear or valve malfunction may be involved.
Field Anecdotes and Practical Advice
A technician in Missouri recalled a 973 that crawled like a turtle until the underspeed override valve was manually disabled. Once bypassed, the machine regained full travel speed. Another operator in Kentucky noted that his 973 performed poorly after a dealer service, only to discover that the wrong oil had been used during refill.
In one case, a farmer noticed that his 973 moved faster when cold and slowed down after an hour of operation. After switching to ISO 46 hydraulic oil and replacing the filter with a genuine CAT unit, the loader’s performance returned to normal.
Recommendations for Long-Term Reliability
To maintain optimal travel speed and hydraulic performance:

• Use only high-quality ISO 32 or ISO 46 hydraulic oil
  
• Replace filters every 250–500 hours and inspect for contamination
  
• Monitor charge pump function and maintain proper oil level in the pump drive box
  
• Avoid mixing oil brands or viscosities
  
• Keep the underspeed override system properly adjusted and lubricated
  

If the machine has over 10,000 hours, consider a full hydraulic system flush and inspection of pump wear surfaces. Hydrostatic systems are sensitive to contamination and viscosity changes, especially as tolerances widen with age.
Conclusion
Slow travel in a CAT 973 is often a symptom of hydraulic mismatch rather than mechanical failure. Oil viscosity, filter condition, and valve calibration all play critical roles in maintaining speed and responsiveness. With proper fluid selection, routine maintenance, and attention to system behavior, the 973 can continue to deliver powerful performance—even decades after its debut. In hydrostatic machines, the right oil isn’t just a detail—it’s the difference between crawling and charging.