Bobcat 753 Drive Control Problems

[MikePhua]

Overview of the Bobcat 753
The Bobcat 753 is a classic skid-steer loader known for its maneuverability and robust hydraulic systems. It uses hydrostatic drive with separate drive motors powering each side, controlled via steering levers that regulate hydraulic swashplate angles. This design allows precise speed and directional control, essential for compact loader performance.
Typical Drive Control Issues
A common problem reported is uneven power delivery where:

• The wheels on one side, for example left side, only creep slowly despite hydraulic pumps running.
  
• The opposite side wheels (right side) exert full power and suddenly “hook up,” causing wheels to dig into soft ground like sand, leading to traction loss and uneven operation.
  
• Swapping drive motors between sides confirms the issue remains with the same side, indicating the problem lies not in the motors but elsewhere.
  
• Symptoms occur consistently rather than intermittently, limiting practical operation time.
  

This behavior disrupts smooth steering and can accelerate component wear due to uneven load.
Potential Causes

• Relief Valve Malfunction: Relief valves situated behind the hydrostatic pumps regulate system pressure to prevent overloading. If faulty, the valve may cause pressure drop or inconsistent hydraulic flow to one side, reducing wheel torque.
  
• Valve Spool or Control Linkage Issues: Wear or sticking valves and loose linkages controlling the swashplate angles can cause delayed or incomplete hydraulic power delivery.
  
• Hydraulic Pump or System Leaks: External or internal leaks reduce available flow and pressure for one drive motor.
  
• Drive Motor Swashplate Problems: Swashplate angle sensors or actuators malfunction, throttling motor output.
  
• Hydraulic Fluid Contamination or Degradation: Dirty fluid could cause valve sticking or pump wear, leading to uneven drive.
  

Troubleshooting Suggestions

• Inspect Relief Valves: Test the relief valves for correct operation and pressure setpoints. Replace if leaking or stuck.
  
• Check Linkages and Control Valves for Wear: Tighten or replace loose parts and clean valves.
  
• Hydraulic Fluid Maintenance: Flush and replace fluid and filters to ensure contamination-free operation.
  
• Pressure Testing: Use gauges to verify consistent pressure delivery across both sides during operation.
  
• Electronic Controls: For models with electronic controls, confirm sensor calibration and wiring integrity.
  

Operator Tips

• Monitor for early indications of uneven steering such as hesitation or jerking movements.
  
• Limit operation duration under known fault conditions to prevent further damage.
  
• Engage experts for repair due to complexity involving hydraulic pressure regulation.
  

Glossary of Terms

• Hydrostatic Drive: A drive system using hydraulic fluid flow controlled by variable pumps and motors.
  
• Relief Valve: Safety valve designed to maintain system pressure within limits by releasing excess fluid.
  
• Swashplate: A rotating disc controlling pump or motor displacement and thus flow or torque.
  
• Valve Spool: The moving part inside control valves that directs hydraulic fluid flow.
  
• Hydraulic Contamination: Pollutants in hydraulic fluid causing damage or malfunction.
  

Conclusion
Drive control problems in Bobcat 753 loaders causing one side to underperform relative to the other often trace back to hydraulic relief valve faults, control valve issues, or fluid contamination. Systematic inspection and maintenance are key to resolving such problems, restoring balanced drive and smooth operation for these enduring skid-steers. Preventive care of hydraulic components and understanding their function prolong equipment life and reduce operational disruptions.