Resolving Drive Motor Shudder on the Bobcat 753

[MikePhua]

The Bobcat 753 and Its Hydraulic Drive System
The Bobcat 753 skid steer loader was introduced in the mid-1990s as part of Bobcat’s B-series lineup, offering a compact footprint, 43-horsepower diesel engine, and a rated operating capacity of 1,500 pounds. It quickly became a staple in landscaping, construction, and agricultural operations due to its reliability and ease of maintenance. Central to its mobility is the hydrostatic drive system, which uses variable displacement pumps to power hydraulic motors on each side of the machine.
Unlike mechanical transmissions, hydrostatic systems rely on fluid pressure and flow direction to control speed and movement. Each side of the loader is independently driven, allowing for zero-radius turns and precise maneuvering. However, this design also means that any imbalance or fault in the hydraulic circuit can cause erratic behavior—such as shuddering, hesitation, or directional weakness.
Symptoms of Right-Side Shuddering
A common issue reported by operators is shuddering or struggling on one side of the machine—typically during forward motion. In one case, a Bobcat 753 with approximately 3,000 hours exhibited sloppy and inconsistent movement on the right side when moving forward, while reverse remained smooth. After swapping hydraulic lines at the pump, the issue reversed—indicating that the problem was not isolated to the motor alone.
Typical symptoms include:

• Shuddering or vibration during forward travel on one side
  
• Smooth operation in reverse
  
• Directional weakness that changes when hydraulic lines are swapped
  
• No visible leaks or external damage
  

These signs suggest a pressure imbalance or internal leakage within the pump or motor circuit.
Terminology and Component Breakdown
- Hydrostatic Pump: A variable displacement pump that controls hydraulic flow to the drive motors based on joystick input.
- Drive Motor: A hydraulic motor mounted to each side of the loader, converting fluid pressure into rotational force.
- Charge Pressure: The baseline pressure in the hydraulic system that ensures pump and motor stability; typically around 300 psi.
- Relief Valve: A pressure-regulating valve that prevents over-pressurization and protects components from damage.
- Check Valve: A one-way valve that maintains directional flow and prevents backflow in the hydraulic circuit.
Diagnostic Strategy and Pressure Testing
Before replacing components, technicians should perform a charge pressure test. This involves connecting a pressure gauge to the test ports on the pump and monitoring readings during forward and reverse operation. If pressure drops significantly in one direction, the fault may lie in the pump’s internal valving or control linkage.
Recommended steps:

• Test charge pressure during forward and reverse strokes on both sides
  
• Cap hydraulic lines at the pump to isolate motor behavior
  
• Compare pressure readings with factory specifications
  
• Inspect control linkages for wear or misalignment
  

If pressure remains consistent when lines are capped, the issue likely resides in the drive motor. If pressure drops in one direction only, the pump or control linkage may be at fault.
Common Causes and Repair Considerations
Shuddering can result from several factors:

• Internal leakage in the motor’s piston or valve plate
  
• Worn or contaminated relief valves
  
• Misadjusted control linkages causing uneven stroke
  
• Air entrainment or cavitation in the hydraulic fluid
  
• Contaminants from previous motor failure lodged in the pump
  

In one documented case, both drive motors had been rebuilt recently, but the issue persisted. This suggests that debris from the original failure may have migrated into the pump or relief valves, causing intermittent blockage or pressure loss.
Recommendations:

• Flush the hydraulic system thoroughly after motor replacement
  
• Replace or clean relief and check valves associated with the affected circuit
  
• Inspect pump control arms and linkages for equal travel
  
• Use OEM-spec filters and fluid to maintain system integrity
  

Historical Context and Lessons from the Field
Hydrostatic drive systems became standard in compact loaders during the 1980s and 1990s, replacing chain-and-sprocket designs. While more efficient, they introduced new diagnostic challenges. One technician recalled a similar issue on a Bobcat 763, where a faulty relief valve caused shuddering only when hot. Replacing the valve resolved the issue, but only after hours of pressure testing and line swapping.
Another operator noted that improperly torqued motor bolts can cause misalignment, leading to vibration under load. Ensuring proper installation torque and gasket seating is essential during rebuilds.
Conclusion
Shuddering on one side of a Bobcat 753 during forward motion is often a symptom of hydraulic imbalance—either in the pump, motor, or associated valves. Swapping lines can help isolate the fault, but true resolution requires pressure testing and component inspection. With careful diagnostics and attention to hydraulic cleanliness, the 753 can return to smooth, reliable operation. In hydrostatic systems, fluid tells the story—and every shudder is a signal worth decoding.